Tropical Forage Tree legumes:
A short version of this paper has been included in Unasylva Vol. 51, No. 200, pages 25-32.
Table of Contents
Much has been written on the role of forage tree legumes. The literature abounds with reports, scholarly papers, conference proceedings, and books which describe traditional uses of indigenous species and new opportunities with exotic species (see Reference list). Tree legumes offer many benefits. But it is the flexibility of their uses that makes them especially significant; they can be found on farms ranging from small-holder subsistence to large-scale commercial.
The purpose of this monograph was to:- (a) Review the benefits derived from planting tree legumes; (b) consider which species are important for forage; (c) analyse the important issues in improvement, management and use, (d) review the level of uptake and commercial potential of forage tree legume technology, and (e) suggest some on-going R & D priorities.
The full text follows. This summary is linked to the main text for those wishing more detail.
Much has been written on the role of forage tree legumes. The literature abounds with reports, scholarly papers, conference proceedings, and books which describe traditional uses of indigenous species and new opportunities with exotic species. Tree legumes offer many benefits. Apart from their value as feed for livestock, tree legumes are recognised for their multi-purpose contributions to the productivity of farming systems, to the welfare of people and to the protection of the environment (see Box 1.1). But it is the flexibility of their uses that makes them especially significant; they can be found on farms ranging from small-holder subsistence to large-scale commercial.
The most well known species, Leucaena leucocephala (leucaena), was once referred to as the miracle tree. This label did great damage to perceptions of the value of leucaena, especially after the movement of the psyllid insect around the world. Following this event, great hardship was suffered by those who depended on this species for their livelihood. Its limitations are now more clearly understood (Shelton and Jones 1995) and have led to a worldwide study of alternative species; both those currently in use, as well as new, but not yet domesticated species.
It is therefore timely to critique our findings as we move towards the new millenium.
The purpose of this monograph is not to review all that is known about forage tree legumes. Detailed information can be found in the literature. A list of many important publications is given in the bibliography. Rather, the aim is to review the progress of R & D on forage tree legumes in tropical and subtropical farming systems; and in particular, to consider the following questions:
(a) Are the claimed benefits from planting tree legumes exaggerated?
(b) Which species are important and why?
(c) What are the important issues that have emerged regarding their improvement, management and use?
(d) What is the level of uptake of forage tree legumes world-wide?
(e) What are the on-going research priorities and potential for further expansion of use of tree legumes for forage?
We tend to think of the world-wide interest in forage tree legumes as being relatively recent, probably because publications and promotion of tree legumes, have greatly increased over the past 15-20 years. In reality, the use of tree legumes in tropical farming systems dates back to the beginning of domestic agriculture, although early use was not for forage. Indigenous peoples had excellent knowledge of the multipurpose value of the various species available.
In Mexico and Central America, where many of our most useful forage tree species originate, there was no tradition of tree forage use. For example, Mesquite (Prosopis spp.) pods were a component of diets of inhabitants of the United States and Mexican border lands for several thousand years, and later on were consumed by the white pioneers of the 1800s (Ibrahim 1992). Its use as a browse has been more recent.
There is evidence of indigenous use of unripe Leucaena pods and seeds for human consumption in the Tehuacan Valley in Mexico since the first domestication of agriculture. Archaeological studies have located Leucaena fragments in prehistoric cave settlement sites dating back to 6800 BC and it seems that Leucaena cultivation may have begun about 2000 years ago (Hughes 1998). It continues to be cultivated for human consumption in Mexico today, but rarely for forage.
In another contrast with present day fodder use, the genus Calliandra has its centre of origin in Central America, where it has little significance for any agroforestry purpose (Arias and Macqueen 1996).
The Spanish conquerors of Mexico observed local people using and cultivating Gliricidia sepium (gliricidia) for a number of non-forage purposes (Stewart et al. 1996). From this time, gliricidia was transported around the world in several waves of introductions, beginning with Spanish colonization in the 1600s, to provide shade for plantation crops (coffee, tea, cocoa). The Spanish are thought to have introduced it to the Philippines and to the Caribbean. Later in the 1800s, it was introduced to Sri Lanka and other Southeast Asian countries and finally to West Africa in the 1900s (Stewart et al. 1996).
A recent study of Erythrina in rural populations of Costa Rica showed that food, fuelwood, medicine, construction, living fences, and shade, but not forage, were the principal traditional uses (Nygren 1993).
There are some examples where the principal indigenous use of tree legumes was and is for forage. These tend to be in the drier regions of the world, e.g. the Sahel and North Africa. Even today, in these arid and semi-arid zones, tree legumes, principally Acacia spp., continue to provide a proportion of total herbage intake, and most of the protein intake, for livestock. This increases during dry periods (Baumer 1992).
More recent movements of tree legume germplasm (over the last 50 years) have largely been for agroforestry purposes, of which forage use was one of the primary proposed objectives.
A review of the qualities sought in a forage tree legume quickly shows that we are indeed looking for miracle trees (see Box 3.1). An analysis of these qualities reveals that none of our current species meet these requirements in their entirety:-
Agronomic. Apart from Sesbania spp., tree legume seedlings rarely show rapid early growth and routinely require ongoing protection against weed competition and predation until the juvenile phase has passed (Shelton 1994b).
Many species require inoculation with specific Rhizobium strains for best results (Leucaena - Lesueur et al. (1998); Sesbania sesban - Dart (1994); Prosopis spp. - Dommergues (1992); Erythrina spp. - Westley and Powell (1993); Calliandra calothyrsus - Evans (1996). Poor agronomic performance, caused by lack of an effective Rhizobium symbiosis, may be erroneously attributed to unsuitable environmental adaptation. Inoculum is often not available commercially and therefore not used in field plantings in the developing world.
While all species tolerate lenient defoliation, many species are weakened by repeated cutting, and are often damaged by direct grazing e.g. C. calothyrsus and S. sesban, especially when grazed by goats (Shelton et al. 1996). Leucaena leucocephala is a notable exception with its exceptional ability to tolerate severe defoliation, by either cutting or grazing, over extended periods of time (10-30 years).
There is a great range of adaptation to environmental stresses. Species are available for cold, acid and waterlogged environments, but we cannot expect this combination of tolerances in all species. For instance, whilst cold tolerance has been identified in Leucaena (L. trichandra, L. diversifolia, L. pallida), adaptation to acid and waterlogged soils has not (Mullen et al. 1998b). Many species are ravaged by diseases and insects (Lenné and Boa 1994, Walter and Parry 1994) and suitable methods of control are not known. Many African Acacia and American Prosopis species are thorny. The abundant seed production of some aggressive species, required for easy propagation, often leads to unwanted invasion of disturbed ecosystems.
Nutritional qualities. Clearly, high nutritional quality is desired for tree legumes managed for forage purposes. Whilst total protein levels in herbage of all species are usually adequate, the availability of this protein is often compromised by low digestibility, due to the formation of indigestible protein-tannin complexes. These are caused by high levels of naturally occurring poly-phenolic compounds (McNeill et al. 1998, Dalzell et al. 1998). The palatability of some species is low for ruminant livestock, especially for animals without prior experience of the fodder. Anti-nutritive compounds commonly occur in many species (see section 3.7).
We must be realistic in our goals when selecting forage trees for farming systems. It is important to match environment and species with need (see Box 3.2). When multiple objectives are required, in multiple habitats, it is likely that an integrated approach using several species will be appropriate.
This paper concerns the utilisation of tree legumes for forage, although this may be just one of several uses.
There can be no doubt that there is increasing demand for high quality legume forage for use in livestock feeding systems where the main objectives are commercial gain and sustainability. This can be argued for both smallholders and large-scale ranchers.
The International Livestock Research Institute expects that demand for milk and meat will double in developing countries between 1993 to 2010 (Dr Hank Fitzhugh, DG of ILRI, personal communication). Inadequate feed resources is the key constraint, especially for smallholders in peri-urban locations. ILRI suggests that the use of forage trees, in conjunction with crop residues, will be one of their key strategies to meet this increased demand.
Workers in other regions see similar opportunities. In the development of agroforestry in grazing areas of Zimbabwe, systems designed to improve forage production will make a significant contribution to farm productivity because of the importance of cattle in the farming systems and the present forage shortage (Cambell et al. 1991). In Australia, large scale leucaena plantings are occurring because farmers appreciate that leucaena-grass systems are both sustainable and highly productive. This production system allows them to produce cattle for high value domestic and export markets in East and Southeast Asia (Larsen et al. 1998) (see Box 3.6).
In southeast Asia, an increasingly affluent urban population is consuming more meat, and demanding higher quality meat. Smallholders are responding to this new and profitable opportunity with feeding strategies designed to fatten animals for slaughter at a younger age. This requires either high quality locally grown feeds or use of expensive concentrates. Farmers are finding that forage tree legumes meet this need enabling them to achieve increasing levels of profitability. Box 3.4 describes the system of "forced feeding" of leucaena in the Philippines. Box 3.3 describes the use of gliricidia for fattening goats in Aceh Province of Indonesia.
