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7.2 Acacia nilotica: a Tree Legume out of Control

J.O. Carter

Distribution and Habitat
History of Invasion in Australia
Seed Production
Seed Dispersal
Effect of Trees on Grass Production
Acacia nilotica as a Browse Plant in Queensland
Quality of Browsed Components of A. nilotica
Management of Grazing Systems with a Browse Component
Anticipating Weediness Problems


Acacia nilotica, a species of thorny acacia introduced into northwestern Queensland late last century or early this century, is spreading at an alarming rate. Large areas of formerly treeless, Mitchell grass (Astrebla spp.) plains are being invaded by this species creating problems with management of sheep and cattle. Although it is serious weed, the species produces reasonable quality browse, high in tannins.


Acacia nilotica (L.) Del. is a thorny wattle native to India, Pakistan and much of Africa. Nine subspecies are currently recognised (Brenan 1983); only one subspecies, Acacia nilotica subsp. indica (Berth.) Brenan, is found in Australia. Until 1940, the Australian species was regarded as Acacia arabica; however, Hill (1940) cleared the confusion with nomenclature and declared the name given by Linnaeus in 1753 as the correct one.

Distribution and Habitat

This acacia is widely distributed in tropical and subtropical Africa from Egypt and Mauritania to South Africa. Some subspecies are widespread in Asia as far east as Burma. Acacia nilotica subsp. indica grows in Ethiopia, Somalia, Yemen, Oman, Pakistan, India and Burma. It has been cultivated in Iran, Vietnam (Ho Chi Min City), Australia (Sydney and Queensland) (Brenan 1983) and the Carribbean. This subspecies is commonly found on soils with a high clay content, but may grow on deep sandy loam in areas of higher rainfall. It commonly grows close to waterways on seasonally flooded river flats and tolerates salinity well. It will grow in areas receiving less than 350 mm of rainfall to areas receiving more than 1,500 mm per annum. The species is reported to be very sensitive to frost, but will grow in areas where the mean monthly temperature of the coldest month is 16°C (Gupta 1970).

In Australia, the major areas of A. nilotica are in Queensland with small infestations reported from the Northern Territory, New South Wales and South Australia. Frost limits the distribution of the plant in southern states. Data derived from Bolton and James (1985) show infestations of about 6.6 million hectares, or 25% of the Mitchell grasslands, with dense areas of about 0.6 million hectares. The distribution and density of the species is increasing (Reynolds and Carter 1990).


In Africa and the Indian subcontinent, A. nilotica is extensively used as a browse, timber and fire-wood species (Gupta 1970, Mahgoub 1979, New 1984). The bark and seeds are used as a source of tannins (New 1984, Shetty 1977). The species is also used for medicinal purposes. Bark of A. nilotica has been used for treating haemorrhages, colds, diarrhoea tuberculosis and leprosy while the roots have been used as an aphrodisiac and the flowers for treating syphilis lesions (New 1984). The gum of A. nilotica is sometimes used as a substitute for gum arabic (obtained from A. senegal) although the quality is inferior (Gupta 1970). The species is suitable for the production of paper and has similar pulping properties to a range of other tropical timbers (Nasroun 1979).

History of Invasion in Australia

Acacia nilotica was probably introduced into Australia in the late 1890s or early 1900s and the first recorded specimen in a herbarium was collected in 1914. The plant was actively spread as a shade tree along bore drains throughout central-western and northwestern Queensland. A series of above average wet years in the early to mid 1950s led to a spectacular increase in tree density especially on town commons. In 1957, the plant was declared a noxious weed following concern about its rapid spread in previous years. Landholders were required to remove all plants, but few did so and planting continued. Many plantings were along bore drains (long open drains distributing artesian water). These plants grew rapidly and the constant water supply ensured a large seed set. Seeds from these trees were spread by animals throughout properties and during above average wet years in the mid-1970s germination of this soil seed bank resulted in up to 1,000 fold increases in the number of plants on individual properties. The properties with greatest density of plants today, are those which had bore drains lined with trees prior to the wet years of the 1970s. Properties with cattle rather than sheep tend to have more serious infestations.

Although most germination and spread of new seedlings occurred in very wet years, some establishment occurred every few years. The recent prolonged drought has reduced tree density in some areas by up to 80%. However, soil seed banks are still large, and much larger than they would have been 20 years ago. A recurrence of above average summer rainfall could result in a further massive increase in plant populations. The pattern of invasion has been exponential in nature but in a stepwise fashion associated with wet years. Calculations of the area of land infested with A. nilotica in the early 1980s, estimates of infestation before 1970 and a projection for the future are presented in Figure 7.2.1.


