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2.2.1 Forage
The desirable criteria for forage are, according to Hines and Eckman (1993):
1. Provision of palatable, non-toxic, nutritious foliage and fruit.
Forage refers to all browse and herbaceous food that is available to livestock and game animals; fodder, which is often wrongly applied to forage, refers to dried, cured plant material of crops such as maize and sorghum, including the grain. Browse refers to the tender twigs and leafy shoots of woody plants (Ibrahim, 1975) and is extended by Skerman et al (1988) to include their fruit. For the purposes of this document the former definition, which excludes pods, is used since, depending on the height of the tree or shrub, fallen pods may be available to some of the smaller mammals whereas the foliage is beyond their reach.
The Acacia trees provide a valuable browse for both game and livestock as well as being a valuable source of highly nutritious pods which can be stored as a dry season supplement for livestock (Table 2.2.1). Depending on the species, a flush of growth is often available at the end of the dry season before the grasses have begun to grow following the first rains (See Annex B for approximate analyses of Acacia browse, etc.). The woody vegetation thus provides a high quality food source at the critical period in late pregnancy of most undulates. It must be emphasized that this first flush of growth, which represents the major seasonal above-ground increase in biomass, is not growth per se but the relocation of stored food resources from the previous growing season. Growth begins after the flush, with the current season's production of photosynthates.
Browse is essential to all herbivores in arid and semi-arid environments since grasses alone are unable to supply maintenance requirements for more than a few months of the year (Table 2.2.1. 1). Indeed, cattle in Niger deprived of dry season browse showed serious signs of vitamin A deficiency. Furthermore, pseudo-contagious amaurosis, another vitamin A deficiency, is a common disease of cattle in deforested areas of the Sahel (Le Houérou 1983b). The crude protein content of most browse is generally considerably higher than that for grasses except during the early growing season (Walker, 1983).
It is worth while quoting a statement from the Commonwealth Agricultural Bureau Publication No. 10 (1947) and cited by Skerman et al. (1988) "It is a humbling fact for grass pasture experts to realize that probably more animals feed on shrubs and trees, or on associations in which trees and shrubs play an important part, than on true grass-legume pastures." The reliance on browse reflects the ecophysiology of the trees and shrubs that permits their survival in semi-arid ecosystems and produce greater quantities of nutritious fodder than can be obtained from grasses and forbs.
Because of their low presence, the indigenous Acacia play a very minor role in the browse regime of North Africa and the Near East, never-the-less they are of considerable local importance where species do occur (see sections 3.5 and 3.6). In the arid, semi-arid and sub-humid zones of subSaharan Africa, woody species are an important source of fodder as well as exerting an influence on the seasonality and productivity of the grass cover growing beneath their canopy.
The forage utilization of Acacia species is not confined to herbivores. A number of species are regarded locally as a good source of pollen and nectar for bees (Table 2.2.1.2. Wild honey is largely collected from natural cavities in trees or from crude hives suspended in the trees. Acacia honey is an underexploited food resource for both local and export markets and should be encouraged.
2.2.1.1 Forage Utilization
The degree to which browse is utilized by herbivores is species dependent. However, the recurrent droughts in subSaharan Africa have amply shown that the use of browse of any given soecies is also very dependent on the circumstances and overall availability of usable biomass. Cattle, sheep, equines, wildebeest, most antelopes, gazelles, white rhino and hippo are mainly grazers, but during the dry season balance their diet by browsing. Other species, such as goats, camels, eland, impala, kudu, elephant, giraffe, black rhino and a number of antelope are mainly browsers and can thrive satisfactorily on a purely browse diet (Le Houérou, 1983c).
The daily diet of herbivores, both wild and domestic, is selected almost entirely from the indigenous vegetation. As Dougall et al., (1964) have pointed out, it is important to know what plants are eaten by the different animals throughout the year, and what nutrients such plants might be expected to provide. For example, an elephant in the Tsavo East National Park Park, Kenya, during a single day in May (dry season), selected food from 64 species represent by 28 families, only 10 species of which were grasses (Dougall and Sheldrake, 1964), although what the choice would be for other months of the year is unrecorded.