Where there is no premium available for the sale of livestock products, the value of forage trees is less appreciated. Indigenous forage tree species have generally been used for subsistence feeding rather than in commercial systems. Wickens et al. (1995) describe how fuelwood and grazing were the principal uses of the former Acacia communities in the Sahel, North Africa and the Near East (Box 3.5). However, due to overuse, some have now deteriorated, almost beyond recovery.
In eastern Sudan, where commercial livestock production is not practised, small-scale industries are almost 100% dependent on mesquite for fuelwood (Elnur A. Elsiddig, unpublished report to FAO) (Box 3.8).
To conclude, whilst forage is just one of the uses of tree legumes, it may be argued that it offers the best opportunity for commercial enterprise, where livestock markets exist. It is applicable and commercially beneficial to both small and large-scale operators. Significantly, most commercial use of forage from tree legumes has been with exotic species. Indigenous species appear to have been confined to subsistence feeding systems perhaps due to their lower productivity and lower quality. The communal management regimes employed in traditional systems place few limits on use, and this has led to over exploitation.
Accurate classification and documentation of genus and species relationships within important plant groups, is a prerequisite for effective plant improvement and utilisation of tree legumes (Hughes 1998b). Plant classifications provide the names that we use in our agronomic, nutritional and farming system programs. Accurate naming, and knowledge of the diversity available, influences the direction of plant evaluation. Without this, key taxa may be omitted and uncertainty is often the main result. This has been the case, at various points, with most forage tree legume plant improvement programs.
In some cases, important genera have yet to be described and evaluated. Only a small proportion of the total diversity in Albizia has been surveyed, and there is considerable unmapped potential in this genus (Hughes and Pottinger 1997). Further critical investigation is required to test the forage potential of Albizia.
There is incomplete understanding and confusion regarding the taxonomy, genetics and ecology of Prosopis (Dutton 1992). Hybridisation, intra-specific polymorphism and heterogeneity make it very difficult to identify some Prosopis spp. (Ibrahim 1992).
After a long period of disagreement, Leucaena has finally been described in detail and 22 species have been named (Hughes 1998a and b), compared with only 17 species in an earlier report (Hughes and Harris 1995).
Another source of taxonomic confusion is the hybridisation which often occurs when species, normally well separated in their native range, are brought together in evaluation programs. Hybrid seed is unknowingly collected and spread. Examples of this phenomenon have been documented in Leucaena (Hughes 1998a), Erythrina (Neill 1993), and Gliricidia (Lavin 1996). In Papua New Guinea, where Leucaena leucocephala and L. diversifolia were introduced separately, a vigorous inter-specific hybrid has appeared spontaneously. It is highly favoured and known locally by the incorrect name L. mexicana.
Unfortunately, in the present economic environment, it is difficult to find support for taxonomic studies, yet such activity underpins all plant improvement programs, and ultimately influences the quality of new varieties made available for farmers.
"Too often in extension work, a few exotic species have been strongly promoted without any attention being given to the rich indigenous flora and local knowledge of it" (Bekele-Tesemma et al. 1993).
Over recent years there has been increasing interest in indigenous species as an alternative to introducing exotic species, and debate concerning the appropriateness of introducing exotic species into indigenous ecosystems. There are many reasons for this trend:
(a) Farming communities have very detailed knowledge of the use and value of indigenous species, and often this has not been documented, assessed or verified (B. Calub, personal communication, Schrempp et al. 1992).
(b) There are clear ecological advantages in using a diversity of indigenous species, compared to a monoculture of exotics.
(c) Concern, sometimes for nationalistic and patriotic reasons, about preserving and conserving indigenous germplasm.
(d) A reduced emphasis on promotion of exotic species and greater in situ use of local tree diversity, may reduce risk of unwanted weed invasion and genetic pollution through hybridisation (Hughes 1994).
There is no simple answer to this debate and decisions have to be made on individual merit. There are arguments on both sides. Combined use of native and exotic species may have merit (Box 3.7).
Often exotic species are more vigorous and produce higher yields than indigenous species. This was the case in Malawi where L. leucocephala, Cassia spectabilis and Gliricidia sepium have been promoted over the indigenous Faidherbia albida which is slow growing (Cromwell et al. 1996). In fact, there are many regions where exotic species have made invaluable contributions. It has been estimated that 150 to 200 M people use gliricidia world-wide, the majority of whom live outside its native range (Simons 1996). Leucaena is now naturalised in the Philippines where it is the principal source of tree fodder and of fuelwood. This species underpins a sustainable, highly productive beef cattle production system in northern Australia (Middleton et al. 1995)(see Box 3.6).
In India, fast growing, multipurpose exotic tree species introduced with the relatively slow growing Acacia nilotica (an indigenous tree) enhance biomass production. However, competition reduces growth of the indigenous tree. Careful planning and thoughtful species selection was recommended before implementation of exotic large-scale afforestation programmes (Neelam-Bhatnagar et al. 1993).
Sometimes indigenous species are better adapted to difficult soils. In Costa Rica, native leguminous species had more potential for reforestation and agroforestry on acid soils high in aluminum and manganese than exotic species (Tilki and Fisher 1998). In contrast, in the mountainous area of Minas Gerais, Brazil, where acid infertile soils predominate, the exotic species Acacia mangium and A. auriculiformis achieved faster growth than indigenous species when introduced into an existing B. decumbens pasture (Carvalho 1997).
Exotic species can have significant effects on associated ecosystem species. In Puerto Rico, regeneration of understorey native and naturalised trees and shrubs under exotic tree species (Casuarina equisetifolia, Eucalyptus robusta, and Leucaena leucocephala), were most abundant beneath Leucaena leucocephala and least abundant under Casuarina equisetifolia (Parrotta 1995). In Hawaii, 4 species of native birds rarely feed on the fruits of the exotic nitrogen-fixing tree Myrica faya which is invading Hawai'i Volcanoes National Park. However, five species of exotic birds were seen ingesting the fruit (Woodward et al. 1990). In South Africa, invasive exotic plants such as Acacia longifolia and A. mearnsii, were detrimental to native, ground-living, invertebrate fauna. There was no significant effect on species richness and diversity, but there was a different assemblage of species associated with exotic compared to indigenous vegetation. Management should therefore be sensitive to the needs of the ecosystem to ensure conservation of desirable species when native vegetation is replaced by exotics (Samways et al.1996).
Over the last three decades, there has been movement of plant material around the world on an unprecedented scale, but there are few restrictions covering movement. Hughes (1994) advocated a more cautious approach to species introduction and a more thorough assessment of the advantages and limitations of native and exotic species to lessen the risks of introduction of a weed.
Many native plants are incompletely studied. Some species are only now undergoing preliminary domestication and are still harvested by the traditional gathering activities associated with wild species. A case in point is Acacia albida (Faidherbia albida) which is now the focus of international collaborative efforts to extend its versatility of utilization (Nouaille 1992). There is large variability in performance of individual trees as little plant improvement has occurred, and little is known of the silviculture of the species (Cromwell et al. 1996).
It is likely that the most appropriate path through this minefield is judicious use of both native and exotic species (see Box 3.7). For instance, in tropical humid Africa, research may continue on species such as Leucaena, Gliricidia and Sesbania, but emphasis may gradually shift to local species as adoption would be more rapid and widespread. Schrempp et al. (1989) noted from their work in the eastern highlands of Ethiopia that preferred species in fields were indigenous species such as A. albida, while preferred species off-field were fast growing exotics such as A. mearnsii, A. saligna, Eucalyptus spp. and P. procera.
(a) Lack of diversity in original introductions
In most cases, the movement of germplasm around the world began a long time ago. Usually the original seed that was introduced into an area was harvested from a few readily accessible trees. This meant that the early developments were based on unimproved, inferior varieties that contained little genetic diversity.
The movement of small amounts of seed of Leucaena leucocephala subspecies leucocephala from Mexico to Southeast Asia in the 1600s is the most celebrated example of this phenomenon. This species is a highly self-fertile polyploid, so that further movement of the introduced variety, was of almost identical genetic material. The spread of readily harvestable seed from country to country, region to region, organisation to organisation, farmer to farmer, has resulted in over-reliance on an extremely narrow genetic base (Hughes 1998a and b). This original leucaena, now known as the "weedy type", has invaded disturbed sites, and become a weed in many countries. We now appreciate a much greater diversity in Leucaena, some of which has significant potential value for agriculture, with much lower weed risk.
Similarly, Dutton (1992) reported that most seed of Prosopis planted around the world was of unknown origin and from a narrow genetic base. Pasiecznik (1999) confirmed that the thorny Prosopis shrubs, widespread in Africa and India, came from introductions of inferior germplasm, and this has lead to a poor appreciation of the genus. Research trials have shown that there is superior germplasm for different rainfall zones and soil types, and information on this new material needs to be disseminated.