There is some evidence that A. nilotica is a weed in its native habitat e.g. South Africa (Holm et al. 1979), but in other areas it is planted for forestry or reclamation of degraded land (Purl and Kybri 1975, Shetty 1977). In both Asia and Africa, the plant and seed pods are eaten by domestic grazers and browsers such as cattle, sheep, goats and camels (Gupta 1970). Other animal species which browse and eat seed pods are impala, Thompson's gazelle, dorcas gazelle, dikdik, elephant, giraffe, kudu and mountain goat (Lamprey et al. 1974). Seed dispersal is mainly via these browse animals. In Australia seed spread is via domestic animals. In Africa and India, there are also large numbers of insects which attack the mature seed.


In Queensland, A. nilotica flowers from March to June with green pods being present in the driest part of the year, July to December (Figure 7.2.2). Leaf production and fall appears to be determined by availability of soil moisture (Carter and Cowan 1988) and up to 75% leaf fall occurs in the dry season. In contrast, in the Sudan, A. nilotica flowers irregularly but usually in the period June to September, with seeding occurring from January to May. Extensive leaf fall occurs in April-May with re-foliation in March-April. Leaf production and fall is similarly influenced by rainfall, whereas temperature affects flowering and fruiting (Khan 1970). In Australia, the intact ripe pods fall from November to February.

Fig. 7.2.1. Spread of Acacia nilotica in the Mitchell grass areas of western Queensland.

Seed Production

Seed production by A. nilotica is very high if trees are well watered. Trees planted along bore drains, dams or creeks produce large numbers of seeds every year. In areas with no permanent source of water, e.g. open downs, seed production is low (perhaps only a few pods per tree) unless there is significant winter rain. Along 3 km of bore drain at Toorak Research Station, estimated seed production in 1986/87 and 1987/88 was 18.6 and 24.0 million seeds respectively. Good relationships between tree basal area and seed production exist (Bolton et al. 1987). The half life of seed produced by trees on bore drains is between 10 and 12 months but may be longer for the smaller, harder seeds produced by trees with no permanent water supply.

When seed pods fall they are rapidly eaten and seed is dispersed by domestic stock. The rapid consumption of the ripe seed reduces insect predation of seed to a low level. Insects such as Caryedon serratus (Coleoptera Bruchidae), a native seed boring beetle, and Bruchidius sahlbergi Schilsky (Coleoptera: Bruchidae), a seed boring insect introduced as an attempt at biological control, do not destroy seed if animals ingest seed soon after ripening (Lamprey et al. 1974). Consequently, biological control by these seed destroying insects will not be effective.

Fig. 7.2.2. Green pods on Acacia nilotica in November near Longreach, Queensland.

Seed Dispersal

Seed dispersal of A. nilotica occurs in several ways. Dispersal over long distances occurs when animals with ingested seed are moved by road transport over large distances (e.g. 1,000 km or more). Some spread of seed also occurs by wind and water.

Cattle are the most effective agents of seed dispersal and up to 81% of seed ingested passes through the animal intact. Tests have shown that at least 41% of this seed is readily germinable (Harvey 1981); furthermore, the presence of seed in a favoured environment (manure) can lead to a survival advantage compared with seed placed on the open ground.

Sheep spread seed by three mechanisms:

· spitting out of seed and pod breakage during eating (35%) (usually under existing trees),
· spitting out seed from regurgitated material (14%), and
· seed passing through the animal in faeces (2%).

Passage through the rumen takes at least 6 days and other ruminants probably have similar or longer retention times. Goats spread seed in a manner similar to sheep with spitting out accounting for 24% of seed ingested, and passage via the faeces only 2.3%. Seed regurgitated or passed by sheep and goats is more than 80% viable. Emus, a major spreader of A. farnesiana (a native thorny acacia) do not appear to eat A. nilotica and no seed has been found in droppings.

Short distance dispersal can be in mud packs formed on animals' hooves during wet periods, or by wind which may blow seed pods from tall trees for distances of up to 25 m. Flood waters can carry seed for significant distances. Many creeks in northwest Queensland have lines of trees which have grown from pods deposited at the edge of flood lines. River systems draining to the Gulf of Carpentaria and Lake Eyre are infested with drainage lines are a significant cause of inter- and intra-property spread.

