Kamal M. Ibrahim
National Agricultural Research Station
It has been estimated that 47% of the earth's land surface is used for grazing and forage production, making this the most expansive form of land use worldwide. Most of this land is covered with some form of woody vegetation. It comprises the world's shrublands, forests, grazing lands of savanna areas, and most desert regions. The world's ruminant and equine livestock population totaled a little less than 3 billion head in 1976 (fao, 1976). The grazing lands of forage provide the major part of their nutrition, as well as that of the world's wildlife population. In countries such as Sudan and those of the Near East, roughly 90% of the total feed for ruminants comes from rangelands. Much of this land is under extensive use and is of low productivity due to physical limitations, such as low and erratic precipitation, poor soils, rough topography, short growing seasons, low temperatures, or other reasons, such as inaccessibility, which make them unsuitable for cultivation.
The role of fodder trees and shrubs in pastoral economies depends largely on climatic conditions. In areas of moderate to good rainfall they are of less importance, since livestock largely depend on the grasses and other herbages. In semi-arid areas, trees and shrubs are of considerable value since they provide the main supply of food for livestock and wildlife. Areas in which fodder trees and shrubs are the principal if not the only source of fodder include large areas of semi-desert shrublands and bushlands throughout Africa south of the Sahara, the maquis of the Mediterranean basin and adjacent Near East countries, the semi-arid regions of Central Asia and the Soviet Union, extensive areas of the Indian subcontinent, the chaparral of California and the forest ranges of the southern states of the United States, as well as large semi-arid areas of South America and southern and western Australia.
Many of the world's poorest people live a precarious existence in the semi-arid and arid tropics and subtropics. Livestock production is often the most economically efficient way to utilize the abundant land and scarce water resources of these arid lands; consequently, plants that can provide a greater quantity and better quality of livestock feed are highly desirable. Adapted fodder trees and shrubs have some of the highest potentials to improve natural arid rangelands in terms of total productivity and nutritive value, as well as a source of feed reserve during dry seasons and prolonged drought periods.
The importance of trees and shrubs in terms of sustained dry matter yield and high nutritive content has often been overlooked, in spite of the fact that throughout the world more animals feed on ecosystems in which the trees and shrubs play an important part than on true grasses or grass-legume pastures. A great deal of research has been carried out on grasses and legumes, but there is still inadequate knowledge about trees and shrubs. In developed countries, many misconceptions about the potential production of fodder shrubs exist, and in developing countries traditions, lack of suitable technology, and harvesting for fuel are also major problems. As a result, shrublands are often either mismanaged, underutilized, degraded, or depleted.
Man's demands on natural resources that sustain his existence, and population pressure on land resources, are increasing at an alarming rate. At the beginning of the present century, the world population was only a little over 1.5 billion. It rose to 4 billion in 1975 and it reached 4.5 billion in March, 1980. It is projected to be about 6.3 billion by the end of this century (United Nations, 1977), with 80% living in developing countries. Livestock population is also rapidly increasing. In the period between 1955–1976, cattle numbers have risen 30% in the world as a whole, with the highest rate of 62% in the Near East, 51% in Latin America, 48% in Africa, and 23% in the Far East. Sheep and goat numbers have risen 21% in the world, with the highest increase of 52% in Latin America, 48% in Africa, and 23% in the Far East. Sheep and goat numbers, in turn, have risen 21% in the world, with the highest increase of 52% in the Asian centrally planned economies and 44% in Africa. A similar or even higher trend is expected during the next 25 years, with the rapidly increasing demands for livestock products, which will rise at about the same rate as the general demand for food in the developing countries (Bommer, 1978).
To meet the demand for food in the developing countries, agriculture and livestock production should be more than doubled. Fortunately for livestock production, there is great still-untapped potential from shrubs in the vast rangelands of the world. In recent years, scientific research has gradually accumulated much information about shrubs in terms of their nutritive value, dry matter yield, palatability to livestock and wildlife, use for wildlife habitat, physical characteristics, and other biological functions.
This paper covers the different factors affecting the dry matter yield, palatability, nutritive value, and utilization of fodder shrubs. A special effort is made to cite examples from different parts of the world. Some recommendations as to the needs and opportunities for research and development programs, particularly for developing countries, are also presented.
Review of Literature
In this presentation no attempt has been made to review all the literature on the fodder trees and shrubs of the world, since that is beyond the scope of this paper. However, it is worthwhile to mention some of the important publications covering this subject.
The first real attempt to summarize the existing information about fodder trees and shrubs on a worldwide basis was included in the Imperial Agricultural Bureaux Joint Publication No. 10 (1947), entitled “Use and Misuse of Shrubs and Trees as Fodder.” In this publication, edible trees and shrubs of each continent were discussed separately by different authors, each with a different background and method of approach to his subject. This publication contributed to our knowledge, but produced no clear picture of resources, utilization, management, and potential of the browse species in any given country.
More recently, the International Symposium on Wildland Shrubs — their biology and utilization (McKell et al., 1972) presented the accumulated knowledge on fodder shrubs with a wider scope as compared with the former publication. The interest in fodder shrub research appears to have quickened, and numerous research and development projects have given further insight to shrub potential as a source of animal feed. The interest is very evident in the United States, but still far from being a reality in many developing contries.
Since the turn of the century, there have been over 45 different publications in the United States dealing with subjects on shrubs. McGinnies (1972) reported very briefly on most of these publications. One of the relevant publications is that by Dayton (1931) on the “Important Western Browse Plants.” In Australia, Corbet (1957) reviewed information on about six introduced species and 50 native fodder trees of Australia. In “The Trees of South Wales,” Anderson (1957) presented notes on the usefulness of many species for fodder production. A booklet by Everist (1969) published notes on 97 native and 16 introduced woody plants used as fodder for sheep or cattle in Queensland. He included proximate analyses for 54 native and 11 introduced species. Chippendale and Jephcott (1960) published an extension booklet on the palatability, fodder value, and proximate chemical analyses for 60 different species of trees and shrubs utilized as “topfeed” by livestock in Northern Australia. More recently, a publication entitled “The Use of Trees and Shrubs in the Dry Country of Australia,” published by the Department of National Development, Forestry and Timber Bureau (1972), gave more detailed information on fodder trees and shrubs with respect to their use for soil erosion control, shade, timber, shelter belt, ecology, silviculture, preservation, propagation, and utilization. Another publication, entitled “Fodder Shrubs and Trees in Pakistan” (Khan, 1965), gives an excellent account on the trees and shrubs of Pakistan and Bangladesh.
Over 2,500 papers have been published in different parts of the world on fodder trees and shrubs. The largest number is in the United States, covering a very wide range of subjects, from factors affecting the dry matter yield, nutritive value including digestibility, palatability, grazing, ecology, establishment including germination, adaptation, regeneration and fertilization, utilization by livestock and wildlife, control, and toxicity of fodder shrubs. Special interest has been given to the techniques of measuring shrub productivity. Major emphasis has been laid on desert shrubs as compared to trees.
In Australia, emphasis has been laid on the description of the important tree and shrub species, as well as on the dry matter yield, nutritive value, ecology, utilization, and the establishment of selected species. In the Indian subcontinent, the main interest is on lopping fodder trees, the chemical analyses of fodder trees and shrubs, and their ecology and production. In South Africa, work has mainly focused on fodder shrubs as related to their palatability, nutritive value, ecology, establishment, and utilization. In the Soviet Union work on the palatability, chemical analyses, dry matter yield, and establishment of desert fodder shrubs received the greatest efforts. In North Africa and the Near East, the main emphasis has been on the ecology of native fodder shrubs and on the assessment of introduced species. In Latin America, work has been mostly on the ecology of fodder shrubs and, to a limited extend, on their dry matter yield.
Shrubs play an important part, in one way or another, in augmenting the grazing values of practically all ranges. They represent from less than 10% to as much as 80% of the plant cover. Generally, owing to relatively lower palatability than associated grasses, fodder trees and shrubs do not furnish a substantial proportion of forage eaten by cattle and sheep. This may vary from 2% to 30% of the total forage consumed, depending upon density of browse, season of growth, availability of species, and palatability. In the case of goats and wildlife, browse usually provides from 25% to 50% of the forage during the growing season, and up to 75% or more during the dry periods. Nearly every shrub provides food for one or more species of wildlife, big game, and domesticated animals.
There appears to be no reliable single index for determining palatability in trees and shrubs, and no absolute scale by which it may be measured. Palatability is usually assessed subjectively by observation of animal behavior. It depends on a number of interacting variables, including class of livestock, breed and normal behavioral pattern of the animals under observation, and the stage of growth, degree of previous utilization, physiological condition of both the species in question and that of the associated species, and environmental factors such as edaphic and climatic conditions as well as burning and shading.
In certain circumstances, considerable differences in palatability between individuals of the same species may exist, even when the plants are growing on the same soil type. For instance, in Australia, Anderson (1974) reported that the plants of Geijera parviflora grown in the same paddock, with no apparent botanical differences, were browsed differently by stock. The factors or constituents which are responsible for such genetic variability are still unknown. These differences may also be due to the incomplete knowledge of the taxonomy of the species.
In general, species of moderate to low palatability (not unpalatable) are important as forage reserve since livestock prefer associated grasses and forbs when these are available. In some situations, however, high palatability can be a distinct disadvantage —not to an animal but to the plant itself and its environment. Excessive grazing on highly palatable species or an ecotype may lead to its disappearance from the community, and its replacement with other less palatable and less productive species. This happened to the palatable species of Rhagodia and Eremophila in the mulga country in arid western Australia (Everist, 1972).
Seasonal Changes in Palatability
Observation of the grazing habits of livestock and wildlife reveal different animal preferences for certain forage at different seasons of the year. Perhaps the most important single factor affecting palatability of a given species or group of species is the seasonal change in plant growth. Some species were grazed only at certain times of the year. Also, animal preference of a given species varies with the seasonal changes in plant growth.