The introduction of Gliricidia sepium from Trinidad to Sri Lanka was made with seed from one tree (Stewart et al. 1996). They suggest that the genetic diversity in many introduced populations will not be sufficient to ensure long-term stability.
There are other examples. Only two seed samples of Calliandra calothyrsus were first introduced into Indonesia (Java) from Guatemala in 1936 to provide shade for coffee. After 1974, seed was further spread by forest rangers for fuelwood use and now covers more than 30,000 ha in Java alone (Kartasubrata 1996). The first introductions of Erythrina species into Europe were for botanical and ornamental purposes and began in the early 1700s. Other introductions into Europe, Australia and the United States occurred in the late 1700s and early 1800s (McClintock 1993).
There is an added problem as farmer choices have narrowed diversity. In Flores Indonesia, a wide diversity of tree legumes was grown in farming systems in the 1960s. Species such as Acacia, Albizia, Calliandra, Cassia, Gliricidia, Pterocarpus, Sterculia and Tamarindus were all grown in diverse mixed farming systems. With intensification and commercialisation, there was greater reliance on few species notably leucaena (Djogo et al. 1995). The arrival of the psyllid in this region was particularly devastating.
The current recommendation for selecting seed from a native range, is to obtain seed from at least 25, and preferably 50 trees, with sufficient distance between them (50 m) to minimise the likelihood of co-ancestry (Allison and Simons 1996). This simple approach was not appreciated when the first introductions were made.
(b) Understanding diversity
Although the genus Leucaena has been well described both taxonomically (Hughes 1998a), agronomically and nutritionally (Shelton et al. 1998), this is not the case with all important genera. Much more investigation is required to test the forage potential of the untapped species and provenances in the Albizia genus (Hughes and Pottinger 1997).
Further, our ability to describe variation in plant material has greatly improved, and this may warrant a re-investigation of some genera. Traditionally, morphological and agronomic traits have been used to characterise patterns of diversity in plants. It is now known that these represent only a small proportion of the genome. Such traits are influenced by environmental factors, thus limiting their use for description of genetic relationships and variability. Molecular approaches such as the use of isozymes, and other genetic markers, which may be independent of environment and production responses, are likely to provide a more powerful method to gauge species relationships and origins (Dawson and Chamberlain 1996).
Macqueen (1996) confirmed that studies of molecular data, polyploidy and hybridisation research, rather than morphological work, were needed to understand the complex patterns of variation in Calliandra.
(c) Accessing high quality germplasm
It is clear that many farmers are using inferior planting material and that overcoming this limitation will not be a simple matter. Cromwell et al. (1996), in surveys of farmers using multi-purpose trees (MPTs) in Honduras, Sri Lanka and Malawi, found that the quality and reliability of MPT germplasm supply was limited. Purchase from formal sources was often expensive, and projects often obtained seed locally, as it was cheaper and more accessible. For this reason, germplasm was often:
This has not been the case with leucaena, as high quality seed marketed under species, cultivar or provenance name, is available.
(d) Problems when introducing new material to farmers
We now also appreciate that it is difficult to introduce new varieties to areas where inferior varieties are already well established. There are two reasons for this.
(a) Farmers may not be prepared to purchase seed of improved varieties e.g. of Gliricida, as they consider it a low value crop (Simons 1996). They can use existing material at no cost. New germplasm would need to be markedly superior. Although seed of the best provenances of Gliricidia is now becoming available from seed orchards around the world, much of the current demand for gliricidia is being satisified by inferior unlabelled material, and it may be difficult to promote superior provenances such as Retalhuleu (Simons 1996). Simons (1996) suggested that new varieties will need to be at least 30% better in terms of woody and leaf biomass to interest farmers.However, other experience is that farmers will plant new varieties if they recognise key benefits. For instance, farmers in Batangas Province in the Philippines were immediately enthusiastic about the new F1 hybrid KX2 leucaena (A. Castillo, personal communiucation). They appreciated its improved growth potential and its resistance to psyllids. Whereas K636 (cv. Tarramba) may not be accepted as quickly as it is more similar to the giant leucaenas already present in the Philippines.
In order to introduce new varieties it may therefore be important to:-
(a) Ensure that the variety has clear benefits e.g. insect / disease resistance, or greatly improved productivity,There are advantages in promoting farmer level (smallholder) seed production schemes to provide income for farmers, and local availability of seed. Their enterprises can also be used to create incentive to conserve the native range of threatened species. Disadvantages are lack of quality control, and difficulty in marketing away from their immediate regions.
Agroforesters require forage tree legumes adapted to a wide range of environments. Species are needed for climates ranging from the humid tropics of some Pacific Island countries, the seasonally dry tropics of Southeast Asia, the cooler high altitude tropics of East Africa, and the arid zones of the Sahel in North Africa. Similarly, adaptation to a great range of edaphic conditions is sought from the alkaline vertisiols of the brigalow soils in Central Queensland to the very acid high aluminium grasslands of South America.
A summary of the general ecological adaptation of key species is given in Table 1.
Source: Roshetko et al. (1996), Shelton (1994)
There is a wide range of ecological adaptation among tree legumes, although there are no single species suited to the entire range of conditions. Managers must select carefully to ensure successful growth of tree species in their environment. The topic of environmental adaptation is treated in detail elsewhere (Shelton 1994a). The vexed question of suitable leguminous trees for acid soils was covered in a workshop and reported in Evans and Szott (1995) and continues to be an important research objective for many workers. Several species such as Cratylia argentia, Desmodium velutinum and Flemingia macrophylla, are valued in South America because of their acid soil tolerance, but need to be more thoroughly tested for nutritional quality for ruminant feeding (Kexian et al. 1998). No tolerance of severely acid soils (pH < 5.0, with high aluminium saturation) was found in Leucaena (Mullen et al. 1998a).
High nutritive value for livestock is an essential pre-requisite for successful adoption of forage species. Without high quality, farmers may not achieve the economic animal responses they require to justify their investment. On the other hand, in areas where feed resources are grossly inadequate, or other uses are equally important, farmers may accept a more modest contribution from browse, especially if the species is indigenous and does not require specific introduction and management.
Having established the agronomic advantages and ecological adaptation of promising new varieties, they must then be assessed for nutritive value. Many researchers view this as a matter of high priority (Dicko and Sikena 1992). Plants which grow well but contribute little to livestock production are of little value as forage species. For instance, the species Cratylia argentia, Desmodium velutinum and Flemingia macrophylla, are valued in South America for their acid soil tolerance, but Flemingia macrophylla has low intake, high condensed tannin content, low digestibility and protein quality (Kexian et al. 1998).
Firstly, what is nutritional quality? Many agroforesters have only a partial understanding of this concept. Judgements are often made based on readily available chemical composition data which may be misleading. Forage quality is complex and is assessed in many ways (Table 2).
Table 2. Methods of assessment of forage quality of tree legumes
The most important measure of forage quality is intake of digestible dry matter (nutritive value), and ultimately the production of animal product. Whilst this is known for well researched species such as Leucaena leucocephala (Middleton et al. 1995), Sesbania sesban (Gutteridge 1994b), Calliandra calothrysus (Shelton et al. 1996), and Gliricidia sepium (Stewart 1996), there is much less information on other species. Much of the data available are chemical composition only, and therefore of limited value. The concepts of nutritive value of tree legumes are described in detail in Norton (1994a,b,c).
One aspect of forage quality that deserves special mention is the secondary plant compounds which are common in tree legumes. They appear to have no functional role, although they may impart ecological advantage by limiting or preventing damage from insects, fungi, bacteria, protozoa or grazing animals.
In particular, many tree legumes species contain condensed tannins (CT). These compounds are highly polymerised proanthocyanidins composed of flavanoid units with molecular weight from 1000-20,000. Tannins may have positive and negative effects on feed quality for ruminants. They bind with protein reducing digestibility of dietary protein in the rumen, but the effect may be positive if protein is released post-ruminally. The relative binding capacity of CTs varies among species eg. L. leucocephala CT appears to be "better" than L. pallida CT (McNeill et al. 1998).
However, it is clear that high levels of CT are detrimental to forage quality. Dalzell et al. (1998) showed that there was a strong relationship between in vitro digestibility and the ratio of crude protein to CT in tissues. Levels of CT above approx. 5-6% reduced digestibility.
Many genera contain high levels of tannins (>10%) e.g. Acacia (Woodward and Reed (1997), Calliandra (Shelton et al. 1996), Prosopis (Ibrahim 1992), Leucaena (Dalzell et al. 1998), and Flemingia (Kexian et al. 1998). However, there is great variation in CT levels both between and within species. This was shown in Leucaena where L. collinsii, L. lanceolata, L. macrophylla, L. magnifica, L. shannonii, L. trichodes, and L. lempirana had low CT content while L. pallida, L. trichandra, and L. diversifolia had high CT contents.