The wealth of indigenous vegetation as a source of food for wildlife and domestic animals is enormous and it is essential that its preservation, regeneration and productivity should be assured for the future survival of the fauna. See Table 2.2.1 for the livestock and wild herbivores utilizing browse; the list is not exhaustive; see also section 2.3.4 for further discussion on herbivore survival and browse.
The structure of the herbivore community in a wildlife area can be quite complex but under pastoral conditions there is a direct ratio between cattle and goats and the effect of herbivory on the vegetation. Matching the herbivore community to the vegetation structure is essential if a stable system is to be achieved (Walker, 1983). Pollarding is widely used in some areas to keep the browse accessible.
Continuous grazing can result in the suppression of natural regeneration so that stands become senile and susceptible to drought and disease. For example, the stands of Acacia along the Senegal River became over-mature and even-aged due to lack of regeneration/or regeneration survival under grazing. Most of the aged stands succumbed to the prolonged drought during the late 1970s and the dead trees used for fuel (M.L. Malagnoux, verbal information).
The spines on many Acacia species can act as a self-regulating defense against excessive browsing; longer thorns can develop on, for example, branches of A. drepanolobium that are regularly browsed by goats than those out of reach of the herbivores (Young, 1987). However, thorns do not prevent giraffe browsing, for example, A. tortilis, in the Serengetti Plains but act by slowing down their feeding until a critical level is reached where it becomes too time consuming to seek the shoots between the spines and the animal moves on (Pellew, 1984). When the thorns are removed, increased browsing was observed among the free-ranging giraffe (Milewski et al., 1991). Excessive browsing while young can even reduce the plant to a densely spiny cushion shrub. It is suggested that spine development should be regarded by the grazier a management tool for evaluating grazing pressure.
Similarly, chemical deterrents may develop in the less spiny species, such as A. oerfoeta. The foliage of this species has a strong, obnoxious smell when bruised and in Kordofan Province, Sudan it was reported to deter even the browsing camel (Hunting Technical Services, 1964), although in recent years there have been several citations that it is now being browsed by livestock in the Sudan (e.g. Seif el Din, 1991). This is a recent phenonomem for the Sudan and is probably due to the shortage of browse as a result of the Sahelian drought from the late 1960s onwards and it would appear the local livestock have now aquired a taste for this species. Similarly, Ibrahim and Barker (1986) working in Somalia report that A. horrida is also sometimes refused on account of its unpleasant odour; while A. oerfota, which is also amongst the browse species present, is not mentioned as being rejected! Dougall et al. (1964) and Skerman et al. (1988) record A. oerfota as being an important browse species in northern Kenya. Palatability is, of course, relative to what is available. It is possible that there are a number of chemical races of A. oerfota may be involved, providing several degrees of palatability.
2.2.1.2 Nutritional Value
The available proximate analyses for the various Acacia species are given in Annex C. The majority of analyses are based on a single sample and, as Walker (1983) has pointed out, such data can be misleading since the variation within a species sampled from different locations can be quite wide, at least for southern Africa and can be far greater than between species within a community. There is no reason to suspect the contrary throughout the continent. In general, the protein content for Acacia browse is high but, from the little information available, only half of the dry matter is digestible. Especially noteworthy is the high calcium content of the bark of A. tortilis subsp. spirocarpa (5.68%) and A. xanthophloea (4.07%).
Cyanogenic glucosides have been reported by Steyn and Rimington (1935) in the pods of A. erioloba, A. hebeclada subsp. hebeclada, A: lasiopetala, A. tortilis subsp. heteracantha and A. robusta. However, the authors conclude that there is no danger provided the pods are ingested slowly, i.e. in small quantities by livestock that are not excessively hungry.
