J.J. Bennison and R.T. Paterson
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The importance of trees and shrubs in the feeding of animals in the tropics and sub-tropics has long been recognized by livestock owners. In arid areas where the growth of herbaceous plants is limited by lack of moisture, leaves and edible twigs of trees and shrubs can constitute well over 50% of the biomass production of rangeland. At high altitudes, tree foliage may provide over 50% of the feed available to ruminants in the dry season, branches being harvested and carried to the animals. Even in regions of higher rainfall where grass supplies the major proportion of the dry matter eaten by ruminants, tree leaves and fruits can form an important constituent of the diet, particularly for small ruminants.
In the last two decades interest in the planting of trees as a source of feed for livestock has been encouraged by workers in research and development, but in contrast to the hundreds of indigenous species which are used as fodder, attention has focussed on a limited number of introduced species. Thus there are many publications reporting the chemical composition of Leucaena leucocephala leaves and suggesting management strategies for utilization of the tree for fodder, but it is more difficult to find information on alternative genera which might be equally, or more, appropriate.
The aim of this series of publications is to bring together published information on selected genera of trees which have the potential to increase the supply of fodder for ruminants. Each booklet summarizes published information on the fodder characteristics and nutritive value of one genus, with recommendations on management strategies, where available. Further, since the leaves of woody species frequently contain secondary compounds which may have an anti nutritional, or toxic, effect, a separate booklet summarizes the effects of a number of these compounds. It is hoped that the booklets will provide useful resource material for students, research and extension workers, interested in promoting the use of trees as a source of fodder for ruminants.
Further copies of this booklet or others in the series can be obtained by writing to the Publications Section at the Natural Resources Institute.
Livestock Production Programme
Family LEGUMINOSAE Subfamily MIMOSOIDEAE Tribe ACACIEAE
Principal phyllodenous species
Acacia trachycarpa (syn A. aff. Iinarioides)
Principal bipinnate species
Faidherbia albida (syn. Acacia albida)
Acacia haematoxylon (syn. A. giraffae)
Acacia hebeclada subsp. hebeclada (syn. A. stolonifera)
Acacia mearnsii (syn. Racosperma mearnsii)
Acacia mellifera subsp. detinens
Acacia oerfota (syn. A. nubica)
Acacia tortilis subsp. heteracantha (syn. A. Iikatunensis)
With the African Acaca spp. the fruit is probably more valuable as a feed than the foliage, because with the exception of F. albida, they tend to shed their leaves at the time of greatest feed scarcity during the dry season. Australian species however, retain their leaves and therefore show greater potential as fodder trees.
The productivity responses of animals reflect both the total intake, and the availability, of nutrients in the diet. Proximate and fibre analyses are unreliable indicators of nutritional value because of the presence of both cyanogenic glycosides and polyphenolics. A cautious approach should be adopted when feeding both A. leucophloea and some of the Australian species.
In some areas of Africa, Acacia spp. are the dominant browse trees within the natural ecosystem. Often there is no choice but to utilize the available feeds even if they do contain anti-nutritive compounds. Consumed as part of a wider diet, the effect of anti-nutritive factors is diluted. They only become serious in periods of extreme feed scarcity or in situations where the opportunity for feed selection is reduced, such as in stall feeding and cut-and-carry systems.
Acacia is a huge genus with pantropical distribution and a notable ability to survive in harsh environments. It has potential for use in environmental protection and for production of fodder, fuelwood, gums, and tannins.
Acacia is a pantropical and subtropical genus with species abundant throughout Australia, Asia, Africa and the Americas. They thrive in a diverse range of habitats and environments. Many species are well adapted to the semi-arid and savannah regions but equally others survive in moist forest and riverine areas, tolerating both high pH and waterlogged soils. With such diversity, Acacia has considerable potential in a range of livestock and agroforestry systems. In Africa and Australia, some naturally occuring species are important in traditional pastoral and agropastoral systems, while imported species have become commercially accepted. Trees provide fodder and shade for livestock, improve soil fertility through nitrogen fixation and the production of leaf litter end stabilize veils. Acacia species provide edible fruits and seeds, gum arabic and timber for fuel, construction and fencing.