Some genera, such as Acacia, are therefore unlikely to contain high quality species. They are clearly valuable for supplemental forage but most could not supply adequate minerals when used as sole feeds (Karachi et al. 1997). Volatile fatty acid (VFA) analysis showed that mixed rumen microbes, after 12 hours, produced only 15 æmol/ml from A. angustissima fermentation compared to 63.9 æmol/ml from Sesbania sesban (Osuji et al. 1997).
Similarly, Prosopis, Flemingia, Calliandra, Erythrina, whilst important, can be regarded as species of lower quality. In contrast, key species from Leucaena, Gliricidia, Sesbania and Chamaecytisus (Osuji et al. 1997) are generally of higher quality, but there can still be significant inter- and intra-specific variation as was found in Leucaena (Dalzell et al. 1998).
There is also evidence that diets containing forage trees can influence rumen microbe composition. Extracts of A. angustissima inhibited the growth of pure cultures of rumen bacteria, while those from Sesbania sesban increased growth. Aacia cyanophylla decreased the numbers of protozoa in Ethiopian highland sheep (Osuji et al. 1997).
Palatabilty is another complex issue with tree legumes. There are reports of low palatability in Gliricidia, Sesbania, and Leucaena whilst similar material at other locations was relished by livestock. It is now clear, that `palatability' is not constant and is influenced by prior learning, time to accustom to new feeds, smell, method of presentation and breed of animal (M. Faint, personal communication).
Length of time of exposure to feeds is an especially crucial parameter in `palatability'. In 5-day trials at ILRI in Ethiopia, MPTs such as Leucaena leucocephala and Sesbania sesban and less well known species such as Acacia venosa, Acacia persiciflora, Acacia melanoxylon, Acacia hockii, Acacia polyacantha, Tamarindus indica, Chamaecytisus palmensis, Tipuana tipu, Indigofera arrecta and Atriplex nummularia, had high palatability. Flemingia macrophylla, Erythrina abyssinica, Acacia salicina, Acacia coriacea, Albizia schimperana, Ceratonia siliqua, Casuarina glauca and Erythrina burana, had poor palatability. Gliricidia sepium and Calliandra calothyrsus, had only a medium palatability ranking (Kaitho et al. 1996).
Educational programs are required to inform researchers, extension workers and farmers of the value of "apparently unpalatable" plants, including methods to overcome the initial reluctance of inexperienced animals to consume new materials.
In semi-arid and arid Africa, cattle, sheep, equines, wildebeast, most antelopes and gazelles graze forage tree legumes in the dry season to balance their diets. During the wet season, they prefer grass. Species, such as goats, camels, eland, impala, kudu, elephant, giraffe, black rhino and a number of antelope, are primarilty browsers of forage tree legumes (Wickens et al. 1995). The Orma people in the Tana and Lamu Districts of Kenya, who keep cattle, goats, sheep, camels and donkeys (in descending order of importance) found that browse preferences varied with species (Anttila et al. 1994). The ability of herbivores to graze browse trees often depends on their ability to handle thorns, woody materials, or high tannin foliage. Goats have greater preference for high tannin species than sheep or cattle, because of their ability to secrete proline rich saliva to reduce the astringency of the tannins (Kaitho et al. 1997, Kexian et al. 1998).
There is opportunity for mixing both livestock and plant species to take advantage of the varying preferences of livestock species.
Monograstric animals consume very little forage from tree legumes although there are many examples of MPTs being used for supplementing diets of monogastric livestock. However, the general consensus is that they have a limited role to play in monograstric feeding, and only as leaf protein concentrate. In general, they are not a suitable feed due to (a) their high content of anti-nutritive compounds, which non-ruminants have greater difficulty handling, (b) their high fibre content (Dutton 1992) and (c) their low energy content. Seeds are sometimes fed to monogastrics but may need detoxification procedures before being fed (D'Mello 1992).
Diseases and insects of forage tree legumes limit productivity worldwide. As the use of tree legumes is expanding rapidly, pest problems are likely to increase in occurrence and severity, yet the extent of knowledge of diseases and insects of tree legumes is generally poor.
There are data providing lists of pathogens but little information on their significance or on pathogenic variability (Lenné 1992). Disease and insect pests are reviewed for specific species and genera including Gliridicia (Boa and Lenné 1996), Leucaena (Boa and Lenné 1995), Erythrina (Westley and Powell 1993) and Sesbania (Murphy 1990). There are some important tree legume species with little information available (Lenné and Boa 1994).
There are also summary tables of insects pests on tree legumes (Walter and Parry 1994) but little is known about insect and host plant relationships and other aspects of their ecology. For this reason, there is often ignorance concerning acceptable control measures. Chemical control may be the easiest approach as a range of broad spectrum insecticides are available. However, in many cases, chemical control may not be an acceptable approach as (a) chemicals are sometimes not accessible to farmers, (b) they are expensive, (c) broad sprectrum chemicals have other harmful effects, and (d) animals may consume the sprayed leaf material with detrimental results. The leucaena psyllid (Heteropsylla cubana) is the most studied insect pest. While chemical control measures are effective and biological control using predatory insects is partially effective, the most practical, the most cost-effective, and the most ecologically sound approach is the use of resistant varieties. Yet we still do not understand the mechanisms of resistance in Leucaena necessary to develop effective screening programs (Mullen et al. 1998a).
The narrow genetic base of many species is frequently reported. The condition arose due to the movement of small quantities of seed from their centres of origin and subsequent multiplication for extended use. This has lead to widespread use of similar genetic material (see section 3.5) which will expedite speedy dissemination of pest species. The most notorious example is the leucaena psyllid.
It is vital that more detailed information is assembled on the diseases and pests affecting the cultivation and productivity of tree legumes. Country and region surveys are needed to describe the location and extent of problems. The existing networks are an appropriate way to gain information on disease and insect problems currently experienced (Lenné and Boa 1994). Catalogues and manuals illustrating the key insect and disease species are required to assist field workers, not only with identification but also with formulation of control measures. Preparation of quarantine guidelines to ensure the safe movement of seed to limit the spread of pests is another priority.
Unfortunately, there is often a lack of specialist expertise to address these problems.
The development and improvement of tree legumes for farm use is contingent on the availability of germplasm from the Centres of Origin of species to underpin improvement programs. It is therefore imperative that the native ranges of tree legumes are protected from exploitation and over use. Some examples of the current status for selected species are given below.
According to Wickens et al. (1995), the former Acacia communities in the Sahel in North Africa and the Near East have deteriorated almost beyond recovery. This has been due principally to excessive demand for fuelwood, but also to overgrazing, and demand for more agricultural land; all driven by increasing population pressures. The result is almost irreversible. Rehabilitation of these areas will be very slow where desertification and soil movement has occurred as there is little soil seed reserve. They stress the need for low cost participatory approaches, emphasising preventative rather than remedial measures.
Stands of Faidherbia albida in Wadi Aribo in western Sudan are endangered due to indiscriminate lopping for browse by camel nomads (Wickens et al. 1995).
Due to its colonising nature, G. sepium is not under threat at the species level. But certain important provenances such as Retalhuleu in Guatemala are under serious threat from human encroachment and river erosion (Stewart et al. 1996).
The majority of Albizia species is severely depleted in their native range in Mexico and Central America. Most species remain abundant in only a few areas. Promotion of greater use of the species would assist with their in situ conservation (Hughes and Pottinger 1997).
In Leucaena, the majority of species are of no conservation concern. However, three species, L. matudae, L. magnifica and L. involucrata are rare and of strong conservation concern. There are less than 400 known individual plants of L. magnifica (Hughes 1998a).
Prosopis africana is seriously threatened in the semi-arid lowlands of West Africa in Burkina Faso, Niger, Mali and Senegal. ICRAF has organized seed collections of this species to capture the genetic diversity before invaluable genetic resources are lost (Tchoundjeu et al. 1998). Patterns of genetic diversity in populations of Calliandra calothyrsus, distributed throughout Central America and southern Mexico, were examined using isozyme analysis. Four distinct population groups were identified and their conservation status was reported by Chamberlain (1998).
There are various methods used to conserve genetic resources. Hughes (1998) discusses the merits of in situ (maintenance of natural population), ex situ (e.g. germlasm banks and botanic gardens) and circa situm (maintenance while in agricultural use e.g. as hedge row) conservation. In vitro techniques for conservation and multiplication of germplasm, and elimination of disease, have been applied to the conservation of Leucaena leucocephala, Erythrina brucei and Sesbania sesban by Ruredzo and Hanson (1988). Perhaps a combination of all approaches may be necessary. As with taxonomic studies, it is now exceedingly difficult to obtain support for conservation of undomesticated genetic resources in their native range.