Tannins are present in most vascular plants and in a number of Acacia species they occur in sufficient concentrations for use in tanning leather (section 2.2.3). Tannins also act as an anti-nutritional factor due to their ability to precipitate proteins from an aqueous solution: In tree leaves tannins are present in both neutral detergent fibre (NDF) and acid detergent fibre (ADF) in significant amounts and are tightly bound to the cell wall and cell protein and appear to be involved in decreasing digestibility. Two groups are recognized, condensed tannins and hydrolysable tannins. While the former are more effective in reducing digestibility, the latter can cause various toxic manifestations due to their hydrolyzing activity in the rumen. They have, therefore, an important influence on the digestibility of browse. Dietary condensed tannins (2-3%) may even assist rumen digestion by forming a protein-tannin complex, thereby reducing wasteful protein degradation. Condensed tannins are known to be present in A. nilotica pods and have been shown to lower growth rate in sheep due to their ability to reduce nitrogen and NDF digestibility; a similar effect is also produced by the hydrolysable tannins in A. sieberiana pods (Kumar, 1992).
2.2.2 Gum
The polysaccharide exudate produced by a number of Acacia species (Table 2.2.2) are used for a number of domestic purposes, including adhesives, a constituent of ink, in crafts, as a cosmetic, in confectionery and as a food. The Hottentots of southern Africa are able to survive on gum for days, while the Moors harvesting gum in the North African desert are reputed to survive on a daily ration of 170 g (Grieve, 1931). Fagg and Stewart (1994) cite the example of the gum from A. gerrardii being in eaten in Oman, and Story (1958) of the Bushmen of the Kalahari eating gums from A. mellifera subsp. A. erioloba, A. erubescens, A. fleckii and A. tortilis subsp. heteracantha.
Gums are also widely used in traditional medicine since Pharaonic times as a soothing and softening agent, being taken internally for coughs, diarrhea, dysentery, haemorrhage, and as well as being used externally to cover inflamed areas.
Acacia gum from A. senegal and some 18 other species was, until the recent tightening of the specifications, a major item of commerce. The gums from this group of species, which include A. seyal (gum talha from West and East Africa), A. xanthophloea (from East Africa) and A. karroo (from southern Africa) are traded on the international markets, but only gum arable, from A. senegal, is now permitted for the food trade, the remainder being for industrial use only. However, acacia gum 'from A. senegal and other African species' is still official in the British Pharmacopoeia (1993) for use as a bulk-forming laxative and pharmaceutical aid. Similarly, while the US specification for the use of acacia gums in the food trade limits use to that from A. senegal (gum arable), the pharmaceutical specification also permits the use of gum talha from A. seyal; the rational being that only small amounts are used per patient and under medical supervision.
Gum arabic was traditionally defined as 'the gummy exudate from Acacia senegal or its related species', embracing a number of species that are not even remotely related taxonomically, despite the fact that the Test Article, evaluated as toxicologically safe as a food additive, refers solely to that from A. senegal. The increasing international pressure towards tighter trade specifications and labelling regulations, identity and purity has led to the Revised Specification (WHO, 1990a, b; FAO, 1990) where gum arabic is defined as originating from A. senegal or closely related species, with a specific optical rotation range of -26° to -34° and a Kjeldahl nitrogen content of 0.27-0.39%. This has limited the designation of gum arabic to members of the subgenus Aculeiferum which, in addition to A. senegal, includes A. laeta, A. mellifera, A. polyacantha subsp. campylacantha, although gum is not available commercially from any of these closely related species. The gums containing tannins and with a positive optical rotation from species such as A. seyal, A. xanthophloea, A. karroo and A. nilotica are now excluded for use in food and consequently attract a lower price (Anderson, 1993); the presence of tannin in these gums is also considered carcenogenic.
Exudates from species other than A. senegal, occur as small tears and dribblets; their collection is consequently extremely time-consuming. This, together with the low price, currently at $US 1000 per tonne, compared to $US 5000 per tonne for gum arable, is likely to kill the export trade in acacia gums but is unlikely to have any serious effect on internal use as an edible comodity, adhesive and ingedient of traditional medicines.