In a large genus of 800-900 species (Allen and Allen, 1981), classification and nomenclature can be complex, particularly as there have been a number of revisions and attempts to subdivide the genus. Pedley (1986; 1987) advocated that it be divided into three separate genera, namely Acacia, with 200 species best represented in Africa and South America, Senegalia, with about 150 species with the same geographical distribution, and Racosperma, with about 850 species virtually confined to Australia. While the suggestion may have merit, it has not yet been generally accepted. This text uses the traditional nomenclature adopted by standard flora. Where a name has been changed as a result of a generally accepted revision, the name used in the original reference is listed as a synonym. The names of the African species are those included in the check-list produced by Lock (1989) as part of the programme of the International Legume Database and Information Service (ILDIS).
One widely accepted recent change is the reclassification of Acacia albida as Faidherbia albida, although Coe and Beentje (1991) stated that the justification for the change was primarily phytochemical. In this publication, for purposes of discussion and comparison, the name F. albida is accepted but it is considered as though it had remained within the genus Acacia, since it shares many agronomic characteristics with species that have not been reclassified.
Various workers have divided the genus into divisions, series and subseries (Bentham, 1864) or sections (Taubert, 1894) but for ease of description and an understanding of the fodder potential, it is simplest to describe Acacia spp. as being either phyllodenous (principally Australian in origin) or bipinnate (mainly from Africa and Asia).
The majority of Australian Acacia spp. lack true leaves. Instead, they exhibit a characteristic known as phyllodony where the petiole is expanded and flattened. The phyllode has the ability to photosynthesize and for all practical purposes, it is the equivalent of a leaf. The Australian exceptions are from the Section Botrycephalae, which maintain bipinnate foliage until maturity and do not develop phyllodes.
Vercoe (1987) noted that species of Acacia and other genera from the dry tropics of Australia had potential in the semi-arid zones of Africa for fuelwood and erosion control. If their usefulness as fodder could also be established, this would enhance their overall value. Certainly the value of trees such as A. aneura as drought reserve feed is well established and recognized by pastoralists in Australia. Unfortunately, success in establishing A. aneura in Africa has been limited. However, a number of other species appear to show potential, notably A. holosericea, A. trachycarpa and A. coriacea. Other species that have been studied include A. salicina and A. cyanophylla, although the phyllodes of the latter contain high tannin levels (Vercoe, 1987; Goodchild and McMeniman, 1987; Reed et al., 1990), which can reduce both the palatability and the feeding value. Unlike African and Asian species, Australian Acacia spp. are evergreen and unarmed.
There are about 200 bipinnate Acacia species in the New World and a further 150 species in Africa and Asia. All 129 African species possess bipinnate leaves. There are other physical and physiological characteristics which separate them from their Australian relatives, the Botrycephalae. This group, the best known of which is A. mearnsii (black wattle), grown throughout the subtropical world for the extraction of tannin, is confined to the cooler and moister parts of south eastern Australia. They are not noted as valuable fodder trees (Boland, 1987). Most bipinnate Acacia spp. are armed with thorns of various shapes and sizes, some of which are modified stipules at the base of the leaves. Many exhibit transient periods of defoliation in the dry season and have been described as deciduous. Before the start of the rains, there is a often a flush of new growth of leaves. Although the stimulus for this activity is uncertain, it may be due to temperature.
Information on leaf yield of African bipinnate Acacia spp. is limited, and comparative data with introduced Australian species, even more so. The work of Cossalter (1986) compared species from the two regions, when grown in Senegal at uniform spacings of 3 x 3 m (Table 1). The results, as means of 20, or 25 individual trees aged between 18-23 months, demonstrated the fodder potential of the Australian Acacia species when compared with African ones. A. holosericea is perhaps the most frequently planted Australian Acacia in development projects because of its superior yield and because it retains a large phyllode biomass during the dry season, while African species shed their leaves during this period (Cossalter, 1986). There is some uncertainty about the palatability of A. holosericea: early trials indicate that only dry phyllodes are accepted. The fresh phyllodes of A. trachycarpa and A. tumida, however, were readily eaten by both cattle and sheep (Hamel, 1980).
Table 1 - Total biomass production (phyllodes, leaves and wood) of Acacia spp.