In Canada, McLean and Willms (1977) studied the diet of beef cattle. In the summer, consumption of forbs and trees increased, while consumption of grasses remained constant and consumption of shrubs decreased. In autumn, grass and shrub consumption increased, while forb and tree consumption decreased. Grazing intensity was generally less on the habitat with trees present than on the others. In Utah, in late spring and early summer, sheep grazed mostly herbs. After mid-July, sheep grazed Purshia tridentata (Jensen et al., 1972).
Fecal samples of mule deer (Odocoileus hemionus) on a winter range in northwestern Colorado were examined by Hansen and Dearden (1975) to study the composition of its diet. Pinus edulis and Juniperus osteosperma comprised 83% of the diet, and Artemisia tridentata, Purshia tridentata, and Amelanchier utahensis comprised another 13%. Ten other plant species composed 4% of the diet. Food habits of white-tailed deer (O. Virginianus) were investigated in Texas by Everitt and Drawe (1974). Average percentage rumen contents by volume in early spring were 23.2% browse, 51.5% forbs, 2.3% grass, 12.9% Opuntia spp., and 9.6% unknown; in late spring, 39.6; 27.5; 2.6; 20.6, and 9.2%, respectively. Differences also occurred between years and between areas differing in soil and vegetation types.
In Arizona, with normal rainfall preceding spring, mule deer and white-tailed deer fed primarily on preferred deciduous shrubs such as Eysenhardtia polystachya, Calliandra eriophylla, and Krameria parvifolia. But, during a yearlong drought, they utilized evergreen and drought-resistant species (Anthony, 1976). Stomach contents from white-tailed deer and mule deer were examined by McCulloch (1973). Major foods were forbs, dwarf shrubs and half shrubs, mast and other fruits, and evergreen browse of both chaparral and desert zones. In most seasons evergreen browse was the least preferred forage, but was important because of stability of food supply. Some of the least preferred browse plants were important because of their fruit crops. In Montana, browse plants averaged 43; 81; 60, and 59% of the total volumes of rumen contents for mule deer in the summer, autumn, winter, and spring, respectively (Dusek, 1975).
Availability of Fodder Plants
Plants of a given palatable species should be available within the reach of animals in reasonable quantity so that they can contribute to their nutrition. In some circumstances, the presence of a few highly palatable trees in a stand of moderately palatable ones may be prejudicial to the success of the feeding. In Australia, Merino sheep being fed on Acacia aneura alone, should maintain a daily intake of at least 1.4 kg of dry matter per day in order to meet their maintenance energy requirements (Everist, 1972).
The availability and dominance of a given species is not an indication of its palatability. In semi-arid areas of Australia, where Atriplex vesicaria is a dominant constituent, it was sparingly grazed, whereas many associated species which often constitute only a minor portion of the total available forage are heavily grazed (Leigh and Mulham, 1966). Acacia excelsa is ranked fairly high as a fodder species in some regions, whereas in other areas it is regarded as of little value (Anderson, 1947). Artemisia tridentata, Larrea divaricata subsp. tridentata, Prosopis juliflora var. velutina, and P. juliflora var. glandulosa are widespread on western United States rangelands and are of low palatability to animals.
Palatability Varies Between Species
In South Australia, almost all the small shrubs of the steppe country and transitional areas are edible, but the chief species of importance and value are Atriplex vesicaria, Kochia sedifolia, K. aphylla and K. pyramidata. Livestock prefer grasses and other herbage to shrubs as long as the former are available. The order of palatability of saltbush and the bluebushes is: Atriplex vesicaria, Kochia planifolia, K. aphylla, K. sedifolia, and K. pyramidata (Wood et al., 1947).
An Atriplex canescens introduced to Iran and two native species, Artemisia herba alba and Kochia prostrata, were fed to sheep throughout a year to determine their relative palatability. Kochia has the highest and Artemisia has the lowest palatability when provided at two amounts on consecutive days Atriplex is of intermediate palatability in comparison with the two native species (Nemati, 1977).
The yearly composition of the diet of crossbred Brahman cows in Pinus palustris/ Andropogon spp. range in central Louisiana averaged 69% grasses (Andropogon, Panicum, and Paspalum spp.), 18% forbs, and 5% browse shrubs. The greatest variation in the diet occurred during the winter, when browse and pine needles composed more of the diet than in any other season. Between May and August, grasses made up more than 80% of the diet, and forbs averaged 14% (Pearson, 1976). In Canada, McLean and Willms (1977) found that the diet composition of the rumen-fistulated beef cattle averaged 93% grasses, 2% forbs, 2% shrubs, and 3% trees.
Plant Structure and Rate of Growth
In general, species with flat, soft leaves are the most palatable. Cattle are deterred from browsing species with thorns or viscid, oily or pungent leaves (Chippendale, 1963). Palatability of new shoots is usually higher than old woody parts. Also preference appears to be more closely related to growth rate or succulence than to plant species. In a comparison between utilization of the seedlings and the sprouts that followed a heavy commercial cut of hardwoods in Florida, Moore and Johnson (1967) found that 57.3% of sprouts and 14.5% of seedlings were browsed by deer.
Selected Feeding by Different Classes of Animals
Marked differences in browse preference by different classes of animals grazing in the same vegetation type at a given season have been reported by many researchers. In semi-arid woodland in New South Wales, Wilson et al. (1975) compared the diet of goat and sheep. The diet of the former consisted largely of browse, with the leaves of Heterodendrum oleifolium a consistent component, although a large proportion of herbaceous material was eaten on occasions. Sheep showed a preference for the pasture species, such as Stipa variabilis and Bassia spp. When these plants were not available, they were replaced in the diet by a higher proportion of browse, mainly of the tree Casuarina cristata.
Food studies in Botswana indicated that bushbuck is capable of single-species feeding if necessary, but feeds on a wide variety of principally woody browse species (Simpson, 1974). Rumen contents of an adult klipspringer at Gilgil, Kenya, consisted of 1.5% grasses, 62.5% Rhynchosia spp., 23.5% woody plants, and 12.5% unidentified dicotyledons (Qvortrup and Blankenship, 1974). The relationship between the selected food and rumen fermentation was investigated in Thomson's gazelle, Grant's gazelle, Impala, and Masai haired-sheep and goats in Kenya. Sheep were almost exclusively grazers. In goats, Thomson's gazelle and Impala, grass constituted about 70% of all plant parts identified. In Grant's gazelle, browse accounted for 68% of rumen ingesta (Hoppe et al., 1977).
Species browsed by Impala (Aepyceros melampus) in the E. Selous Game Reserve, Tanzania, were determined by Rogers (1976) from field observation and identification of fragments from rumen contents. Browsing increased from July to December as the dry season progressed and grazing became less available in the short grassland. In semi-arid areas of Kenya, as the grasses mature, eland gradually concentrate on browse plants while the other species continue to graze and to browse a little. When deciduous browse plants lose their leaves, eland rely on evergreen shrubs. Analyses of browse leaves show that during dry periods they were richer in protein as compared with grasses (Field, 1975).
Effect of Fertilization
Fertilizer application at different levels and combinations could change the browse utilization patterns by altering the palatability of plants (Wood and Lindsey, 1967). Bayoumi and Smith (1976) reported that nitrogen application increased the forage yield and crude protein content of both Purshia tridentata and Artemisia tridentata and their preference by elk during winter, whereas phosphate had no effect on their yield or their utilization.
Effect of Site
Edaphic conditions influence the growth characteristic of plants and thus indirectly influence their nutritive value (Cook and Harris, 1950). Shrubs and forbs, when they approach naturity, are considerably more leafy on less favorable growing sites. Plants on unfavorable sites are more palatable and are more nutritious than plants on the more favorable sites (Cook and Harris, 1950; Cook, 1959).
Physical Change in Palatability
Increase in palatability and intake of the plant material by livestock can be induced by altering its physical condition or by combining it with other additives. Euphorbia coerulescens, commonly known as “noors”, is a spiny leafless succulent that grows up to 1.5 m in height in East Cape Province, South Africa. It is almost unpalatable in natural stands, but is readily eaten by stock when cut, chopped, and fed fresh or slightly wilted (Van der Walt, 1968).
In feeding trials with mule deer fawns, adding pelleted alfalfa to Purshia tridentada and Cerocoarpus lowered the digestibility percentage and tdn content of the browse species, but gave improved palatability and weight gains. Fawns would not eat A. tridentata unless fed alfalfa in a combined pellet (Dietz, 1968).
In New Hampshire, snowshoe hares (Lepus americanus) consumed significantly more Thuja occidentalis and Acer rubrum browse when fed in dried, ground, pelleted form than when it was kept frozen until feeding. Digestibility of nutrients were reduced by pelleting of T. occidentalis but increased by pelleting of A. rubrum (Mautz et al., 1976).
Effect of Chemical Content
In Western Australia, as reported by Nichols (1938), a negative correlation exists between ether extract of Acacia aneura and palatability. In Queensland, a tendency towards the same correlation was also reported by Roe (1947). Of a number of trees cut down on the same day, those eaten by sheep and those untouched showed ether extract figures of 4.85 and 5.06 percent, respectively.
In Western Australia, the mulagas (Acacia spp.) provide a high protein fodder admirably suited to wool-growing sheep. There is a difference in the palatability of the various species, those with a high resin content usually being unattractive to stock. Some species, although unpalatable when young, are readily eaten when mature (Meadly, 1947).
Local variations in forage preferences are important, and generalizations about preferred food items without confirming data for any given area are risky. A combination of methods or obtaining food habit data appears the most useful, since any given method in use had limitations.
Among the methods used in assessing palatability are: fecal analysis, percentage rumen contents, visual assessment, hand-plucked sampling, analysis of samples from esophageal fistulas, number and size of bites, time spent on grazing, and the before-and-after-grazing method. The selection of the method depends on vegetation type, class of animals, level of accuracy needed, and labor cost.