Gum arabic is used in the food industry to fix flavours and as an emulsifier, to prevent the crystalization of sugar in confectionery products, as a stabilizer in frozen dairy products; its viscosity and adhesive properties find use in bakery products, and as a foam stabilizer and clouding agent in beer. In the pharmaceutical industry gum arabic is used as a stabilizer for emulsions, binder and coating for tablets, and as an ingredient in cough drops and syrups. In cosmetics it finds use as an adhesive for facial masks and powders, and to give a smooth feel to lotions.
Industrially, gum arabic is applied as an adhesive, as a protective colloid and safeguarding agent for inks, sensitizer for lithographic plates, coatings for special papers, sizing agent to give body to certain fabrics, and anti-corrosive coating for metals; it is also used in the manufacture of matches and ceramic pottery (Cossalter, 1991).
Gum arabic is a major export crop of the Sudan and, through the Gum Arabic Company, which has a statutory monopoly of the gum arabic trade from the Sudan, effectively controls about 85% of the World Market, with the West African countries Senegal, Mauritania, Mali, Chad, Niger and Nigeria supplying much of the remainder (Anderson, 1993). In the past, the rural population of the Sudan "gum belt" practiced a tree fallow rotation system with A. senegal, forming practically pure stands during the fallow period, thereby increasing soil fertility and reducing soil erosion while, at the same time, providing a dry-season source of income from the gum harvest. With increasing human and livestock populations and pressure on the cultivatable and grazing lands, the length of the tree fallow was decreased or, in some instances, eliminated. The carrying capacity of the range land also decreased, resulting in the necessity to fell the young trees for forage. The poor prices paid to the farmers for their gum has also encouraged the cutting of trees for firewood and charcoal for an albeit short-term source of income.
Despite its advantages of low cost and superior performance over possible substitutes, in the past gum arabic has suffered from variations in quality due to the varying composition of each batch and also from the uncertainty of maintaining a regular supply, especially since the loss of so many of the trees in the Sahelian droughts of 1973-74 and 1982-83. As a result there has been a steady decrease in demand over the past 15-20 years. Gum arabic sales peaked at around 1970 at approximately 70000 t, of which about 70% went into confectionery products. The high prices and world shortage as a result of the 1973-74 drought resulted in some major users seeking alternative modified starches. Annual sales fell to about 40,000 t during 1978-82. The following disastrous drought created a further world shortage and loss of markets, with annual sales reduced to about 20000-24000 t. World consumption of hydrocolloids by the food industry is fairly stable but, due to the fluctuations in the supply of gum arabic and the increasing availability of modified celluloses and fermentation products such as xanthin and gellan, major users are certainly beginning to pay serious attention to any cheaper alternatives to gum arable. Furthermore, gum importers and users consider that the Sudan, with its almost monopolistic control of the market, as bad for trade confidence. The spreading of sources of supply to give the Sudan control of only 50% of the market would lessen the risk of a major crop failure and improve confidence (Anderson, 1993).
A great deal of wasted effort has gone into the selection for higher gum yields. Elite trees have been identified and their seed sown; a somewhat pointless exercise with insect-pollinated trees. Furthermore, even the yields of elite trees are susceptible to unpredicatable fluctuations in yield. The chemo-physiology of gum production has not been thoroughly investigated and until it is known how and why gum is produced the proper selection and management for higher yields is not scientifically possible.
2.2.3 Tannins
The Acacia species whose bark or pods have been recorded as sources of tannin for taning leather are shown in Table 2.2.3, although it is suspected that many other Acacia species could also be used. The tannins are mainly used in the local tanning industries; very little is exported. Although more efficient solvents are available, for economic reasons it is the water-soluble tannins that are used in the commercial and local tanning industries. As already briefly discussed in section 2.2.1.2, there are two groups of tannins, condensed and hydrolysable tannins present in plants, and their chemistry affects their tanning properties. However, there appears to be no information available regarding the percentages of condensed and hydrolysable tannins present although, according to Siegler et al. (1986), most Acacia tannins are of the condensed type.