A number of introduced tree legumes have become serious weed pests. Given the large number of introductions to many new environments, this is not surprising. Weediness of introduced exotic trees has generally occurred when:-
Over the past 80-100 years, mesquite (Prosopis spp.) has become an aggressive invader of desert grasslands in the southwest United States (Ibrahim 1992) due to interference in the natural ecological balance by man and his activities. Strategies for control and management of this problem are still not available. Grazing livestock and reduced occurrence of fire were key factors in the increase in density of mesquite. The original movement of Leucaena leucocephala subspecies leucocaphala around the world commencing in the 1600s has lead to this inferior but seedy variety becoming a weed in many tropical environments (Hughes 1994).
Species may also become a weed in their own environment. Albizia tomentosa is a weed in disturbed areas in Mexico (Hughes and Pottinger 1997) and Acacia aneura is often weedy in southwest Queensland when poorly managed (Beale 1994).
Free movement of seed in international R & D activities may result in related taxa being planted in close proximity leading to natural hybridisation perhaps increasing weed risk (Hughes 1998a and b). This appears to be supported by molecular studies of genome structure, which indicate that gene exchange between cultivated plants and wild and weedy relatives is often considerable (Nouaille 1992).
The question of weed risk raises many difficult questions with few easy answers. Some suggest that only indigenous species should be considered in agroforestry programs. But this is an unrealistic constraint on farming systems and indeed the environment (see Box 1.1). Forage tree legumes are already major contributors to our farming systems. They have the potential to become even more important in our livestock industries thus enhancing the quality of life of rural communities. However, it will be imperative to pursue our objectives actively, but with environmental responsibility. The key is to carefully evaluate the level of risk, reject high risk introductions, and carefully manage introductions to minimise the chances of weed outbreak.
When introducing new species to an environment it may be necessary to first:-
Perhaps the most important step is to ensure that the rural community adopting the new species have the tools to make full use of the MPT. There are many examples of apparent weediness occurring because villagers may be unaware of the many uses of new plants.
Nevertheless, tree legumes should not be introduced where risk is high, or where nearby disturbed vegetation might be ecologically threatened.
Examples of successful adoption of exotic and indigenous tree legumes, for multi-purpose uses including forage, are too numerous to list. Outstanding examples are Leucaena leucocephala in Australia (Middleton et al. 1995) and Asia (Moog et al. 1998), Gliricidia sepium in southeast Asia (Stewart 1996), Sesbania grandiflora in Indonesia (Gutteridge 1994b), Calliandra calothyrsus in Indonesia (Palmer et al. 1994), and Acacia spp. in Africa (Wickens et al. 1995), to mention a selected few (see Boxes 3.2 to 3.5).
Nevertheless, despite high levels of promotion, farmer uptake has been lower than anticipated . Recent attempts to achieve adoption of new varieties and agroforestry packages, and more complex agroforestry packages such as alley cropping, have been only partially successful, and in some cases unsuccessful (Gutteridge 1998). Difficulties in achieving high levels of adoption for Leucaena are reported for Africa (Dzowela et al. 1998), South America (Argel et al. 1998) and Asia (Moog et al. 1998).
Although the forage bank concept has been shown to be feasible in tropical Africa, rate of adoption has been low due to socio-economic constraints such as land tenure insecurity and lack of infrastructure support (Cromwell et al. 1996).
The value of alley cropping especially, is hotly debated in Africa (Cromwell et al. 1996). Research groups have become cautious about the sustainable benefits of the system and in particular the value of the mulch in terms of increased crop yields on farm. Part of the problem has been identified by Grist et al. (1999) who found, using an alley cropping model, that while the use of Gliricidia in Imperata grasslands can increase soil fertility, farmers were likely to incur a loss in the first year of development, and that it would take approx. 4 years to begin making a profit.
There are many reasons put forward for the lower than anticipated levels of adoption (Smith 1992, Cromwell et al. 1996) including :-.
(a) Projects usually assume which species and agroforestry system are required by farmers. In reality, reasons for farmer choices are often complex and determined by their specific needs and resource constraints. There needs to be a range of options available to meet their various needs.On the other side of the ledger, many suggestions have been made to increase levels of adoption:-
(a) The priority is for participatory on-farm testing of new varieties and systems. It is vital that farmers are brought more directly into the decision making process to ensure that social constraints (risks, relevance, labour, environment) and economic constraints (incentives, markets and returns) are adequately addressed. It is important that exotic species fit into their existing year round feeding systems and are sustainable and persistent under regular use.Without improved levels of adoption, and more explicit demonstration of the relevance and benefits of forage tree legumes, the good will and support of funding and donor agencies will be limited.
There are several hundred species of leguminous trees with potential for forage listed in the literature (Houérou 1980, Atta-Krah 1989). Most have not been investigated and few are in current use in any significant way. Of the 5000 known nitrogen fixing woody species, Brewbaker (1986) suggested that only about 80 leguminous tree and shrub species may have potential multipurpose agroforestry roles, including fodder, in tropical farming systems. Roshetko et al. (1996) listed 46 species suitable for fodder, but many fewer have found significance in world animal production systems as key sources of forage supply.
The species and key references are given in Table 3. There may be additional
species which have forage potential, and within each species there is
genetic variation which can be exploited. However, in this brief review
only those species in significant use for forage are listed. Selection
for membership of this list was a subjective process although fodder
value was the pre-eminent selection criteria.
(key references in parenthesis)
Other species have potential but are not yet in significant use. Examples include the Leucaena pallida x L. leucocephala KX2 hybrid, L. collinsii and L. trichandra, the latter species for the high altitude tropics (Shelton et al. 1998).
Allison, G.E. and Simons, A.J. (1996). Propagation and husbandry. In: Stewart, J.L., Allison, G.E. and Simons, A.J. (eds), Gliricidia sepium - Genetic resources for farmers. Tropical forestry Paper 33. Oxford Forestry Institute. pp. 49-71.
Anttila, L.S., Alakoski Johansson, G.M., Johansson, S.G., Odera, J.A. (ed.); Luukkanen-MO (ed.); Johansson-SG (ed.); Kaarakka-V (ed.); Mugah-JO. (1994). Browse preference of Orma livestock and chemical composition of Prosopis juliflora and nine indigenous woody species in Bura, Eastern Kenya. Forestry in irrigation schemes with special reference to Kenya. East African Agricultural and Forestry Journal, 58: Special Issue, 83-90.
Argel, P.J. and Lascano, C.E. (1998). Cratylia argentea (Desvaux) O. Kuntze: una nueva leguminosa arbustiva para suelos acidos en zonas subhumedas tropicales. Pasturas Tropicales, 20, 37-43.
Arias, R.A and Macqueen, D.J. (1996). Traditional uses and potential of the genus Calliandra in Mexico and Central America. In Evans, D.O. (ed) International Workshop on the Genus Calliandra. Forest, Farm and Community Reports Special Issue, 1996. Winrock International. pp. 108-114.
Attah-Krah, A.N., Sumberg, J.E. and Reynolds, L. (1986). Leguminous fodder trees in farming systems - an overview of research at the humid zone programme of ILCA in Southwestern Nigeria. In: Haque, I., Jutzi, S. and Weate, P.J. (eds). Potentials of forage legumes in farming systems of sub-saharan Africa. ILCA, Addis Ababa, pp. 307-329.
Audru, J., Labonne, M., Guerin, H. and Bilha, A. (1992). Acacia nilotica: a traditional forage species among the Afar of Djibouti. In: Speedy, A and Pierre-Luc Pugliese (eds). Legume trees and other fodder trees as protein sources for livestock. Proceedings of the FAO expert Consultation held at the Malaysian Agricultural Research and Development Institute (MARDI) in Kuala Lumpur, Malaysia. FAO of the United nations, pp. 277-293.
Audru-J; Labonne-M; Guerin-H; Bilha-A. (1993). Acacia nilotica: its fodder value and exploitation by the Afar pastoralists in the Madgoul valley, Djibouti. Bois-et-Forets-des-Tropiques, No. 235, pp. 59-70.
Baumer, M. (1992). Trees as browse to support animal production. In: Speedy, A and Pierre-Luc Pugliese (eds). Legume trees and other fodder trees as protein sources for livestock. Proceedings of the FAO expert Consultation held at the Malaysian Agricultural Research and Development Institute (MARDI) in Kuala Lumpur, Malaysia. FAO of the United nations. pp. 1-10.
Beale, I.F. (1994). Management of mulga (Acacia aneura) scrublands in southeast Queensland. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 325-337.
Bekele-Tesemma, A., Birnie, A. and Tengnäs, B. (1993). Useful tress ans shribs for Ethiopia. Regional Soil Conservation Unit. Swedish International Development Authority. 474 pp.
Benson, L. and Darrow, R.A. (1981). Trees and shrubs of the south-western deserts. University of Arizona Press, Tucson, Arizona, 416pp.
Boa, E.R. and Lenné, J.M. (1995). Diseases and pests of Leucaena. In: Shelton. H.M., Piggin, C.M. and Brewbaker, J.L. (eds), Leucaena - Opportunities and Limitations. Proceedings of workshop held in Bogor, Indonesia. ACIAR Proceedings No. 57, pp. 129-134.