Effect of Trees on Grass Production

In the Mitchell grasslands of northwest Queensland, A. nilotica suppresses pasture production by 50% at 25-30% tree canopy cover or 2 m2 basal area per hectare (Figure 7.2.3). Maximum canopy cover and basal areas for A. nilotica in northwest Queensland are about 35% and 3.5 m2/ha respectively.

These relationships for an average year, illustrated in Figure 7.2.3, are similar to those found in eucalypt and mulga communities and follow the form of a Mitscherlich equation , where y is the yield of herbaceous species, x is the tree basal area and k is the slope of the line. The generalised relationship developed by Scanlan and Burrows (1990) shows that the slope of the curve (k value) can change with moisture and nutrients (site potential). Under ideal conditions the curve is flat, with suppression of grass being linear with tree basal area; however, as conditions worsen during drought, trees increasingly suppress grass production. The yields of annual and ephemeral plant species do not appear to be affected by tree density.

Fig. 7.2.3. Effect of A. nilotica on pasture production (kg/ha).

Acacia nilotica as a Browse Plant in Queensland

In the Mitchell grasslands, the plant is commonly browsed when there is an absence of green feed. The Mitchell grass system is driven by summer rainfall and in most years there is little green growth from May through to the first summer rainfall. The protein content of early green Mitchell grass leaf may be as high as 18%, but this rapidly declines and by the end of the season, ranges from 2.5 to 4.5% which is insufficient to maintain animal productivity. Mitchell grass pastures may also provide diets low in phosphorus, sodium and copper (McMeniman et al. 1986a). However, though old dry Mitchell grass will not maintain body weight in sheep or cattle, the presence of a robust grass species does provide a base diet during those months when more palatable feed is absent.

Browse and pod fall (November to February) can be an important supplement for animals grazing the low quality Mitchell grass pastures. Pod production occurs during the driest months of the year, and there is usually little pod set away from permanent water. However, trees growing along flowing bore drains can produce c. 1,000 kg of pods per kilometre. The average property in northwest Queensland has about 7 km of these drains.

In dry times, sheep, cattle and goats actively seek leaf and stem. Goats, and to a lesser extent sheep, also eat the bark of young trees, and sheep eat fallen flowers. Landholders cut trees down or lop branches to provide extra material for animals during drought, although leaf availability is at a minimum during these times.

Quality of Browsed Components of A. nilotica

Acacia nilotica leaf is very digestible and has high levels of protein. In some samples protein levels were higher than those for lucerne hay and much better than those for dry Mitchell grass (Table 7.2.1). Micronutrients, with the exception of sodium, are adequate for animal requirements.

The amino acid profile of A. nilotica leaf is similar to that of Mitchell grass leaf. However, the fruit is higher in glutamic and aspartic acid and lower in most other amino acids (Table 7.2.2). The amino acid methionine (an amino acid essential for wool growth) was absent from the fruit of Australian material but present in the seed of African material.

In the Mitchell grass/A. nilotica system, the tree legume acts primarily as a protein supplement. When pasture conditions deteriorate the pasture protein content falls to a level where nitrogen supply to the rumen microbes limits their activity. Provision of rumen digestible nitrogen will correct this situation, improve the protein status of animals and probably increase intake. However, the low digestibility of Mitchell grass pasture may limit energy availability and significant improvement in animal production may also demand additional energy supplements. The protein: ME ratio in drought affected pasture is often below the 6.5 g/MJ needed for maintenance and well below the 12:1 ratio needed for optimum wool production (McMeniman et al. 1986b). This compares with a ratio of 16:1 for A. nilotica leaf and 12:1 for pod material. An intake of about 1 kg of leaf or 0.9 kg of pods would be needed to supply the energy requirement of sheep for maintenance at 0.22 MT/kg W/day (McMeniman et al. 1986b).

Table 7.2.1. Nutrient levels in A. nilotica leaf and fruit.