Using the time-spent-on-grazing technique, Kelly et al., (1976), in Rhodesia, found that cattle spent 74% of their feeding time on grasses, 16.2% on herbs, and 9.3% on woody plants; the respective values for goats were 29.3%, 19.9%, and 50.3%.
Howard and DeLorenzo (1975) fed Mexican bighorn sheep (Ovis canadensis mexicana) primarily on forbs and shrubs during the spring and summer. Direct observation of feeding using a bite-count technique, showed that forbs and shrubs constituted 46.4% and 31.5%, respectively, af all bites recorded.
Two-dimensional ascending paper chromatography is used to differentiate between the subspecies, varieties, or even ecotypes of a given species. Hanks et al., (1975), using chromatographic variations within subspecies of Chrysothamnus nauseosus, found that browsing preference, determined in the field by an estimate of the green weight of herbage removed, were correlated with the presence of spots associated with phenolics characterizing the different subspecies.
Comparing the rumen and fecal analysis to describe deer diets, Anthony and Smith (1974) found that the mean number of plant species/samples was lower for rumen than for fecal samples and that fecal analysis was very time-consuming.
Dry Matter Production
The role of fodder trees and livestock in pastoral economies depends largely on climatic conditions. In areas of moderate to good rainfall they are of less importance, as the carrying capacity of the land depends on grasses and other herbages including legumes. In semi-arid areas, trees and shrubs are of considerable value. Although herbage is valuable as forage, trees and shrubs provide the main supply in these areas, especially as a reserve fodder during the prolonged dry period. The dry matter yields of fodder trees and shrubs are affected by many ecological and physiological factors. Some of these factors are also reported above under the discussion on palatability and they do affect dry matter yields.
Steppes of the Betpak-Dala desert, Kazakhstan, are used by livestock migrating between summer and winter pastures. Kirichenko (1966) recognized four desert shrubs types, dominated by: (a) Salsola rigida, Artemisia terrae-albae, and A. turanica; (b) Salsola arbusculiformis; (c) Anabasis salsa; and (d) Atriplex pauciflora. The fodder yields of these four types depended mainly on seasonal rainfall and ranged from 130–670, 360–880, and 450–610 kg/ha, respectively. The crude protein content varied from about 6 to 17%, depending on season, and was generally satisfactory although in some species it fell to 4% in summer and autumn. In the semideserts of Uzbekistan, under an annual rainfall of 108–334 mm, pastures were sown with S. rigida, S. subaphylla, and Kochia prostrata at 6, 10, and 3 kg seed/ha of these species, respectively, in 12-m-wide ploughed strips alternating with 12- or 24-m-wide uncultivated strips. During a three-year period, the dry matter yields of these species ranged from 700–2870, 1140–2300, and 760–1320 kg/ha per year, respectively, as compared with 160–580 kg from natural pastures of ephemeral grasses in spring and 80–290 kg in autumn (Shamsutdinov and Korsun, 1968).
|Drought-resistant trees offer a valuable fodder reserve during prolonged dry periods.|
|Camels browsing on P. juliflora in Niger.|
Artemisia pauciflora is another valuable fodder plant in Kazakhstan. Its dry matter yield ranges from 620 kg/ha in a year of good rainfall to 390 kg/ha in dry years (Gordeeva, 1957). In the Caspian semidesert areas, yield was 515 kg/ha (Gordeeva and Larin, 1965).
In Western Australia, the potential dry-matter production of Kochia brevifolia was at least 152–203 kg/ha. The yield was positively correlated with annual rainfall (Dept. Agric. W. Aust., 1960). In a saline region having 325 mm annual winter rainfall, A. brevifolia yielded 254 kg/ha, and Atriplex nummularia and A. semibaccata yielded 203 kg/ha (Dept. Agric. W. Aust., 1962). In Queensland, the best strain of Leucaena leucocephala Peru yields 3,800 kg protein/ha (Hutton 1961) and about 16 tons green herbage/ha (Div. Plant Ind., 1959). It provides fodder in winter and spring and makes rapid summer regrowth.
Variation Between Species
As in any other group of forage plants, the fodder production from shrubs varies between species In the Kara Kum desert, ussr, grazed or browsed material was measured for 7 shrubs and semishrubs. The amounts varied from a minimum of 0.1 to 0.3 kg green material/plant for Astragalus vilosissimus. to a maximum of 1.2 to 8.3 kg/plant for Haloxylon aphyllum (Petrov, 1966). Windbreak strips of H. aphyllum in Uzbekistan (ussr) are recommended for wind protection of pastures and sheep, for snow accumulation, and for browsing. Five-year-old plantations produce 800–1,000 kg/ha of browse material compared with 160–200 kg/ha from the original Artemisia ephemeral grass pastures (Shamsutdinov et al., 1968).
As in many other forage species, fodder shrubs grow a great deal of ecotypic variation. Many field observations revealed considerable variations among natural fodder shrub populations in regard to such characteristics as germination, growth habits, phenology, palatability, productivity, disease resistance, nutritional value, insect susceptibility, and drought, cold, and fire tolerance.
Kochi prostrata grown in Uzbekistan showed considerable ecotypic variation in terms of adaptation, seed production, and morphology (Colovchenko, 1968). In Queensland, strains of Leucaena leucocephala from Peru, El Salvador, Guatemala, and Hawaii showed great variation in dry matter yield and protein content (Hutton and Bonner, 1960).
In the United States, strains of Artemisia tridentata showed distinct differences in palatability adaptation, establishment, and production (McArthur and Plummer, 1978). In Syria, over 15 ecotypes of Atriplex nummularia were collected from range improvement areas. Introductions of this species from Australia, Egypt, Syria, and Tunisia show a great deal of variation in phenology and production when grown in Kenya. In a varietal trial of 9 forage strains of Crotalaria spp., the highest yields of green material/ha were obtained from Giant Striata, 33.4 tons, and Intermedia, 24.9 tons (Wallace, 1960).
Effect of Shading
In Texas, Segelquist and Pennington (1968) found that browse yield ranged from 92 to 197 kg/ha in undisturbed and disturbed oak/pine timber-type habitats, respectively. There was little correlation between total browse yield and tree basal area, and factors such as position on slope, direction of exposure, and soil fertility appeared to account for more of the variation on browse yield than did stand density.
In the Coastal Plain forests of Louisiana, Texas, and Arkansas, Ilex vomitoria provides browse for deer in winter. It grows well beneath trees and provides up to 162 kg/ha browse (green weight) in pine stands in eastern Texas. The yields from plants grown in the open are 5.57 times higher than under trees (Halls and Oefinger, 1968).
Forest cutting increases the understory of deer browse available to animals. In Michigan, Verne (1961) found that clear-cutting of white cedar (Thuja occidentalis) produced 5,825 kg/ha of cut deer browse (sufficient for 11 deer/acre for 100 winter days). Cutting to a 25-cm minimum stump reduced the yield by 60%. In undisturbed oak pine timber-type habitats in Texas, deer browse yield was 95 kg/ha as compared with about 200 kg/ha in areas disturbed by timber stand improvement and harvest cutting. Vaccinium spp. produced the greatest amount of browse. The species most frequently browsed were Euonymus americanus, Acer rubrum, Smilax spp., and Vaccinium spp. (Segelquist and Pennington, 1968).
In South Dakota, Kranz and Linder (1973) found that the annual understory forage dry matter production from shrubs, forbs, and grasses averaged 640, 235, and 450 kg/ha in Populus tremuloides, Pinus ponderosa, and mixed P. tremuloides/ P. ponderosa communities, respectively, and was inversely related to overstory density.
In a mixed-coniferous forest in Oregon with 3 classes of shade (open, moderate, and heavy shade), Young et al. (1967) found that the development of shrub layer was related to tree cover. They reported that open canopy areas produced significantly more herbage than the other classes. The mean shrub weight was 8,640 ± 1,891 kg/ha, and current annual growth was estimated at 92 ± 12 kg.
Prescribed burning of forested areas may increase the biomass production of leaves and shoots of woody species. This effect may be held constant for several seasons after the fire. In Pennsylvania, Hallisey and Wood (1976) found that prescribed burning of scrub oak (Quercus ilicifolia) habitat doubled the browse production. They noted an increase in the concentrations of crude protein and minerals of different scrub species after burning. Summer browsing of scrub oak by deer was greatest on the most recently burned plots. Burning the vegetation in a southern pine forest in Texas increased browse production in two years from 286 kg/ha to 359 kg/ha. The increases were shown by Ilex vomitoria, Symplocos tinitoria, Callicarpa americana, and Cyrilla racemiflora. Utilization of browse by the deer was increased from 24 to 40% due to burning (Lay, 1967).
Investigations concerning the effect of fertilization on the forage production of fodder trees and shrubs have not received as much attention as those on other herbage grasses and legumes. Perhaps the expected low economical return of fertilization did not encourage workers to investigate this subject.
Atriplex canescens is an important member of arid and semi-arid rangeland communities in the Western United States, supplying high quality forage for herbivores. A greenhouse study showed that small additions of balanced fertilizer could greatly increase growth. The results appear to warrant further study in the form of field tests to evaluate the practicality of large-scale fertilization of saltbush plantings. Japanese honeysuckle (Lonicera japonica) is one of the most valuable plants to white-tailed deer (Odocoileus virginianus) in the southeastern United States. Segelquist and Rogers (1975) reported that its yield increased significantly by 175 kg N/ha, but the further increase produced by 300 kg N/ha was not significant.
Also, Bayoumi and Smith (1976) found that the seed production, forage yield, and crude protein content of Purshia tridentata and Artemisia tridentata increased with each increment of applied N in the range 33.6–168 kg/ha. In winter, elk preferred shrubs on plots given fertilizer. The application of phosphate had no effect on yields or utilization.
Lopping and Hedging
In Bangladesh there is fodder scarcity during the dry period and most of the livestock are fed on straw. Leaves and twigs of trees and shrubs are lopped, particulary when the plains are flooded. Ficus hispida is generally cultivated in the villages for the production of fodder.