Boa, E.R. and Lenné, J.M. (1996). Diseases and insect pests. In: Stewart, J.L., Allison, G.E. and Simons, A.J. (eds), Gliricidia sepium - Genetic resources for farmers. Tropical forestry Paper 33. Oxford Forestry Institute. pp. 73-76.
Brewbaker, J.L. 1986. Nitrogen-fixing trees for fodder and browse in Africa. In: Kang, B.T. and Reynolds, L. (eds), Alley farming in the humid and subhumid tropics. proceedings of a workshop held at Ibadan Nigeria, 10-14 March 1986, IDRC Ottawa, pp. 55-70.
Campbell,B.M., Clarke,J.M. and Gumbo,D.J. (1991). Traditional agroforestry practices in Zimbabwe. Agroforestry Systems, 14, 99-111.
Carter, J.O. (1994). Acacia nilotica - a tree legume out of control. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 338-351.
Carvalho, M.M. (1997). Asociaciones de pasturas con arboles en la region centro sur del Brasil. Agroforesteria en las Americas 4, 5-8.
Chamberlain, J.R. (1998). Isozyme variation in Calliandra calothyrsus (Leguminosae): its implications for species delimitation and conservation. American Journal of Botany, 85, 37-47.
Cook, C.C., Grut, M. and Christoffersen, L.E. (1989). Agroforestry in Sub-Saharan Africa: a farmer's perspective. World Bank Technical Paper No. 112, World Bank, Washington, 99pp.
Cromwell, E., Brodie, A. and Southern, A. (1996). Germplasm for Multipurpose Trees: Access and Utility in Small-Farm Communities. Case studies from Honduras, Malawi, & Sri lanka. Overseas Development Institute. 93 pp.
Dalzell, S.A., Stewart, J.L., Tolera, A. and McNeill, D.M. (1998). Chemical composition of Leucaena and implications for forage quality. In: Shelton, H.M., Gutteridge, R.C., Mullen, B.F. and Bray, R.A. (eds), Leucaena - Adaptation, Quality and Farming Systems. Proceedings of workshop held in Hanoi, Vietnam. ACIAR Proceedings No. 86. pp. 227-246.
Daniel, J.N., Hedge, N.G. and Relwani, L. L. (1997). Performance of Albizia lebbeck in semiarid India. In: Zabala, N.Q. (ed), International Workshop on Albizia and Paraserianthes Species. Proceedings of workshop held in Bislig, Suriago del Sur, Philippines, 1994. Farm, Forestry and Community Tree Research Reports Special Issue. Winrock International. pp. 22-26.
Dart, P.J. (1994). Microbial symbioses of tree and shrub legumes. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 143-157.
Dawson, I.K. and Chamberlain, J.R. (1996). Molecular analysis of genetic variation. In: Stewart, J.L., Allison, G.E. and Simons, A.J. (eds), Gliricidia sepium - Genetic resources for farmers. Tropical forestry Paper 33. Oxford Forestry Institute. pp. 77-91.
Dico, M.S. and Sikena, L.K. (1992). Fodder trees and shrubs in range and farming systems in dry tropical Africa. In: Speedy, A and Pierre-Luc Pugliese (eds). Legume trees and other fodder trees as protein sources for livestock. Proceedings of the FAO expert Consultation held at the Malaysian Agricultural Research and Development Institute (MARDI) in Kuala Lumpur, Malaysia. FAO of the United nations. pp. 27-41.
Djogo, A.P.Y., Siregar, M.E. and Gutteridge, R.C. (1995). Opportunities and limitations in other MPT genera. In: Shelton. H.M., Piggin, C.M. and Brewbaker, J.L. (eds), Leucaena - Opportunities and Limitations. Proceedings of workshop held in Bogor, Indonesia. ACIAR Proceedings No. 57, pp. 39-43.
Djogo, P.Y. (1997). Use of Albizia species in small-scale farming systems in Indonesia. In: Zabala, N.Q. (ed), International Workshop on Albizia and Paraserianthes Species. Proceedings of workshop held in Bislig, Suriago del Sur, Philippines, 1994. Farm, Forestry and Community Tree Research Reports Special Issue. Winrock International. pp. 27-36.
D'Mello, J.P.F. (1992). Nutritional potentialities of fodder trees and fodder shrubs as protein sources in monograstric feeding. In: Speedy, A and Pierre-Luc Pugliese (eds). Legume trees and other fodder trees as protein sources for livestock. Proceedings of the FAO expert Consultation held at the Malaysian Agricultural Research and Development Institute (MARDI) in Kuala Lumpur, Malaysia. FAO of the United nations, pp. 115-127.
Dommergues, Y. (1992). Maximising nitrogen fixation in Prosopis. In: Dutton, R.W., Powell, M. and Ridley, R.J. (eds), Prosopis species - Aspects of their value, research and development. Proceedings of Prosopis symposium held by CORD, University of Durham, UK. pp. 207-218.
Dutton, R.W. (1992). A research and development strategy for Prosopis. In: Dutton, R.W., Powell, M. and Ridley, R.J. (eds), Prosopis species - Aspects of their value, research and development. Proceedings of Prosopis symposium held by CORD, University of Durham, UK. pp. 3-27.
Dzowela et al. 1998. see leucaena proceedings.
Escalente, E.E. (1997). Saman (Albizia saman) in agroforestry systems in Venezuela. In: Zabala, N.Q. (ed), International Workshop on Albizia and Paraserianthes Species. Proceedings of workshop held in Bislig, Suriago del Sur, Philippines, 1994. Farm, Forestry and Community Tree Research Reports Special Iissue. Winrock International. pp. 93-97.
Evans, D.O. (1996). International Workshop on the Genus Calliandra. Forest, Farm and Community Tree Research Reports Special Issue, 1996. Winrock International. 268 pp.
Evans, D.O. and Szott, L. (1995). Nitrogen Fixing Trees for Acid Soils. Nitrogen Fixing Tree Research Reports (Special Issue). Winrock International and NFTA, Morrilton, Arkansas, USA. 328 pp.
Fagg,-C. (1991). Acacia tortilis: fodder tree for desert sands. NFT Highlights, No. 91-01, Nitrogen Fixing Tree Association, Hawaii, USA. 2 pp.
Grist, P., Menz, K. and Nelson, R. (1999). Multipurpose tress as improved fallow: An economic assessment. International Tree Crops Journal, 10, 19-36.
Gutteridge, R.C. (1994a). Other species of multipurpose forage tree legumes. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 97-108.
Gutteridge, R.C. (1994b). The perennial Sesbania species. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 49-64.
Gutteridge, R.C. (1998). Leucaena in alley cropping systems: Challenges for development. In: Shelton, H.M., Gutteridge, R.C., Mullen, B.F. and Bray, R.A. (eds). Leucaena - Adaptation, Quality and Farming Systems. Proceedings of workshop held in Hanoi, Vietnam. ACIAR Proceedings No. 86. pp. 337-341.
Hammel, R. (1992). L'importance de l'arbre fourrager pour les eleveurs de la reserve naturelle de l'Air Tenere. Flamboyant, No. 22, 6-9.
Hughes, C.E and Harris, S.A. (1995). Systematics of Leucaena: Recent findings and implications for breeding and conservation. In: Shelton. H.M., Piggin, C.M. and Brewbaker, J.L. (eds), Leucaena - Opportunities and Limitations. Proceedings of workshop held in Bogor, Indonesia. ACIAR Proceedings No. 57, pp.54-65.
Hughes, C.E. (1994). Risks of species introductions in tropical forestry. Commonwealth Forestry Review, 73, 243-252, 272-273.
Hughes, C.E. (1998a). Leucaena. A genetic resources handbook. Oxford Forestry Institute, Tropical Forestry Papers No. 37. 274 pp.
Hughes, C.E. (1998b). Taxonomy of Leucaena. In: Shelton, H.M., Gutteridge, R.C., Mullen, B.F. and Bray, R.A. (eds), Leucaena - Adaptation, Quality and Farming Systems. Proceedings of workshop held in Hanoi, Vietnam. ACIAR Proceedings No. 86. pp. 27-38.
Hughes, C.E. and Pottinger, A.J. (1997). Albizia species from Mexico and Central America. In: Zabala, N.Q. (ed), International Workshop on Albizia and Paraserianthes Species. Proceedings of workshop held in Bislig, Suriago del Sur, Philippines, 1994. Farm, Forestry and Community Tree Research Reports Special Iissue. Winrock International. pp. 57-65.
Ibrahim, K.M. (1992). Prosopis species in the south-western United States, Their utilisation and research. In: Dutton, R.W., Powell, M. and Ridley, R.J. (eds), Prosopis species - Aspects of their value, research and development. Proceedings of Prosopis symposium held by CORD, University of Durham, UK. pp. 83-115.