Dry leaves, (g/tree)
Source: Cossalter (1986)
When assessing the fodder value of African Acacia spp., the importance of the fruit and pods cannot be ignored. This fraction may be of more value than the foliage in some cases. A major problem in estimating the yield of fruit is the variation in individual trees between years. Trees may bear a heavy crop one year and little the next. The fruits of F. albida are relished by game, cattle and small ruminants and NAS (1979) suggests that yields of 125-135 kg are possible from a single tree. When ripe, the fruit can be harvested and stored for future use, although there is a tendency for the pods to shatter and disintegrate.
The bark of A. seyal is particularly valuable as a fodder, although complete removal will result in the death of the tree. Skerman et al., (1988) suggested that the bark, which is extensively used for feeding cattle and small ruminants in the dry season, was the most valuable part of the plant. Cattle could consume up to 5.5 kg bark/day, sufficient for maintenance and the production of 4.5 litres of milk.
Table 2 - Mean proximate and fibre analyses of Acacia leaves, phyllodes, fruits and seeds
In Table 2, the mean proximate and fibre analyses of leaves, phyllodes and fruits of Acacia spp. are summarized from data of individual species (Appendix 1). Absolute values should be treated with caution since there is considerable variation in the literature. For example, the mean crude protein (CP) level of Acacia leaves is 18.9% of dry matter (DM), with a reported range of 6.5-42.8%. However, the data provide an indication of the nutritive value and in general both leaves and fruits of all species are valuable sources of protein, particularly when compared with mature grasses, where CP levels in forage dry matter (DM) during the dry season can fall to 3-4% or less, well below the 7% CP which is suggested as a lower limit for fibre digestion in cattle (Van Soest, 1982).
While the crude fibre (CF) levels of Acacia are relatively high (15-40%) compared with other fodder tree species such as Leucaena leucocephala and Gliricidia septum, they are still markedly lower than for tropical grasses and common crop residues. Even the fruit, consisting of the dry dehiscent pod and the seeds contained in it, are a valuable feed source, with moderate CF (25%) and high CP levels (15%). The phyllodenous Acacia spp. tend to have marginally higher CF and lower CP levels than bipinnate species (Appendix 2) though the available data consist of fewer observations. The lower nutritive value is offset by both the higher biomass yield and the retention of the phyllodes throughout the dry season, when bipinnate species tend to shed their leaves.
Information on the digestibility of DM and organic matter (OM) of Acacia leaves and phyllodes is limited but the available data indicate that, with sheep, they are relatively low and in the range of 40-605 to respectively (Table 3). The digestibility of phyllodes appears to be lower than that of bipinnate leaves. Goodchild and McMeniman, (1987) suggested that the low fibre digestibility could be associated with the high lignin content of the cell wall, since fibre digestibility is inversely related to the lignin content of the fibre fraction.
Information on the digestibility of DM and organic matter (OM) of Acacia leaves and phyllodes is limited but the available data indicate that, with sheep, they are relatively low and in the range of 40-60% respectively (Table 3). The digestibility of phyllodes appears to be lower than that of bipinnate leaves. Goodchild and McMeniman, (1987) suggested that the low fibre digestibility could be associated with the high lignin content of the cell wall, since fibre digestibility is inversely related to the lignin content of the fibre fraction.
While the digestibility of the whole fruit (seed and pod) is higher than that of the leaves and phyllodes, it can be adversely affected by the proportion of seeds that pass undigested through the gut of the animal. Tanner et al., (1990) reported that 46% of A. tortilis seeds were voided in the faeces of sheep. Similarly, Goodchild and McMeniman (1987) noted that when feeding yearling cattle with seeds of A. tortilis subsp. spirocarpa, 12% passed unchanged through the gut. Legume seeds are physically hard and notoriously resistant to digestive juices, although the small size of Acacia seed may be a contributory factor in their passage through the gut (Tanner et al., 1990).
The digestibility of the leaf and fruit fraction can also be depressed by the presence of polyphenolic compounds, including tannins.
Acacia spp. have acquired a variety of physical and chemical defence mechanisms aimed at reducing the palatability and nutritive value of the plant to predators including both insects and animals.