In the uplands of Pakistan, livestock owners resort to lopping of trees and shrubs in the forest to sustain the excessive number of livestock. The indiscriminate lopping and the absence of a proper management system and rotation as well as wasteful methods of feeding cattle have all resulted in the disappearance of fodder trees in such areas (Khan, 1965). In the inner Himalaya, each farm has its own alloted broad-leaved trees, particularly the evergreen Quercus incana, Q. dilatata, and Q. ilex. These are lopped on a regular rotation and carefully preserved, but those in common village lands and forests are excessively lopped for fodder (Khan, 1965).
Among the most important trees lopped in the lower hills of Pakistan are: Aegle marmelos, Anogeissus latifolia, Bauhinia racemosa, B. vahlii, Dalbergia sissoo, Grewia oppositifolia, Garuga pinnata, Kydia claycina, Ougenia dalbergioides, Nyctanthes arbotristes, Sterospermum suaveolens, Terminalia belerica, T. chebula, and T. tomentosa. Among the fodder trees recommended by Khan (1965) for planting in Pakistan for lopping are: Azadirachia indica, Albizzia lebbek, A. procera, Bassia latifolia, Ficus glomerata. Moringa pterygosperma, Morus alba, and Pterocarpus marsupium.
The basic principle underlying lopping regulations is that the trees get the necessary rest to enable them to recover from the damage caused by lopping. Saplings and small poles should not be lopped. The upper one-third of the crown of all trees is protected from lopping; branches over three inches in diameter in the lower two-thirds of the crown may not be cut (Khan, 1965).
In India, high-grade cattle are fed regularly with pods of Acacia leucophloea in Madras and the leaves of Albizzia amara in parts of Salem and North Arcot Districts (Khan, 1965). Compared with alfalfa, Desmanthus virgatus has similar contents of ash, crude protein, and phosphorus. In India, it is readily eaten by cattle and is recommended for hedging, with the prunings used for silage. About 275 kg green matter was obtained at 1 cutting from a 10-m-long hedge (Sundararj and Nagarajan, 1963).
The interplanting or sowing of palatable native or introduced shrubs in overgrazed areas may provide a means for improving their overall fodder production. They could actually enhance the nutritional balance of the range feed available for livestock and wildlife consumption, particularly during the dry period of the year.
In southwest Kyzylkum (ussr), poor natural pastures produced 50 to 500 kg/ha of dry matter, depending on rainfall. When these pastures were sown with Salsola rigida, Eurotia eversmanniana Aellenia subaphylla, Artemisia turanica, and Kochia prostrata, they yielded 420–1,100 kg/ha, and when local ephemeral grasses (Poa bulbosa, Eremopyrum distans, E. buonapartis) were added, the mixture yielded 780 kg, of which the grasses contributed 460 kg. Oversowing empheral grasses into established shrubs sown in the previous year increased the yields of S. rigida, A. subaphylla and E. eversmannia by 340 kg/ha compared with pure stands of the same plants (Alimzhanov, 1967). Under experimental cultivation in the North Caucasus (ussr), Camphorosma monspeliacum, a shrub 0.5–1 m in height, produced 5 tons of fresh material/ha in the first year and 15–20 tons in the second year, with a crude protein content of 17.5% in summer and 12.1% in winter in the first year of growth, and 14.1 and 12.6%, respectively, in the second year (Dudar and Dudar, 1968).
On the foothills of Turkmenistan (ussr), a winter pasture was established under 170–250 mm annual rainfall. The main species sown were Artemisia badghysi, Aellenia subaphylla var. arenaria Haloxylon persicum, Salsola paletzkiana, and Astragalus unifoliatus. These pastures yielded 500–1,800 kg/ha in comparison with 100–200 kg/ha from the natural range (Nechayeva, 1965). Mukhammedov (1974) found that the most suitable browse species for cultivation in the Kara Kum desert, ussr, were: H. persicum, H. aphyllum, S. richteri, A. subaphylla, and Ephedra strobilacea. These species attained a height of 100–150 cm in 5–6 years. In ten years, they provided a twofold increase in the production of aboveground biomass of grasslands and a threefold increase in the production of palatable fodder.
In semi-arid desert of Samarkant (ussr), Salsola rigida is regarded as an important plant for summer and autumn grazing, particularly for sheep and camels. Shamsutdinov (1966) reported that the dry matter production of sown pasture using this species was 800, 2,800, and 3,200 kg/ha in the first, second and third years after sowing, respectively. The crude protein content in the herbage ranged from 20.3% in the early spring growth to 11.4% at seed ripening. In Khazakhstan (ussr), K. prostrata gave an average of 1,690 kg/ha as compared with 957 kg/ha from Agropyron cristatum (Pryanishikov, 1968). In Utah (usa), this species in a mixture with A. cristatum yielded almost as much as alfalfa in the first harvest year (Keller and Bleak, 1974).
In New South Wales, Muirhead and Jones (1966) found that Atriplex vesicaria was the best among various native shrub species sown into scalded areas on degenerated pastures. This is mainly due to its relatively reliable establishment by seeds, vigorous growth, persistence, and ability to spread under both ungrazed and grazed conditions. They suggested that seed of A. nummularia can be included as supplement when sowing A. vesicaria, but that it cannot replace the latter.
In South Africa, spineless varieties of Opuntia megacantha, Monterey, Robusta, Nudosa, and Chico were found to be resistant to cochineal insect (Dactylopus opuntiae). The forage yield of O. megacantha averaged 320 t/ha (Myburg, 1958). In Tunisia, plantations of the spineless cactus (Opuntia ficusindica) are being increased by 3,000 to 5,000 ha/yr. The green fodder yields are from 10 to 30 t/ha in areas having light rainfall and up to 80 to 100 t/ha under 350–450 mm rainfall. Sheep can consume at least 10% of their body weight of cactus (Le Houerou, 1972). Domingues (1960) reports that O. ficusindica (for zero grazing) and Napalea cochenillifera (for grazing) are grown in northeast Brazil in an area over 1/4 million ha. In Tunisia, Atriplex nummularia and A. halimus can produce 1,000 to 1,500 feed units/ha under 200 mm rainfall (Le Houerou, 1972). The utilization and management of A. halimus and Artemisia herba-alba as well as other fodder shrubs in northwestern Egypt are reviewed by Ibrahim (1968).
During the last twenty years or so there has been an increasing interest, in Australia as well as in other countries, in the use of Leucaena leucocephala as a fodder plant for cattle in tropical and subtropical regions. The species is widely used also as a shade tree in plantation crops in some tropical countries, and for several other purposes.
In Mauritius, Leucaena is one of the most promising fodder shrubs, giving a green production of about 86 t/ha under a rainfall of 1,700 mm. Silage was also made with the addition of molasses (Mauritius Dept. Agric., 1957). In an attempt to evaluate the herbage production of hedges when grown between adjacent contour strips of rotational crops, Anslow (1959) obtained 500 kg per 30 m of Leucaena hedge from 5 cuttings during a period of ten months. In Hawaii, when it is grown in rows 1.75 m apart and cut when 1.80 m high, it produced 76 tons green matter/ha/yr. The crude protein content was similar to that of alfalfa but the carotene content was 50–100% higher (Kinch and Ripperton, 1962).
In semi-arid areas of Rajasthan, India, Prosopis spicigera and Zizyphus nummularia were considered to be the best browse plants for cattle, sheep, and camels on the basis of availability, palatability, and nutritive value. At medium density (14%) in natural grazing lands, these species provide about 125 kg/ha of dry fodder (Ganguli et al., 1964). However, Shankarnarayan et al. (1965) reported higher productions of 450 and 280 kg/ha for these two species, respectively.
Elgueta and Calderón (1971) reported that the growth of Prosopis tamarugo in Chile was dependent on age and spacing. The most economic spacing for production of browse leaves and pods was 13.5 m × 13.5 m. This spacing allowed a carrying capacity of up to 13.7 sheep/ha.
Factors Affecting Nutritive Value
Many environmental conditions, such as shading, available soil moisture and minerals, edaphic and climatic factors, fire, plant competition (Kozlowski, 1964), and genetic factors affect the growth of woody plants and their morphology, metabolism, and consequently the digestibility of the different shrub tissues (Short et al., 1972).
Stage of Growth
Nutrient quality of the current year twigs varies with the growth stage (Blair and Hall, 1968). Foliar nutrient content varies with leaf age (Blair and Epps, 1967), leaf positions within the crown, and season of growth (Lowry and Avard, 1965). In general, leaves are more succulent and higher in crude protein, crude fat and ash, as compared with twigs. For both leaves and twigs, relative digestibility is generally affected by the change in nutrient content, which is associated with the phenological stages of growth. The rapid decline in the digestibility of twigs occurring when twig elongation nears completion is due to development of a secondary cell wall and the increase in lignin (Isenberg, 1963). Leaves also vary in their nutrient quality (Blair and Epps, 1969).
|Fodder trees and shrubs are a major component of vegetation communities in semi-arid regions.|
|Prosopis sp. can be used successfully for range improvement.|
In South Africa, the chemical content of Opuntia inermis during different growth seasons has shown that, for all practical purposes, the plant has a constant chemical composition on a wet-weight basis (Walters, 1951). In South Africa, Bonsma (1942) confirmed that, on the whole, large trees and bushes do not change much in their vital mineral content, an opinion which was previously expressed for large and small Karoo bushes (Henrici, 1934; 1940). For this reason they are a reliable feed source, particularly deciduous shrubs and trees which are drought-resistant and less variable in their chemical composition than grasses. There is no doubt that stock graze on grass if this green material is available in any form, but in areas where drought occurs often, green grass is not always available, and then the bushes are of value.
In a study of digestibility of native and introduced rangeland species in Iran, Chadaki et al. (1974) found that the in vitro digestibility of the shrubs decreased from 71.8% at the initial growth stage to 42.6% in the mature stage, as compared with 93.1% and 72.2% for grasses, respectively. In Ghana, leaf-lopping of Antiaris africana is a palatable and nutritious fodder for cattle. It is probable that during a growing period of 9–10 months, the protein content of Antiaris africana is at least twice that of rainy-season grass and four times that of dry-season grass (Mabey and Innes, 1966).