Ir. Mansyur, Ghazali Zainal and Maimunah Tuhulele (1999). Planting gliricidia from stem cuttings. SEAfraD News. Southeast Asia Feed resources Research and Developmewnt network. March 1999. p. 10.
Kaitho, R.J., Umunna, N.N., Nsahlai, I.V., tamminga, S., van Bruchem, J. and Hanson, J. (1997). Palatability of wilted and dried multipurpose tree species fed to sheep and goats. In: Kaitho, R.J. Nutritive value of browses as protein supplements(s) to poor quality roughages. PhD Thesis. Wageningen Agricultural University. pp. 59-73.
Kaitho-RJ; Umunna-NN; Nsahlai-IV; Tamminga-S; Bruchem-J-van; Hanson-J; Wouw-M-van-de; Van-Bruchem-J; Van-de-Wouw-M. (1996). Palatability of multipurpose tree species: effect of species and length of study on intake and relative palatability by sheep. Agroforestry Systems, 33, 249-261.
Karachi-M; Shirima-D; Lema-N. (1997). Evaluation of 15 leguminous trees and shrubs for forage and wood production in Tanzania. Agroforestry Systems, 37, 253-263.
Kartasubrata, Junus (1996). Culture and uses of Calliandra calothyrsus in Indonesia. In: Evans, D.O. (ed), International Workshop on the Genus Calliandra. Forest, Farm and Community Tree Research Reports Special Issue, 1996. Winrock International. pp. 101-107.
Kass, D.L. (1994). Erythrina species - Pantropical multipurpose tree legumes. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 84-96.
Kexian, Yi, Lascano, C.E. Kerridge, P.C. and Avilia, P. (1998). The effect of three tropical shrub legumes on the intake and acceptability by small ruminants. Pasturas Tropicales, 20, 31-35.
Kishan Kumar, V.S.. (1999). Prosopis cineraria and Ailanthus excelsa - fodder trees of Rajasthan, India. International Tree Crops Journal, 10, 79-86.
Larsen, P.H., Middleton, C.H., Bolam, M.J. and Chamberlain, J. (1998). Leucaena in large-scale grazing systems: Challenges for development. In: Shelton, H.M., Gutteridge, R.C., Mullen, B.F. and Bray, R.A. (eds), Leucaena - Adaptation, Quality and Farming Systems. Proceedings of workshop held in Hanoi, Vietnam. ACIAR Proceedings No. 86. pp. 324-330.
Lavin, M (1996). Taxonomy. In: Stewart, J.L., Allison, G.E. and Simons, A.J. (eds), Gliricidia sepium - Genetic resources for farmers. Tropical forestry Paper 33. Oxford Forestry Institute. pp. 3-17.
Le Houérou (1980). Chemical composition and nutritive value of browse in tropical West Africa. In: Browse in Africa, the current state of knowledge. Le Houérou, H.N. (ed). ILCA, Addis Ababa, pp. 261-289.
Lenné, J.M. (1992). Diseases of multi-purpose woody legumes in the tropics: a review. Nitrogen Fixing Tree Research Reports, 10, 13-29.
Lenné, J.M. and Boa, E.R. (1994). Diseases of tree legumes. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 292-308.
Lesueur, D., Date, R.A. and Mullen, B.F. (1998). Rhizobium specificity in Leucaena. In: Shelton, H.M., Gutteridge, R.C., Mullen, B.F. and Bray, R.A. (eds), Leucaena - Adaptation, Quality and Farming Systems. Proceedings of workshop held in Hanoi, Vietnam. ACIAR Proceedings No. 86. pp. 86-95.
Lowry, J.B. (1989). Agronomy and forage quality of Albizia lebbeck in the semi-arid tropics. Tropical grasslands, 23, 84-91.
Lundgren, D. (1992). Foreward, In A selection of useful trees and shrubs for Kenya. International Centre for research in Agroforestry. pp. 9-10.
Macqueen, D. (1996). Calliandra taxonomy and distribution, with particular reference to the series Racemosae. In Evans, D.O. (ed), International Workshop on the Genus Calliandra. Forest, Farm and Community Tree Research Reports Special Issue, 1996. Winrock International. pp. 1-17.
Maghembe, J.A. (1991). Special issue: Agroforestry research in the African miombo ecozone. Malawi. Proceedings of a regional conference on agroforestry research in the African miombo zone held in Lilongwe, Forest Ecology and Management, 64, pp. 105-292.
McClintock, E. (1993). A botanical history of Erythrina introductions. In: Westley, S.B. and Powell, M.H. (eds). Erythrina in the new and old worlds. Nitrogen Fixing tree Research Reports Special Issue. pp. 68-71.
McNeill, D.M., Osborne, N., Komolong, M. and Nankervis, D. (1998). Condensed tannins in the genus Leucaena and their nutritional significance. In: Shelton, H.M., Gutteridge, R.C., Mullen, B.F. and Bray, R.A. (eds), Leucaena - Adaptation, Quality and Farming Systems. Proceedings of workshop held in Hanoi, Vietnam. ACIAR Proceedings No. 86. pp. 205-214.
Middleton, C.H., Jones, R.J., Shelton, H.M., Petty, S.R. and Wildin, J.H. (1995). Leucaena in northern Australia. In: Shelton. H.M., Piggin, C.M. and Brewbaker, J.L. (eds), Leucaena - Opportunities and Limitations. Proceedings of workshop held in Bogor, Indonesia. ACIAR Proceedings No. 57, pp. 214-221.
Moog, F.A., Bezkorowajnyj, P. and Nitis, I.M. (1998). Leucaena in smallholder farming systems in Asia: Challenges for development. In: Shelton, H.M., Gutteridge, R.C., Mullen, B.F. and Bray, R.A. (eds). Leucaena - Adaptation, Quality and Farming Systems. Proceedings of workshop held in Hanoi, Vietnam. ACIAR Proceedings No. 86. pp. 303-310.
Mullen, B.F., Gabunada, F., Shelton, H.M., Stur, W.W. and Napompeth, B. (1998a). Psyllid resistance in Leucaena. In: Shelton, H.M., Gutteridge, R.C., Mullen, B.F. and Bray, R.A. (eds), Leucaena - Adaptation, Quality and Farming Systems. Proceedings of workshop held in Hanoi, Vietnam. ACIAR Proceedings No. 86. pp. 51-60.
Mullen, B.F., Shelton, H.M., Basford, K.E., Castillo, A.C., Bino, B., Victorio, E.E., Acasio, R.N., Tarabu, J., Komolong, M.K., Galgal, K.K., Khoa, L.V., Co, H.X., Wandera, F.P., Ibrahim, T., Clem, R.L., Jones, R.J., Middleton, C.H., Bolam, M.J.M., Gabunada, F., Stur, W.W., Horne, P.M., Utachak, K., and Khanh, T.T. (1998b). Agronomic adaptation to environmental challenges in the genus Leucaena. In: Shelton, H.M., Gutteridge, R.C., Mullen, B.F. and Bray, R.A. (eds), Leucaena - Adaptation, Quality and Farming Systems. Proceedings of workshop held in Hanoi, Vietnam. ACIAR Proceedings No. 86. pp. 39-50.
Murphy, S.T. (1990). Pests of Sesbania species and Sesbania species as pests: A review. In: Macklin, B. and Evans, D.O. (eds). Perennial Sesbania Species in Agroforest Systems. Proceedings of Workshop, Naironi, Kenya. Nitrogen Fixing tree Association, Hawaii. pp. 123-130.
Neelam Bhatnagar; Bhandar, D.C., Promila Kapoor; Bhatnagar, N. and Kapoor, P. (1993). Competition in the early establishment phases of an even aged mixed plantation of Leucaena leucocephala and Acacia nilotica. Forest Ecology and Management, 57, 213-231.
Neill, D.A. (1993). The genus Erythrina: taxonomy, distribution and ecological differentiation. In: Westley, S.B. and Powell, M.H. (eds). Erythrina in the new and old worlds. Nitrogen Fixing tree Research Reports Special Issue. pp. 15-27.
Newbold-CJ; El-Hassan-SM; Wang-J; Ortega-ME; Wallace-RJ. (1997). Influence of foliage from African multipurpose trees on activity of rumen protozoa and bacteria. British Journal of Nutrition, 78, 237-249.
Niang-A; Ugiziwe-J; Styger-E; Gahamanyi-A. (1996). Forage potential of eight woody species: intake and growth rates of local young goats in the highland region of Rwanda. Agroforestry Systems, 34, 171-178.
Norton, B.W. (1994a). The nutritive value of tree legumes. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 177-191.
Norton, B.W. (1994b). Tree legumes as dietary supplements for ruminants. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 192-201.
Norton, B.W. (1994c). Anti-nutritive and toxic factors in forage tree legumes. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 202-215.
Nouaille, C. (1992). At the frontiers of domestication. Biofutur, No. 111, pp. 43-46.