Table 3 - In vivo digestibility (%) of Acacia forage
Perhaps the most obvious physical characteristic is the spiny armature of the African species, which evolved as a defence against large herbivores. A. mellifera subsp. detinens derived its common name (wait-a-bit tree) from the pairs of small hooks on the branches. A. tortilis is protected by two types of thorns, long white stipular spines and small curved hooks, a formidable protection against animals. Coe and Beentje (1991) suggested that as thorns restrict the availability of forage, a critical level is reached below which the herbivore no longer finds it nutritionally efficient to seek shoots between the spines, since this invariably involves movement between each bite. There is also evidence that in A. drepanolobium, thorns on branches which are regularly browsed by animals are significantly longer than those on undefoliated branches (Young, 1987).
Tree size is a form of physical protection that can be harnessed as a desirable characteristic in certain agroforestry systems. Domestic animals are unable to browse tall trees which then maintain a standing reserve of foliage which can be made available by strategic lopping at critical periods when other feed is scarce.
A fascinating defence mechanism of the genus is the symbiotic relationship with ants known as myrmecophily, where the plant lives in association with an ant colony. In the tropical American species A. cornigera, association with ants from the genus Pseudomyrmex is a notable example of myrmecophily. The trees have modified, swollen spines or galls, which provide shelter for the ants, and extrafloral nectaries and modified leaflet tips which provide food. The ants, in return, protect the trees from insects and other predators by swarming to attack them when the tree is disturbed (Janzen, 1966). In Africa, A. drepanolobium is commonly known as the 'whistling thorn', a name derived from the noise of the wind blowing into the holes of the galls that have been occupied by ants.
Many Acacia spp. produce a range of potentially toxic compounds which prevent herbivores from eating the foliage, although it is thought that the chemicals originally evolved as defence mechanisms against insect and fungal attack. Some members contain specific anti-nutritive factors, for example, A. georginae has been implicated in heavy stock losses in Australia due to its content of fluoracetic acid (D'Mello, 1992). In general terms the genus contains two major groups of anti-nutritive factors, cyanogenic glycosides and polyphenolic compounds.
When cyanogenic substrates are broken down under the action of a hydrolyzing enzyme, hydrogen cyanide (HCN) is formed. Plants that contain both cyanogenic glycosides and an endogenous, hydrolytic enzyme are the most dangerous but broad-spectrum enzymes may be present in other fractions of the diet. Symptoms of HCN poisoning include respiratory dyspnoea, intense red conjunctive, frothing at the mouth, bloat, a staggering gait, convulsions and violent death. On post-mortem examination, a characteristic almond smell is often noted in the contents of the gut.
In Asia, there are a number of reports of HCN poisoning of livestock when fed on the pods and leaves of A. Ieucophloea (Gupta and Naurival, 1966; Prasad et al., 1977; Vihan and Panwar, 1987). In South Africa, Steyn and Rimington (1935) reported the presence of cyanogenic glycosides in A. haematoxilon, A. Iasiopetala, A. tortilis subsp. heteracantha, A. robusta and A. hebeclada subsp. hebeclada. They found higher concentrations of HCN from leaves and immature pods than from mature pods. In A. robusta, the glycoside was present without the corresponding hydrolytic enzyme. They isolated the cyanogenic compound acacipetalin from A. Iasiopetala and A. hebeclada. Siegler et al., (1975) isolated a further compound, dihydroacipetalin, from the leaves and shoots of A. sieberiana var. woodii.
Toxicity appears only in sporadic outbreaks, being dependent on a number of factors, including the species and variety of Acacia, the rate of ingestion by the animal and the availability of alternative feeds. It is more likely to occur during periods of drought or feed scarcity when hungry animals consume large quantities of feed in a short period of time. Access to cold water during feeding appears to promote the release of HCN from the substrates. Practical measures can be taken to reduce the risk of poisoning such as restricting the intake, particularly of leaves and immature pods, separating the time or place of feeding and watering, and feeding dangerous material with molasses or sulphur. These substances combine with HCN to form harmless cyanhydrin and sulphocyanides respectively (Steyn and Rimington, 1935).