When shrubs and forbs approach maturity, they are considerably more leafy, palatable, and nutritious on less favorable growing sites than those growing on more favorable sites (Cook, 1959; Cook and Harris, 1950). The high leaf-to-stem ratios of plants growing on unfavorable sites account for higher ether extract, protein, ash, calcium, phosphorus, and nitrogen-free extract, and less lignin, crude fiber, and cellulose as compared with plants grown on more favorable sites.
Sharif (1968) found that shading increased moisture content by 78% in grasses, 89% in forbs, and 28% in shrubs. Seasonal moisture variations ranged from 145% to 317% in forbs, 50% to 187% in grasses, and 48% to 108% in shrubs. Shading sometimes reduces the nutrient quality of woody twigs. Hall and Epps (1969) showed that nutrient quality of several shrub species was greater in the open than under a pine overstory. Burkart and Watterston (1968) found that reduced cambial activity by shade may affect earlywood-to-latewood ratios in softwoods. As a result, suppressed pines or those from poorer sites contain higher carbohydrate-to-lignin ratios than do dominant pines or those from more favorable sites. Since suppression modifies the cell wall components of shrubs, the relative digestibility of shrub tissue is also affected.
In general, fires suppress shrubs in grasslands, promote them in forests, and stimulate them in chaparral communities (Wright, 1972). Prescribed burning might increase forage availability, palatability, and digestibility of shrubs. Wild fires may destroy woody plants. In African savannah and bushland, fire can effectively suppress the bush stands and help to maintain an open grassland. When both grazing and fire operated, the major forage species decreased as under grazing alone, and the shruby species decreased as under fire alone (Moomaw, 1957).
In Idaho, during the first year after burning, the crude protein in Acer glabrum, Amelanchier alnifola and Salix spp. was generally higher than in unburned sites, but no effect was reported in the second year (Asherin, 1974). In a pine culture in eastern Texas, comparable twig segments obtained from recently burned sites of Sassafras albidum were significantly higher than segments from nonburned sites (Short et al., 1972).
As in any group of forage plants, the nutrient content of fodder trees and shrubs varies significantly among species and even related groups of species such as Atriplex and Kochia spp. Also, variations in nutrient content occur due to ecotypic variation within a given species. Seasonal variation affects the chemical content of a given ecotype and the different chemical constituents have a different pattern change. Also, the nutrient content of a given species at a certain season differs due to edaphic and climatic factors, as well as vegetation composition of associated species, and also due to previous utilization. The concentration of the different chemical characters varies widely between different portions of the stems and between leaves and shoots at a given time.
However, a general conclusion might be drawn at least for a certain group of fodder shrubs such as the salt-desert shrubs. Shrubs, in contrast with grasses, are lower in cellulose but higher in ether extract, protein, lignin, phosphorus, and calcium. In most cases, with the advance of the dry season, both grasses and desert shrubs increase in lignin and cellulose and generally decrease in ether extract, carbohydrates, crude protein, and ash content (Cook et al., 1959). For individual nutrient content the following examples are reported.
Protein is considered the most important nutrient component. However, nitrogen and its various substances are of more concern in ruminant nutrition than proteins and amino acids, which are synthesized by both domestic and wild ruminants. The amount of nitrogen compounds present in trees and shrubs varies with the environmental factors, the kind of tissue, the age or stage of development, and the season of growth. The leaves and the meristematic tissues of the fodder shrubs contain higher percentages of crude protein than stems. The tips of the stems contain higher protein levels than the mid and butt sections (Aldous, 1945).
Much of the nitrogen in the leaves of trees and shrubs is translocated to the stems during autumn before leaf fall occurs (Dietz, 1972). This flow of nitrogen into leaves and then back into the stems coincides with physiological activity, and is quite important in the tree or shrub conserving nitrogen rather than losing it in leaf fall (Dietz, 1972). It is assumed that a considerable part of nitrogen might be lost during leaf fall. This situation does occur in the evergreen fodder shrubs. Since crude protein content is significantly correlated to digestible protein content, determination of the crude protein level of a plant can give a reasonably reliable indication of feed value (Sullivan, 1962). A slight deficiency in its intake adversely affects reproduction, lactation, and growth, while a serious deficiency results in the failure of the animal body to maintain itself.
The importance of fodder trees and shrubs as a source of crude protein is noted in numerous publications from different parts of the world. In Kazakhstan, Artemisia transiliensis, which dominates good pastures grazed by sheep and cattle, contains 10–12.6% crude protein (Esnova, 1966). The crude protein content of A. pauciflora varies from 10% during the winter to 14% during the early stages of regrowth (Gordeeva, 1957). It reaches 17% in Salsola rigida (Makhmudov, 1966). Kirichenko (1966) studied four types of desert shrubs in Betpak-Dala deserts (Kazakhstan) dominated by: (a) Salsola rigida, A. terrae-albae, and A. turanica; (b) S. arbusclifornis; (c) Anabasis salsa; and (d) Atriplex cana and Artemisia pauciflora. He found that the seasonal crude protein content varied between 6 and 17%, which was generally satisfactory, although in some shrubs it fell to 4% during the summer and autumn periods.
In Australia, Kochia brevifolia yielded 1.2 t/ha containing 20% crude protein (Dept. Agric. W. Aust., 1962). Wilson (1966) reported that Atriplex nummularia, A. vesicaria, K. ciliata, and K. aphylla had a crude protein content of 14–18%.
In Zambia, the following shrubs and trees were browsed by cattle: Acalypha ambigua, Adenolodichos rhomboidues, Albizia adianthifolia, Anisophyllea boehmii, Baphia Bequaertii, Brachystegia longifolia, Eriosema engleranum, Julbernardia panicultata, Similax kraussiana, Strychnos cocculoides, and Syzygium guineense. The crude protein content in the edible parts of the plants ranged from 10.63% to 29.13%, the highest being in A. adianthifolia (29.13%) and B. bequertii (22.38%) (Lawton, 1968).
In Uganda, Wilson and Bredon (1963) evaluated the chemical composition of young shoots, leaves, and pods of 22 shrubs and trees, and 19 herbs. All samples tended to be high in crude protein and ash, and low in crude fiber. The crude protein of the shrubs and trees was higher than that of the herbs, and remainded so during the dry season. Also, they found that all species tended to be high in crude protein and ash but low in crude fiber.
It would appear that many shrubs translocate protein from stem tip to stem base during the dormant period of winter. A greater protection from cold seems to permit maintenance of some green material. In some species, bud swelling may occur in midwinter. All of these processes tend to make some nutrients available near the bases of certain species rather than at the stem tips (Cook et al., 1959). Protein content is usually highest in plant parts that are growing rapidly (Halls, 1966).
In Iraq, the crude protein content of Rhanterium epapposum in the current year's growth ranged from 100 to 450 kg/ha. It decreased during the growth period, from about 30% in sprouts and 6% in old stems in December, to about 18–19% in the leaves and 4–11% in the stems in May (Thalen, 1974). It was concluded that R. epapposum could be established and produce forage under environmental conditions—particularly climate—where no other perennial shrub could survive. The forage was produced during the last three months of the year and thereby provided an alternative and afforded protection for the annual species. The nutritive value of the parts eaten by sheep, camels, goats, and donkeys was high, and the amount of forage produced in a year with optimum rainfall was extremely high.
In South Africa, Leister (1967) reported that low-growing shrubs usually contain 3–6% crude protein, although some species such as Leucospherma bainesii contain 9–12%. The value of the low shrubs lies in their ability to retain most of their feeding value throughout the dry season. Leaves and shoots of trees and shrubs are rich in protein and provide browse in the dry season. The high protein and low carbohydrate content indicated that Atriplex species should be used in conjunction with other sources of feed high in carbohydrate in order to provide a balanced ration for livestock. In South Africa, Opuntia ficus-indica was found to be an alternative source of carbohydrate.
Carbohydrates provide most of the energy for both domesticated and wild ruminants, and also provide the needed bulk in their diets. Reports on proteins are numerous, but reports on carbohydrates of importance in specific shrubs are scarce. Carbohydrates include simple and complex sugars, starches, cellulose, hemicellulose, gums, and others. The simple hexose-sugar, glucose, is found in plant tissues and xylem sap in large quantities, and fructose in lower concentrations. Pentose sugars are important cell-wall constituents. Disaccharides such as sucrose are abundant in trees (Kramer and Kozlowski, 1960). Complex carbohydrates such as starch and cellulose are the most important polysaccharides in trees and shrubs, and are the most common reserve carbohydrates in shrubs. Starch grains occur in phloem cells and sapwood of inner bark and leaves (Kramer and Kozlowski, 1960).
Cellulose is the chief constituent of cell walls of shrubby vegetation and provides supportive structures. Hemicelluloses occur in all woody tissues and sometimes are used as reserve foods by plants (Kramer and Kozlowski, 1960). Woody plants transform sugar to starch in the summer and fall, and change starch back to sugar in winter. For ruminants this probably makes little difference in the nutritional value present, but it might have an effect on palatability and, thus, on nutritive intake (Dietz, 1972).
Despite many years of research on lignin, neither its composition nor synthesis has been completely understood. Its concentration increases in fodder shrubs with the advance of the season of growth. It is negatively correlated with both protein and dry matter digestibility. The general understanding is that it likely acts as a barrier to the ruminant micro-organisms attempting to attack the cellulose in the cell wall and various intracellular nutrients.
Cook et al. (1959) observed that the buds of some shrubs swelled during the fall and winter, whereas in others they awaited warmer weather. At the time of swelling some of the carbohydrates in the stems were converted into protein, since new growth was high in protein. Inasmuch as budding in many shrubs first takes place in the base of stems, it is believed that a concentration of protein in this portion of the stem is associated with initial budding. At least in some species, protein storage in the basal portion of the stem appears to be a normal process.