The exploitation of tannins from Acacia species is effected in many locations; in Senegal, Mauritania and Mali, the pods of Acacia nilotica subsp. nilotica are intensively used under the name 'Nep Nep'; also in the Sudan as 'sunt grains'. Sunt grains, which were formerly exported from the Sudan, are obtained mainly from the crushed pods of A. nilotica subsp. nilotica or subsp. tomentosa, after first removing the seeds; most of the pod cases are then removed by sifting. The residue contains 50-60% tannin, which gives a soft, plump, very light coloured leather. However, contamination of the pods with iron and mineral matter causes undesirable spotting of the leather (Howes, 1953). There is considerable confusion in the literature regarding the tannin content of A. nilotica because authors fail to mention the subspecies involved; the writer suspects that there could be considerable variation in the tannin content between the subspecies and possibly between individuals within populations. Even Howes (1953), who admits to the existence of subspecies (as varieties of A. arabica), failed to identify the source of sunt grains, although his description of the pods and the vernacular name indicate subsp. nilotica or tomentosa. In Kano, Nigeria subsp. adstringens is reported to be the preferred subspecies for tanning goat skins for the Moroccan leather trade, the pod residues are afterwards fed to cattle (W.J. Howard, O.D.A., verbal information 1994). In Kenya the bark of the Australian Acacia meansii is processed for tannin at Thika.
2.2.4 Fibre
The Acacia species used for root and stem bark fibre are shown in Table 2.2.4. The fibres are used in the domestic economy and there appears to be no commercial potential. The general impression obtained is that such usages could also be carried out by other genera and that Acacia species are not an important fibre source.
2.2.5 Local medicine
The local medicinal uses of Acacia species are shown in Table 2.2.5. Their uses are not supported by any clinical studies and while the successful treatment of verereal diseases, diabetes or use as an aphrodisiac, etc. is problematic, there are active ingredients present which may be efficacious. Gum, for example, has an emollient activity, resulting in a softening soothing action on the skin or irritated internal surfaces. The astringent activity of tannins causes a contraction of mucous surfaces, coagulates proteins and is useful in stopping bleeding of small wounds and other discharges (Brown, 1977). Hagos et al. (1987) have demonstrated that the stem bark of A. tortilis subsp. raddiana, which is used in Somalia against asthma, contains pharmaceutically active compounds that inhibit muscular contractions of the lower small intestine of the guinea pig. Further research will doubtless reveal the presence of other pharmaceutically active compounds within the genus.
Acetic acid, alcohol and water extracts of the fruits of A. dudgeoni, A. nilotica subsp. adstringens and subsp. nilotica have been shown to have molluscocidal activity. The planting of the latter subsp. along waterways could prove beneficial in the control of schistosomiasis (Ayoub, 1982, 1985; Kloos and McCullough, 1987).
2.2.6 Food
The seeds of several African species are known to be eaten, either cooked or raw (Table 2.2.6.1), but very little is known about their nutritional value (Table 2.2.6.2). What little evidence is available suggests that Acacia seeds could be a very valuable and underexploited food resource. Certainly the Australian species have been found to be generally high in protein (17-25%), fat (416%) and carbohydrate (30-40%), i.e. comparing very favourably with such cereals as wheat and rice and even higher than some meats (Brand and Cherikoff, 1985; Thomson 1992). See also section 3.7 for discussion on the edible seeds from introduced species.
2.2.7 Handicrafts
The uses of Acacia species for handicrafts are shown in Table 2.2.7. These are mainly domestic although there is a small commercial potential for sales to the tourist industry. The general impression is that such usages could be carried out equally well by other genera and that Acacia species are not essential sources of handicraft materials.
2.2.8 Miscellaneous domestic uses
The domestic uses of Acacia species in the rural economy are shown in Table 2.2.8. Again the general impression is that such usages could be carried out by other genera and that Acacia is not a vital species.