There are numerous reports of the presence of polyphenolic compounds in the various components of plants of the genus Acacia. From the standpoint of nutrition, the most important of these are the tannins, compounds with molecular weights in excess of 500, which are able to form complexes with, and to precipitate proteins (McLeod, 1974; Mangan, 1988). Hydrolyzable tannins are readily broken down by acids, bases and enzymes to form sugars and phenolic carboxylic acid and are therefore easily degraded in the gut. In contrast, condensed tannins are not degraded and so are available to form complexes with proteins, resulting in reduced feed intake and digestibility. McNaughton (1987) suggested that at levels above about 5%, increasing the tannin content in a diet reduced its palatability.
Reed et al., (1990) assessed the effects of polyphenolics on intake, growth, digestibility and nitrogen utilization by sheep. Three Acacia forages, (phyllodes of A. cyanophylla, leaves of A. seyal, and fruit of A. sieberiana), all containing high levels of polyphenolics, were compared with three protein supplements and another fodder tree Sesbania sesban which is low in polyphenolics. All diets were fed in combination with teff straw (Eragrostis abyssinica). The diets containing Acacia led to the highest levels of feed refusal by the sheep their and lowest intake of teff straw. Fibre digestibility was lower for these diets and the tannins complexed with proteins, resulting in higher levels of faecal nitrogen.
A complementary study by Tanner et al., (1990) reported significant differences in the growth rates of sheep supplemented with fruits from four different Acacia spp. (A. tortilis, F. albida, A. nilotica and A. sieberiana) even though there were no significant differences in total nitrogen intake. The tannin-rich fruits, particularly of A. sieberiana and A. nilotica, reduced feed intake and the efficiency of nitrogen utilization by forming complexes with proteins which again resulted in a shift of nitrogen excretion from urine to faeces. The growth rates of sheep fed the diets low in polyphenolics (F. albida and A. tortilis), were not significantly different from sheep supplemented with noug meal (Guizotia abyssinica), indicating their suitability as dietary supplements.
The literature abounds with conflicting reports on the suitability of Acacia spp. for livestock feed.
Dumancic and Le Houerou (1981) considered the phyllodes of A. cyanoophylla to be sufficient to meet the protein maintenance requirements of sheep, while Reed et al., (1990) reported a negative nitrogen balance for animals fed on this legume. Predictions of the productive responses of livestock are problematical and uncertain. There are a number of possible explanations for the conflicting reports:
· the chemistry of tannins is complex and analytical results do not always correlate with biological effects, e.g. nitrogen digestibility;
· a single feed may contain more than one antinutritive factor. A. nilotica is known to have both tannins and catechin gallates (Mueller-Harvey et al., 1987);
· the concentration and type of phenolic compound both vary with season and in response to stimuli such as defoliation;
· there is much variation between both species and provenance in tannin content, the effect being both genetic and environmental.
Where the principal use of Acacia spp. is for browsing by livestock, little is done to manage the resource, except through some measure of defoliation control by herding the animals. High branches are lopped during times of feed shortage, particularly in the dry season. At other times of the year, the trees are subjected to uncontrolled browsing, often by both wild and domestic animals.
Many legumes produce seed which has a hard, impermeable seed coat. Such seed will not germinate until moisture is able to penetrate it. This characteristic, known as 'hard seededness', prevents the simultaneous germination of the entire stock of seed in the soil at the start of the wet season. This adaptation favours the long-term survival of the population where rainfall is unpredictable. It is, however, a disadvantage when attempting to establish new stands from seed, particularly in areas with a short growing season. Acacia shows extreme variability in the degree of hard seededness, both between and within species; the proportion of hard seed in a sample depends on the environmental conditions during plant growth, the maturity of the seed at harvest and the length of time it has spent in storage. In general, Australian species respond to seed scarification. Recommended treatments (Doran and Gunn, 1987) include:
· manual nicking of the seed coat. Because of the amount of labour involved, this is suitable only for seed for germination tests, or for valuable research lots;
· placing the seed in boiling water for one minute. This is recommended for many hard-coated seed lots;
· placing the seed in hot water (80-90°C) for one minute. This is recommended where sensitivity to boiling water is suspected.