Lipids, Fats, and Related Substances
The term crude fat (ether extract) includes a wide variety of substances which are not digestible to any extent, and may even inhibit rumen function (Nagy et al., 1964), such as terpenes, resins, and essential oils. The high ether extract values obtained from laboratory analysis can give a misleading nutritional result (Dietz et al., 1962). The true lipids include simple, compound (phospho-lipides), and derived lipids (saturated and unsaturated fatty acid), and true fats and oils, most of which are digestible in varying degrees by ruminants.
From a nutritional point of view, fats are highly important reserve foods in shrubs because they contain almost twice as much energy per unit of weight as proteins or carbohydrates (Kramer and Kozlowski, 1960). Also, fats serve as an important food reserve for the shrubs themselves. Fats may account for as much as 70% in shrub seeds, but they rarely make up more than 5% of the stem and leaf components. Fats tend to decrease in shrubs with seasonal progression through midsummer, then increase in fall and winter (Dietz et al., 1962). Ruminants are not dependent upon fat in shrubs since fat is synthesized in the rumen from carbohydrates and proteins. However, range animals seem to benefit from grazing on shrubs such as Eurotia lanata, which contain good fat levels (Dietz et al., 1962).
With the possible exception of protein and phosphorus deficiencies, the most common deficiency affecting range livestock is lack of or insufficient level of available energy and/or digestible energy. Taking into account that more nutrient is required to maintain normal metabolism than for all other purposes combined, energy is a highly indicative measure of the nutritive value of forage plants and feeds. In general, fats produce more than twice as much energy-producing substance as carbohydrates.
On the other hand, proteins have only slightly higher energy values than carbohydrates (Maynard and Loosli, 1956). Gross energy values could be misleading, especially if nondigestible oil content is high, which releases a considerable heat upon combustion but is not usable by animals. Generally, browse species are good sources of energy, but some —as reported by Cook et al. (1954)— are definitely low in energy-furnishing constituents. The deciduous shrubs appear to provide less energy during dormancy than evergreen species.
Vitamins are usually characterized by physiological function. The chemical role is largely catalytic, as they usually form a part of or act with various enzymes. Vitamins a, d, and e are needed in the diet of ruminant animals, but the water-soluble vitamins, c, b-complex, k, etc., can be synthesized from other food or constituents by rumen bacteria (Dietz, 1972). Vitamin d can be synthesized from sunlight in the ruminant's body. Beta carotene is the main source of Vitamin a and approximately 50% of it in forage is converted to Vitamin a by the animal's body (Anderson, 1953).
Vitamin a is particularly important since it cannot be stored in animal bodies except for a short period. It is deficient in dry grasses and most likely in dormant shrubs. Its content in shrubs is usually high during early growth, especially in developing leaves, but declines rapidly after maturation. Evergreen shrubs tend to hold up well in carotine, therefore they are a valuable source of Vitamin a for livestock and wildlife on winter ranges.
Jameson (1952) found the carotine content in Atriplex confertifolia, Eurotia lanata, and Artemisia tridentata during the winter period ranges between 5 and 20 μg/g. Acacia aneura is the main source of fodder for sheep during drought in southwest Queensland. Its carotine content is high, 47.3 ± 19.2 mg/kg, which is adequate to meet the Vitamin a requirements of sheep under conditions where this tree is the only source of feed and the sheep are eating sufficient quantities for survival (Anson and Gartner, 1969).
Ash gives an indication of the total mineral content, but could be misleading in some fodder trees and shrubs because of high levels of silica or other non-nutritious elements. The two most important minerals normally reported in routine feed analyses are calcium and phosphorus. Other minerals such as sodium, potassium, magnesium, iron, manganese, copper, cobalt, zinc, chlorine, sulfur, and iodine, are necessary for many body processes. However, they are usually supplied in adequate amounts by most fodder trees and shrubs and are not usually reported in routine feed analyses except when it is specifically requested. Most of these minerals are needed in very small amounts and function as enzyme constituents or activators (Morrison, 1957).
The ruminant animal must have access to adequate calcium supplies. On western ranges in the United States, calcium supplies are usually ample in shrubs and may be high enough to adversely affect the metabolism of phosphorus (Morrison, 1957). Phosphorus is deficient in many shrub species on ranges throughout the United States during the winter season. During the dormant season, wide calcium-phosphorus ratios occur, to the detriment of browsing animals. In general, the ash content of desert shrubs is higher in the upper portion of the stems, but in grasses ash was higher in the lower portion (Cook et al., 1959).
In saline areas north of Lake Aral region (ussr), the microelements were determined in the ash of Kochia prostrata, Artemisia pauciflora, A. incana, Atriplex cana, Camphorosma monspeliacion, and Stipa capillata. The contents of A. pauciflora ranged from 100 to 440 ppm and were highest in Kochia prostrata; of Pb from 10 to 76 ppm, highest in A. cana; of Cr from 36 to 210 ppm, highest in C. monspeliacion; of Cu from 80 to 590 ppm, highest in A. pauciflora; of Ni from 22 to 38 ppm; of Co from 10 to 16 ppm; and that of Mo was not more than 10 ppm (Bolyshev et al., 1967).
There are marked seasonal changes in leaf sodium and potassium content. These changes are associated with the aging of the leaves, and with an increase in the concentration of salt in the soil moisture during the dry period. Atriplex spp. have both surface and deep roots, and it is possible that during the dry summer period they depend on saline soil moisture at depth. The sodium content in the Atriplex spp. and Kochia spp. is usually high, particularly during the dry period. In Morocco, Foury (1952) found that mineral substance of Kochia scoparia, K. pyramidale, and K. indica appeared to be high. In New South Wales, Wilson (1966a) found that the sodium content of A. nummularia, A. vesicaria, and K. pyramidata was 3.2–8.2% of dry matter, with the higher value in summer. K. ciliata and K. aphylla contained 2.6 and 1.8% sodium, respectively. Where drinking water contained 0.9 or 1.2% sodium chloride, the intake of A. nummularia decreased to less than half than when fresh water was available. The oxalate content of these species appeared to be low enough to avoid the danger of poisoning the sheep.
In Utah, Keller and Bleak (1974) found that the oxalate content in the leaves of 4 accessions of K. prostrata was 4.8%–6.3%, which, although not necessarily dangerous, was high enough to warrant further investigations. In Australia, Wilson (1966b) investigated the sodium content of the diets of penned sheep when fed ad libitum on the fresh leaves of various species of Atriplex and Kochia spp.; it varied from 3.2%–8.2% of the dry weight, the higher values being recorded in summer and the lower values in winter. The sodium intake of the sheep varied from 25 to 97 g/day. The voluntary water consumption of these sheep was related to the sodium chloride intake; the ratio of salt and water was within the range 1.82%–2.17%.
Although data on the proximate composition of shrubs may serve as an indicative guide for their probable nutritive value, feeding trials are needed to provide a more definitive reference. The in vivo or classic digestion trial on fodder shrubs involves feeding known amounts to animals either caged or fitted with feces collection apparatuses. This is a relatively accurate but very time-consuming method. The collection of sufficient leaves and stems of shrubs to conduct such digestion trials is often an insurmountable barrier due to the fact that it involves high expenditures for animals and feed.
The in vitro technique using rumen fluid has greatly simplified the determination of digestible dry matter. Large numbers of samples can be digested simultaneously. In a review of in vitro digestibility trials, Pearson (1970) recommends that in vitro measurements are most useful when compared directly with animal performance rather than with in vivo determination. The true in vitro dry matter digestibility depends on the source of rumen inoculum and the processing of the residues. The average in vitro digestibility as a percentage of that in vivo ranged from 93 to 99.9% for browse (Robbins et al., 1975).
Nutrient analyses of composite samples that simulated diet composition were fast and inexpensive and gave results generally in agreement with values from weighted means of individual plants. The latter procedure is recommended for obtaining more intensive data on forage quality for management purposes (Urness et al., 1975). The value of the composite sample depends upon the accuracy of compositing it to represent the animal diet.
Lignin and Digestibilities
The relationship between lignin content and digestibility is different in woody tissues compared to herbage tissues. Lignin protects carbohydrates in woody tissues better than in legumes (Pew and Weyena, 1962). The lignin/cell wall content ratio in current-year shrub twigs (Short and Harrell, 1969) is higher than the ratio obtained from grasses and legumes (Van Soest, 1964).
The lignin-carbohydrate relationship in wood is incompletely understood. Presumably, lignin surrounds the cell wall of the wood tissues and acts as a barrier against cellulolytic enzymes. Lignin in bark is chemically bonded to carbohydrates and differs in composition from lignin in wood (Jensen et al., 1963). Its protection of carbohydrates in bark is not as in wood tissue.
The chemical composition of the current annual growth of 18 deciduous browse species (Acer, Cornus, Quercus, Viburum, Betula, Carya, Corylus, Crataegus, Fagus, Hamamelis, Magnolia, Rhus and Tilia spp.) collected from the northeastern United States during the winter was determined by Robins and Moen (1975), Average in vitro digestibilities of dry matter, cell wall, hemicellulose, and cellulose were 52%, 15.4%, 20.9%, and 16.9%, respectively. Cellulose and cell wall digestibility decreased as the lignin/cellulose or lignin/acid-detergent fiber ratio increased.
In Australia, the intakes of digestible dry matter of Atriplex nummularia and A. vesicaria (about 550 and 800 g/day, respectively) are sufficient for maintenance when fresh water is available. Intake of A. nummularia is relatively low for fodder of 68–74% dry matter digestibility. The intake of protein-deficient roughage is not altered by addition of about 150 g dry matter/day of Atriplex and Kochia spp., and neither is live-weight, in spite of the high crude protein content (14.18%) (Wilson, 1966).
Although Artemisia tridentata has low palatability, it is considered a good forage when occurring in minor quantities with other forage species. Atriplex canescens is a dominant species in the salt-desert shrub association and furnishes a considerable amount of forage for sheep in early winter. The content of total digestible nutrients and digestible protein was 33.4 and 4.6%, compared with 57.5 and 10.7% for alfalfa hay.