Nygren, A. (1993). Traditional uses and cultural significance of three Erythrina species among rural population of Tuís District, Turrialba, Costa Rica. In: Westley, S.B. and Powell, M.H. (eds). Erythrina in the new and old worlds. Nitrogen Fixing tree Research Reports Special Issue. pp. 62-67.
Osuji, P.O., Odenyo, A.A., Acamovic, T., Stewart, C.S. and Topps, J.H. (1997). The role of legume forages as supplements to low quality roughages - ILRI experience. Selected papers from an international conference on Evaluation of forages for ruminants in the tropics, Harare, Zimbabwe, 28 August-1 September 1995. Animal Feed Science and Technology, 69, pp. 1-3, 27-38.
Palmer, B. Macqueen, D.J and Gutteridge, R.C. (1994). Calliandra calothyrsus - a mutltipurpose tree legume for humid locations. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 65-74.
Parrotta, J.A. (1995). Influence of overstory composition on understory colonization by native species in plantations on a degraded tropical site. Journal of Vegetation Science, 6, 627-636.
Pasiecznik-NM; Vera-Cruz-MT; Harris-PJC. (1995). Prosopis juliflora withstands extreme aridity and goat browsing in the Republic of Cape Verde. Nitrogen Fixing Tree Research Reports, 13, 89-91.
Peltier, R. (1996). Les parcs a Faidherbia. Cahiers Scientifiques No. 12, CIRAD Foret, Montpellier, France, 312 pp.
Preston, T.R. (1992). The role of multi-purpose trees in integrated farming systems for the wet tropics. In: Speedy, A and Pierre-Luc Pugliese (eds). Legume trees and other fodder trees as protein sources for livestock. Proceedings of the FAO expert Consultation held at the Malaysian Agricultural Research and Development Institute (MARDI) in Kuala Lumpur, Malaysia. FAO of the United nations, pp. 193-209.
Roshetko, J.M. (1995). Albizia saman : pasture improvement, shade, timber and more. NFT Highlights, NFTA 95-02. Forest, Farm, and Community Tree Network (FACT Net), Winrock International.
Roshetko, J.M. (1997). Profiles of selected Albizia and Paraserianthes species. In: Zabala, N.Q. (ed), International Workshop on Albizia and Paraserianthes Species. Proceedings of workshop held in Bislig, Suriago del Sur, Philippines, 1994. Farm, Forestry and Community Tree Research Reports Special Issue. Winrock International. pp. 157-162.
Roshetko, J.M., Dagar, J.C., Puri, S., Khandale, D.Y., Takawale, P.S., Bheemaiah, G. and Basak, N.C. (1996). Selecting species of nitrogen fixing trees. In: Roshetko, J.M. and Gutteridge, R.C. (eds), Nitrogen Fixing Trees for Fodder Production - A Field Manuel. Winrock International, Morrilton (AR), USA. pp.23-23.
Ruredzo, T.J. and Hanson, J. (1988). The use of in vitro culture in germplasm management. ILCA Germplasm Newsletter, No. 18, 4-9.
Russo, R.O. (1993). The use of Erythrina in the Americas. In: Westley, S.B. and Powell, M.H. (eds). Erythrina in the new and old worlds. Nitrogen Fixing tree Research Reports Special Issue. pp. 28-45.
Samways, M.J., Caldwell, P.M. and Osborn, R. (1996). Ground-living invertebrate assemblages in native, planted and invasive vegetation in South Africa. Agriculture, Ecosystems and Environment, 59, 19-32.
Schrempp, B., et al. (1989).
Schrempp, B., Tato, K. and Hurni, H. (1992). Non-conflicting multipurpose tree integration: a case study in the Harerge Highlands, eastern Ethiopia. Soil conservation for survival. A selection of papers presented at the 6th International Soil Conservation Organisation held in Ethiopia and Kenya. pp. 109-117.
Shelton, H.M. (1994a). Environmental adaptation of forage tree legumes. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 120-131.
Shelton, H.M. (1994b). Establishment of forage tree legumes. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 132-142.
Shelton, H.M. and Jones, R.J. (1995). Opportunities and limitations in leucaena. In: Shelton. H.M., Piggin, C.M. and Brewbaker, J.L. (eds), Leucaena - Opportunities and Limitations. Proceedings of workshop held in Bogor, Indonesia. ACIAR Proceedings No. 57, pp.16-23.
Shelton, H.M., Gutteridge, R.C., Mullen, B.F. and Bray, R.A. (eds) (1998). Leucaena - Adaptation, Quality and Farming Systems. Proceedings of workshop held in Hanoi, Vietnam. ACIAR Proceedings No. 86. 358 pp.
Shelton, H.M., Norton,B.W., Mullen, B.F., Gutteridge, R.C. and Dart, P.J. (1996). Utilisation and nutritive value of Calliandra calothrysus for forage: a review of research at the University of Queensland. In Evans, D.O. (ed), International Workshop on the Genus Calliandra. Forest, Farm and Community Tree Research Reports Special Issue, 1996. Winrock International. pp. 210-221.
Simons, A.J. (1996). Seed orchards and breeding. In: Stewart, J.L., Allison, G.E. and Simons, A.J. (eds), Gliricidia sepium - Genetic resources for farmers. Tropical forestry Paper 33. Oxford Forestry Institute. pp. 119-125.
Sissoko K., Soumare, S. and Soumare, A. (1993). Woody species, a trump to protect. Projet PSS, BP 22 Niono, Mali. Lettre du Reseau Recherche Developpement. No. 19, GRET, Ministere de la Cooperation; Paris, pp. 4-7.
Smith, O.B. (1992). Alley farming and protein banks for tropical Africa. In: Speedy, A and Pierre-Luc Pugliese (eds). Legume trees and other fodder trees as protein sources for livestock. Proceedings of the FAO expert Consultation held at the Malaysian Agricultural Research and Development Institute (MARDI) in Kuala Lumpur, Malaysia. FAO of the United nations, pp. 245-256.
Smith,O.B. (1992). Fodder trees and shrubs in range and farming systems in tropical humid Africa. In: Speedy, A and Pierre-Luc Pugliese (eds). Legume trees and other fodder trees as protein sources for livestock. Proceedings of the FAO expert Consultation held at the Malaysian Agricultural Research and Development Institute (MARDI) in Kuala Lumpur, Malaysia. FAO of the United nations. pp. 43-59.
Snook, L.C. (1982). Tagasaste (tree lucerne) Chamaecytisus palmensis. A shrub with high potential as a productive fodder crop. Journal of Australian Institute of Agricultural Science, 48, 209-213.
Stewart, J.L. (1996). Utilization. In: Stewart, J.L., Allison, G.E. and Simons, A.J. (eds), Gliricidia sepium - Genetic resources for farmers. Tropical forestry Paper 33. Oxford Forestry Institute. pp. 33-48.
Stewart, J.L., Allison, G.E. and Simons, A.J. (1996). Gliricidia sepium - Genetic resources for farmers. Tropical forestry Papers 33. Oxford Forestry Institute. 125 pp.
Tchoundjeu, Z., Weber, J. and Guarino, L. (1998). Germplasm collections of endangered agroforestry tree species: the case of Prosopis africana in the semi-arid lowlands of West Africa. Agroforestry Systems, 1998, 39, 91-100.
Tiedeman,-JA; Johnson,-DE. (1992). (Need title). Natural Resource Sciences, Washington State University, Pullman, WA 99164-6410, USA. Agroforestry Systems, 17, 169-180.
Tilki, F. and Fisher, R.F. (1998). Tropical leguminous species for acid soils: studies on plant form and growth in Costa Rica. Forest-Ecology-and-Management, 108, 175-192.
Walter, G.H. and Parry, W.H. (1994). Environmental adaptation of forage tree legumes. In: Gutteridge, R.C. and Shelton, H.M.(eds), Forage Tree Legumes in tropical Agriculture. CAB International, pp. 309-321.
Westley, S.B. and Powell, M.H. (1993). Erythrina in the new and old worlds. Nitrogen Fixing tree Research Reports Special Issue. 358 pp.
Wickens,G.E., Sief El Din, A.G., Sita, G. and Nahal, I. (1995). Role of Acacia species in the rural economy of dry Africa and the Near East. FAO-Conservation-Guide, No. 27, 56 pp.
Woodward, A. and Reed, J.D. (1997). Nitrogen metabolism of sheep and goats consuming Acacia brevispica and Sesbania sesban. Journal of Animal Science, 75, 1130-1139.
Woodward, S.A., Vitousek, P.M., Matson, K., Hughes, F., Benvenuto, K. and Matson, P.A. (1990). Use of the exotic tree Myrica faya by native and exotic birds in Hawai'i Volcanoes National Park. Pacific Science, 44, 88-93.
Zabala, N.Q. (1995). Albizia and Paraserianthes Species. Forest, Farm, and Community Tree Research reports - Special Issue. Winrock International and NFTA, Morrilton, Arkansas, USA. 164 pp.