In Central Australia where annual rainfall is about 300 mm, native Acacia spp. naturally regenerate after exceptionally high rainfall at populations of some 5000 stems/ha, and many species produce multiple stems. Subsequent fires and drought will thin the stand to 100-400 stems/ha. Under these conditions,
the optimum stocking level for planted stands may be in the region of 1000 seedlings/ha, followed by thinning as the stand matures. Alternatively, where thinning is not practical, an initial stand of about 400 trees/ha would be appropriate (Kube, 1987). The seed is scarified in hot water (80°C) and germinated in trays. Seedlings are transplanted after a few days into individual tubes 5 cm in diameter by 15 cm deep, filled with a potting mixture of equal parts of coarse sand, fine sand or sandy loam and peat moss. Treatment with fungicide is usually necessary and the seedlings are grown in decreasing shade for about three months before planting out. Supplementary irrigation is applied for 6-12 months, depending upon rainfall. Growth is often rapid, many species attaining heights of 2 m in about two years.
In Tanzania, A. mearnsii plantations have been established for fuelwood, charcoal and building poles by direct seeding scarified seed at rates of 1.22.4 kg/ha in rows 1.8 m apart in fully cultivated soil (Kessy, 1987). The major problem with this species has been its susceptibility to competition from weeds. Harvesting has been on an 8-12 year cycle and natural regeneration has taken place after the original stand has been clear felled and burned.
While Acacia is the largest of the mimosoid genera and the second largest of all the legume family, only about 75 members have economic value, and of these only about 50 are actually cultivated. Nevertheless, they are used in various ways, as described by Allen and Allen, (1981) and Boland, (1987).
Many Acacia spp. form spiny, slow-growing scrub which provides protection against soil erosion caused by wind and rain. Their tolerance of harsh conditions makes them valuable for stabilization and revegetation of difficult sites. A. armata, A. glaucescens, A. pycnantha and A. suaveolens tolerate salt spray and so can be used to stabilize coastal sand dunes. A. decurrens is planted along roadsides in Australia and on denuded hills in Rwanda. A. mearnsii and A. dealbata are used to control gulley and hillside erosion in New Zealand.
Acacia is an important source of fuelwood or charcoal in arid regions. A. mearnsii is used for both domestic fuel and tobacco curing in Indonesia (NAS, 1980). It also provides poles for mining and agriculture in many parts of the world.
As timber, Acacia wood is extremely durable and although it is hard to work, it finishes well. Smaller species provide wood which is used to make implement handles, pipes and furniture, while larger species are used for paneling, boats and musical instruments. Hawaian mahogany (A. koa) was once prized for making surfboards, canoes and bowls while A. segal and A. tortilis were the principal trees of the Arabian Desert region. A. glaucescens and A. melanoxylon are major timber trees today. A. dealbata and A. mearnsii are used for pulping in Australia and Japan, while the latter species is used in the production of rayon in South Africa and India.
A. mearnsii is the world's most important source of vegetable tannin, 35-40% of the air-dry weight of bark is extracted for leather manufacture. Yields in excess of 15% are also obtainable from other species such as A. albida and A. cyanophylla. Wood adhesives and polyurethane foams can be prepared from bark extracts of A. mearnsii, and used as substitutes for petroleum products.
Gum arable, an important emulsifier, protective colloid, adhesive and binder, is produced in commercially exploitable quantities by A. arabica, the principal historical source, and more recently by A. senegal and some 20 other species of Acacia. It is used in foods, pharmaceuticals, cosmetics, adhesives, paints, polishes and inks and in lithography, photography, textile-sizing and other processes.
Species such as A. dealbata, known as mimosa by florists, A. albida and A. tortilis, are valued as ornamentals while A. farnesiana and A. dealbata are used in the production of base oils for perfumes. In Australia, several species are valuable sources of pollen in apiculture.
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Primate and fibre analyses of bipinnate Acacia spp.
Primate and fibre analyses of bipinnate Acacia spp. (continued 1)
Primate and fibre analyses of bipinnate Acacia spp. (continued 2)
Primate and fibre analyses of bipinnate Acacia spp. (continued 3)
Primate and fibre analyses of bipinnate Acacia spp. (continued 4)
Primate and fibre analyses of bipinnate Acacia spp. (continued 5)
Primate and fibre analyses of bipinnate Acacia spp. (continued 6)
Proximate analyses of Acacia phyllodes