Data on dry matter production of shrubs do not provide sufficient information to achieve the needed level of expertise to arrive at effective management decisions (Burdick, 1957). Carrying capacity estimates for livestock and wildlife could be refined by combining the dry matter yield and nutrient production data with in vitro digestibility information. Data on available nutrients, when evaluated in the light of animal nutrient requirements, provide an effective method for the proper allocation and manipulation of fodder shrubs and other forage producing plants.
The periodical botanical analyses of rumen contents of fecal material would assess the seasonal animal diet composition. This should be followed by in vitro digestible dry matter for each species using rumen juice from animals grazing on a given area. Comparing daily requirements of nutrient factors for a given class of animal with nutrients produced on the range would give a good prediction of the carrying capacity. Nutrient analyses of composite samples that simulate diet composition are fast and inexpensive and give results that are generally in agreement with in vivo production.
Managing semiarid desert ranges and subtropical bushland for the greatest sustained productivity requires a basic knowledge of the physiological reaction of plants to grazing. The comparative chemical composition of the fodder shrubs and their physiological response to defoliation has received less attention as compared with the extensive studies on grasses. Except for the unpalatable species nonuse is not normal for fodder shrubs. Most of these shrubs have evolved under different levels of grazing pressure and have adapted themselves or adjusted their structure and physiology to withstand normal use. However, any grazing use which prevents or delays the completion of a growth cycle, affects negatively the plants' vigor and their productivity (Stoddart et al., 1975).
Grazing impact consists of a multiplicity of factors, including the intensity, season, frequency of herbage removal, and the plant parts removed (Willard and McKell, 1973). Good range management practice requires the knowledge of how these various factors interact to affect the ability of plants to produce usable forage. On the basis of this information, proper utilization with the optimal amount of forage and nutrient on a sustained yield basis could be established.
The effects of grazing on the various shrubs differ between species, but it is generally agreed that some degree of grazing is beneficial. The point at which defoliation by grazing has a detrimental effect on the productivity and physiology of the plant is largely determined by the individual species concerned. Though all these factors are closely interrelated, they are treated here more or less separately for better understanding of their influence. Also, the results of simulated grazing through artificial defoliation by clipping, sometimes differs from those achieved from actual grazing trials. For the convenience of presentation in this paper, the effect of both grazing and clipping are discussed jointly.
Intensity of Utilization
The effects on vegetation of different intensities of clipping or grazing have frequently been reported under different levels, usually arbitrary and commonly labeled as light, moderate, and heavy. In Western Australia, in saline areas with 325 mm annual rainfall, the best yield obtained from Kochta brevifolia (980 kg/ha) was under heavy stocking, and from Atriplex nummularia (780 kg/ha) was under moderate grazing (Dept. Agric. W. Aust., 1960; 1962).
The degree of utilization affects the nutritive value of the shrubs. In general, as the degree of utilization increases, the content of desirable nutrients and their digestibility decreases and the total consumption of forage also decreases. In salt-desert shrubs in Utah, Cook et al. (1959) studied the effect of four intensities (0–25%, 26–50%, 51–75%, and 76–100%) on the chemical composition of seven shrubs and four grasses. For the shrubs, A. nova, Eurotia lanata, K. vestita, and Chrysothamnus stenophyllus, all showed an increase in crude protein content percent as clipping intensity increased, while Ephedra nevadensis, Atriplex canescens, and A. confertifolia showed a decrease.
In the North American salt desert range, in a comparison of moderately grazed and ungrazed pastures in 1935–1967, E. lanata increased on both grazed and ungrazed sites, while A. confertifolia decreased on ungrazed sites and was replaced by A. spinescens (Holmgren and Hutchings, 1972). In Montana, under heavy stocking rates, the percentage of twig-length removed was 63% for E. lanata, 52% for Sacrobatus vermiculatus, nil for A. confertifolia, and nil for A. tridentata; under light stocking, the respective figures were 13, 16, 3, and 1 percent, while under moderate stocking rates, respective figures were 15, 17, 22, and 1 percent (Jameson, 1952). In Wyoming, Artemisia frigida, A. tridentata, and Chrystamnus nauseosus also increased with grazing pressure (Hyde and Beetle, 1964).
In eastern Oregon and Washington, Garrison (1953) studied the resistance on five common browse species to different intensities of twig removal: 25, 50, 75, and 100% removal of annual twig growth. Highest productivity of Purshia tridentata was obtained by 75% twig utilization, 50–60% by Holodiscus discolor, 50% for C. nauseosus, 35–40% for Cerocarpus velutinus, and 50–60% for Cerocarpus ledifolius. All species were stimulated to greater forage production when many or all twigs were clipped during the dormant season (Garrison, 1953). In Oregon, Cercocarpus breviflorus, Ceanothis greggii, and Cowania mexicana var. stansburiana were pruned to remove 0, 25, 50, 75, and 100% of current-annual twig growth. Growth of all species was initially stimulated by severe pruning; this persisted for five years in C. mexicana, and to a lesser extent in C. breviflorus, and the most severe treatment slowly killed C. greggii. Production under moderate use was higher than in untreated plants (Neef, 1864).
In Southern Australia, moderate grazing of Atriplex vesicaria leads to a more compact, leafier bush, whereas complete defoliation will almost certainly result in its death. Intermittent but heavy stocking is more beneficial as regards to plant vigor than no grazing, light or continuous heavy grazing (Osborn et al., 1932). Under heavy grazing this species is replaced by Bassia spp., principally B. uniflora, and grasses, mainly Enneapogon avenaceus (Div. Plant Ind., 1958). In east Pamir, ussr, the plants which withstood heavy grazing and are present in degraded pastures are dwarf shrubs, Artemisia pamirica, A. rhodantha, and Eurotia ceratoides (Kasach, 1968).
The response of shrubs and trees to intensity of topping varies drastically between species; while topping might increase forage productivity in some species, it could be detrimental to others. Ferguson and Basile (1966) reported that topping Purshia tridentata from about 2 to 1 m above ground level resulted in nearly 9 times as much twig growth as the control shrubs during the first year. In subsequent years, topped shrubs outproduced control shrubs but at a declining rate. Topping old shrubs may be a useful, though perhaps temporary, way to increase production of this species. In a study of leaf and shoot growth on certain Australian arid zone shrubs, and in particular species of Acacia, Maconochie (1973) found that light pruning of A. kempeana was not detrimental to the plant, but rather increased the size of new shoots produced on the treated shoots.
Season of Utilization
The dry matter yield, nutritive value, vegetation composition, rate of growth, and vigor of fodder shrubs are affected by the various regimes on seasonal grazing. Different species react to seasonal grazing differently, depending upon environmental and habitat factors. A thorough knowledge of carbohydrate synthesis, translocation, utilization and storage as influenced by various ecological parameters is valuable in determining when and to what extent a given species may be utilized for optimum productivity with minimum damage. The depletion of carbohydrate reserves is believed to be a primary factor for loss in plant vigor and, subsequently, fodder production. Since plant vigor may be closely associated with carbohydrate reserves, it is worthwhile to determine the phenological growth stage in which forage species can withstand defoliation without severe reserve depletion (Trilica and Cook, 1971).
In general, in northwestern United States, autumn and winter grazing is the least detrimental, and late spring and the middle of the growing season are the most detrimental period of shrub utilization (Garrison, 1971). Grazing desert shrubs during winter nearly doubled their carrying capacity (Cook and Stoddart, 1963). However, ranges grazed in late winter, year after year, may lose some of the valuable plants for spring forage (Holmgren and Hutchings, 1972).
In Utah, in order to favour the growth of Chrysothamnus stenophyllus and Symphoricarpos spp., browsing should take place in late summer or early autumn or at least be deferred to midsummer (June and July). C. stenophyllus may be browsed lightly in July, August, or September to increase sprouting the following spring, and browsing Symphoricarpos spp. in August or September may give a high number of sprouts (McKell and Willard, 1974).
In Colorado, Atriplex canescens and Purshia tridentata are detrimentally affected by severe defoliation at fruit development and seed stages, while Artemisia frigida was affected by defoliation at any stage. These three species require one year to recover from defoliation. C. viscidiflorus and Sphaeralcea coccinea recovery takes place during a rest period following defoliation (Menke, 1974). In Texas, Bredemeier (1974) found that resting rangeland during the growing season and grazing with cattle or sheep in autumn or winter permitted secondary plant succession, reduction of A. tridentata, and recovery of grasses and forbs.
In Nebraska, changes in the botanical composition of a range under the different grazing treatments showed that Artemisia tridentata was dominant in a disclimax induced by overgrazing, indicating that ecological degeneration had set in. With moderate grazing, climax grasses, including Agropyron spicatum and A. smithii, largely replaced A. tridentata on this range within a period of less than ten years (Copper, 1953). Grazing of winter range by elk and mule deer in the Rocky Mountains of North America appeared to have an adverse effect on tree cover. C. viscidiflorus and Purshia tridentata were more abundant where grazed, while other shrubs including A. tridentata were less abundant where grazed (Gysel, 1960).
In Kazakhstan (ussr), Artemisia sublessingiana and Kochia prostrata are regarded as superior fodder to all other steppe shrubs and grasses. Autumnal use lowers the yield of these plants in the following year. Early spring utilization also has an adverse effect on yield. The use of the steppe during the budding of Artemisia appears to be most beneficial in the summer (July) (Andreev, 1938).
The most detrimental time to topping shrubs and trees correlates with the termination of leaf growth (Wright and Stinson, 1970). This stage of phenology is also related to a low accumulation of carbohydrates (Fisher et al., 1959). Prosopis cineraria is an important fodder tree in the arid region of Rajasthan. Pruning this species each year in September–October decreased the formation of galls on new branches and increased the fodder yield (Sachan and Pals, 1973). In the United States, top removal reduced the yields of Prosopis species at least 75% during all seasons of the year. The most detrimental time was the month of May (Fisher et al., 1959).
Season and Intensity of Clipping and Grazing
Many studies have been conducted separately on the effect of the intensity and season of clipping and grazing on fodder shrub production, vegetation composition, and plant vigor, but there are few studies on the nutritive value of these shrubs. Still fewer studies were conducted on the combined effect of both the intensity and season of grazing or clipping on the harvest yield. These effects are variable and they depend on the physiology of the species involved.
In Utah, Cook and Stoddart (1963) tested the response of Artemisia tridentata to three intensities of clipping during four seasons. They found that this species could tolerate 30% at any one time during winter, early spring, or late spring, but could not withstand 30% use during both winter and late spring (60% total use). They also found that 50% use was too severe during spring. Moreover, a greater loss in vigor and more dead plants resulted from late spring use than from early spring use.
Wright (1969) found that 80% clipping treatment reduced yields of A. tridentata most when applied during midsummer (July), moderately when applied during spring, and least when applied during late summer through winter months. A. tripartita responded similary during midsummer (July), but it was most tolerant to clipping during spring (April and May). During the fall and winter months, the latter species appeared less tolerant to clipping than the former.
Cook and Child (1971) found that defoliation of desert shrubs in the winter and again in the spring at only moderate intensities was considered deleterious to plant welfare. Late spring harvesting was significantly more harmful to plants than early spring harvesting. Comparing individual species, the authors found that Atriplex confortifolia made a faster recovery than either A. tridentata or A. nova. In general, desert shrubs defoliated to the extent that vigor is even moderately reduced, require long periods of nonuse for complete restoration.
Buwai and Trilica (1976) found that when Artemisia frigida and Purshia tridentata were subjected to two moderate defoliations during quiescence and rapid growth (or flowers developing) they remained in fair to good vigor at the end of the growing season. However, both species were detrimentally influenced if clipped during the latter part of the growing season. Defoliation effects were generally more severe when plants were defoliated at a heavy intensity than when defoliated at a moderate intensity during the same phenological stages. McConnell and Smith (1977) found that when Purshia tridentata was heavily grazed during the spring and early summer, it produced more forage than when moderately grazed during late summer and fall. Under the heavy grazing treatment, however, plant longevity was sharply reduced.
Frequency of Clipping
Some shrubby vegetation is not browsed unless it receives a frequent defoliation. As an example, eradication of Tamarix pentandra is probably not economically feasible in many parts of the southwestern United States. On flood plains and reservoir deltas in central Arizona, mowing tamarisk at 8-week intervals encouraged browse production. Tamarisk resprouts, which were usable as fodder with Bermuda grass, markedly increased the production of available forage (Campbell, 1966).
Needs and Opportunities for Fodder Shrub Research
In view of the many present and potential uses of fodder trees and shrubs, a sound knowledge is needed to assess their utilization and management as well as development and selection of superior strains of wider adaptation in different ecological zones.
Misconceptions About Shrub Potential
The importance of shrubs and trees as fodder plants has too often been overlooked, primarily because of inadequate knowledge on the subject. The following misconceptions are just some of those commonly expressed by livestock owners and plant ecologists alike. It should be noted, however, that in many developing countries, livestock owners would not agree with these misconceptions. They have long realized the importance of browse plants for their animals, but they lack the knowledge to make their natural shrublands more productive. It is of utmost importance, therefore, to erase any misconceptions where they exist, and to expand our knowledge on all aspects of browse plant potential and apply it in the areas of the world where it is most needed. McKell (1975) lists the following as common misconceptions about shrubs:
Fortunately such thoughts are changing. Throughout this paper many examples have been cited that contradict the above statements. It is true that man's misuse of grazing land has often led to the invasion of shrubs of low palatability, and thus decreased the total herbage productivity per unit area, as with the invasion of Prosopis juliflora in the southern United States and Artemisia tridentata in the western United States. Two factors are involved here which contribute to the misconception that these shrubs are worthless. The value of any plant is relative. If it dominates land that could support a more useful plant species, then its value is lower, but if it thrives where other plants connot adapt, then its value is higher. Prosopis spp. is one of the most valuable shrubs in North Africa and eastern Kenya. Its pods are fed to all livestock and are relatively high in nutritive value. Prosopis spp. has also shown great resistance to drought and can survive periods of heavy grazing. Without similarly productive small trees and shrubs, these lands could hardly support any human or animal population. Also, as our knowledge of different browse plants increases, we must reassess their worth. A. tridentata was considered an undesirable plant when it invaded large tracts of land in the western United States. Recent studies have shown that on the contrary, A. tridentata has much potential as a browse plant. Certain strains of A. tridentata were found to be highly palatable to deer while others were not eaten at all. Studies on other shrubs have also shown wide variations in palatability within certain populations of shrubs. Therefore, indiscriminate eradication of these shrubs would be undesirable.
Other recent studies on different browse species have shown that many, once thought to be worthless or of only limited value, can be important contributors to certain plant communities if understood and managed properly. Season of grazing, intensity of grazing, age of plant, competing vegetation, selective removal, controlled burning, and resting of land are all factors to be taken into account in order to realize the greatest potential from shrublands. The worth of a shrub should not be assessed only in terms of its food value to animals. Other important uses include erosion control, soil stabilization, and production of certain commercial raw material, such as fibers and oil.
On the question of palatability and feed value, it has been proven that a significant amount of the herbage consumed by all classes of livestock and wildlife comes from shrubs. The nutritive value of this material has been the subject of many recent studies. It is generally accepted that shrubs can be high in protein, phosphorus, and sometimes carbohydrates, when grazed at the proper time. In some studies, animals have been fed only on browse plants and results have shown that their condition could be maintained very satisfactorily. For example, in Australia sheep grazed on mulgas (Acacia), and Atriplex and Kochia spp. in the semiarid regions of the Soviet Union. Many factors affect palatibility, among which are: the presence of spines or thorns, leaf surface roughness, smell, taste, previous use, availability, and environmental factors. Generally speaking, most shrubs are proven palatable to different classes of animals.
Prosopis pods are fed to all livestock in North Africa and eastern Kenya, and are relatively high in nutritive value.
Spiny plants should not be overlooked or considered generally undesirable. In South Africa, the spiny, sweet “noors” (Euphorbia coerulescens) is valuable stockfeed when chopped and can support one sheep/ha (Roux, 1953). Several thorny Acacia spp. represent a significant proportion of the important browse plants. Goats, camels, and wildlife can readily eat the leaves of the spiny plants. In certain circumstances cattle and sheep eat the pods. As mentioned, Prosopis juliflora is a noxious shrub which infests over 16 million hectares of grassland in the southwestern United States. It is a very valuable fodder shrub in northwestern Egypt and semiarid regions of Kenya. A spineless strain of Prosopis cf. juliflora has been recently isolated in Kenya. In South Africa, milled pods of P. juliflora proved as good as alfalfa and oat hay (Kargaard and Van der Merwe, 1976). Cactus has also been proven to be of value. Spiny cactus (Opuntia spp.) can be made acceptable to livestock by burning to remove the spines. In certain dry areas of the world or during prolonged periods of drought, this can provide a very important feed supplement. In areas of northern Mexico, nepal (prickly pear cactus) is harvested for feeding dairy cows. In Tunisia, sheep are fed extensively on spineless cactus, and in northern Brazil cattle are fed on it.
Range improvement programs in the past frequently advocated eradication of shrubs. Fortunately such ideas are changing, and it is more generally recognized that some browse plants on range are important when they occur in a proper balance with grasses and forbs. In addition to providing winter grazing and drought feeding, their presence in mixed shrub, forb, grass rangelands have often been a deterrent to certain epidemic-level insect invasions. Control of shrubs should be selective. More awareness and more studies are needed so that guidelines can be drawn up for different areas of the world.
Undoubtedly, some shrubs and trees contain chemical or mechanical agents harmful to livestock and in severe cases they may cause death. Chemical analyses for toxicity should be carefully assessed. The alkaloid dimethyltryptamine was found in Acacia polycantha subsp. campylantha, A. senegal, and A. nubica, grown in Sudan (Khalil and ElKheir, 1965); cyanogenic glucoside was reported in A. sieber and A. sieberiana var. woodii (Siegler et al., 1976).; mimosine occurs in Leucaena spp.; and oxalates were reported in Atriplex nuttallii. There are very few cases reported about the harmful effects of these species to livestock and wildlife.
Autecology and Physiology
Fodder trees and shrubs are a very important component of major vegetation communities, particularly in semiarid and subtropical regions of the world. Although their importance was recognized several decades ago (Van Dersal, 1983a,b), they have never been adequately studied in different parts of the world except for a few species of Artemisia, Purshia, Atriplex, Acacia, Prosopis, Kochia, and Leucaena. Comprehensive ecological and physiological knowledge is needed on the important species with regard to their seed germination, seedling establishment, survival, growth, nutritive value, tolerance to different levels of grazing, compatibility with associated species, seed production, extent of root systems, and optimim and tolerable range of environmental factors as salinity, nutrients, temperature, moisture, and light. Data on several of these factors, if not all, are needed for a better understanding of shrubland management.
Fodder shrubs, as many other groups of plants, could be genetically improved to develop strains for better production, higher nutritive value, more disease and drought resistance, wider range of adaptation, more tolerance to grazing, and lower or absence of toxic or harmful agents to animals. To achieve this stage, a great deal of information on genetics, taxonomy, physiology, ecology, and pathology of the selected species should be available. Usually, breeding takes a considerable time to develop a superior strain. Very limited breeding work has been carried out on fodder shrubs in North America and Australia and almost none in developing countries. Though breeding may be regarded as an important long-term objective for developed countries, it is impractical for developing countries to undertake it when there is a shortage or lack of qualified forage breeders, forage agronomists, and range management specialists. As an alternative, screening native fodder shrub collections from different ecological zones and new introductions from similar ecological regions of the world would certainly help, in many cases, to identify species of strains suitable for rangeland improvement.