Fruit (pod and seed)


Mean ± SD


Mean ± SD


Protein (%)

13.92 ± 2.53


12.30 ± 2.03


Fat (%)

6.63 ± 3.41


1.93 ± 1.14


NFE (%)

60.99 ± 3.41


63.68 ± 7.35


CF (%)

10.35 ± 2.85


15.36 ± 5.85


ADF (%)

20.38 ± 6.35


25.44 ± 4.16


Ash (%)

9.29 ± 2.95


5.26 ± 1.29


Tannin (%)

7.62 ± 1.00


5.45 ± 1.48


Lignin (%)

6.95 ± 2.17


P (%)

0.23 ± 0.22


0.26 ± 0.21


Ca (%)

2.53 ± 1.13


0.64 ± 0.19


Mg (%)

0.18 ± 0.08


0.13 ± 0.02


Na* (%)





K (%)

1.25 ± 0.79


1.28 ± 0.22


Si (%)

0.45 ± 0.47


0.24 ± 0.21


S (%)

0.26 ± 0.03


0.59 ± 0.11


Cl (%)

0.70 ± 0.26


0.36 ± 0.04


Cu (mg/kg)


6.43 ± 0.90


Zn (mg/kg)

25.63 ± 9.20


28.50 ± 9.76


Mn (mg/kg)

90.25 ± 19.00


2650 ± 0.71


Fe (mg/kg)

428 ± 205


100.00 ± 86.27


ME (mg/kg)

8.69 ± 1.09


10.19 ± 0.16


OMD (%)

69.9 ± 5.20




*Some values below limit of detection (0.05%)


nitrogen free extract


ether extract


acid detergent fibre


crude fibre


organic matter digestibility


metabolisable energy

Condensed tannins are high in all browsed components. The relative tannin levels in A. nilotica from least to most are pods (5.4%), leaves (7.6%), bark (13.5%) and twigs (15.8%). Total polyphenolics in the fruit range from 32 to 34% (Kumar 1983) and in leaf range from 30 to 60% (Ehoche et al. 1983, Reed 1986, Tanner et al. 1990). The high levels of tannins in plant parts may bind protein and, at high levels, suppress animal production. Bullocks fed 45% oil-extracted seeds of A. nilotica in their diet showed reduced weight gain (68 g/day to 16 g/day) and a 5% decrease in intake (Pande et al. 1982). Acacia nilotica tannins have been used to treat cottonseed cake to prevent rumen degradation of protein. At 5% inclusion of tannin (probably total polyphenolics), liveweight gains of lambs were increased by 36% and feed intakes by 6%; however, at 10% inclusion average daily gain was reduced by 18% and intake reduced by 4% (Ehoche et al. 1983). Sheep fed A. nilotica pods and roughage at 204 and 347 g/day respectively had lower growth rates than control feeds (Tanner et al., 1990).

Table 7.2.2. Amino acid composition (%) of A. nilotica and Astrebla lappacea.

Amino acid composition (%)

Amino add

Green fruit































Aspartic acid
























Glutamic acid
























































































Grass is Mitchell grass dead leaf (McMeniman et al. 1986c). Kernel' hull and seed (Kumaresan et al. 1974)

Management of Grazing Systems with a Browse Component

The ecological implication of using A. nilotica as a browse source while maintaining inappropriate stocking rates is land degradation. Trees compete with grasses for limited soil moisture, reducing feed supply and increasing stocking pressure on the remaining pasture, particularly the palatable perennial grasses. Small amounts of tree legume stimulate rumen function resulting in reduced weight loss and probable increased intakes. However, maintenance of animals through dry periods leads to reduction in ground cover and high animal numbers at the beginning of the growing season, putting maximum pressure on new grass tillers and seedlings.

The mulga (A. aneura) lands of southwest Queensland are an example of over-use of browse trees contributing to species change and soil erosion. Failure to adjust stocking rates to available feed is a common management problem in these continuously grazed savannah pastures. Managers often do not realise that competitive effects of woody plants are magnified as growing conditions become harsher.

There is some evidence from field monitoring sites which suggests that strategic heavy grazing by sheep can control small seedlings of A. nilotica. Cattle have less effect on seedlings but can utilise browse to a greater height. A survey by Bolton and James (1985) indicated that sheep-only properties have shown significantly less rapid increase in A. nilotica than cattle-only or mixed animal enterprises. A recent trial using goats to control A. nilotica showed that, in drought times, goats reduced seedling numbers and tree canopy cover with little effect on enterprise profitability (Carter et al. 1990). There is clearly potential for optimising the mix of domestic species to use A. nilotica more effectively while still providing some control of seedling recruitment.

Anticipating Weediness Problems

Planned and unplanned introductions of exotic tree legumes can sometimes lead to environmental disasters. The reasons for the invasion of A. nilotica are summarised below together with some suggestions for minimising the chance of similar invasions of other species in other areas.

Acacia nilotica was able to spread rapidly because:

· seedlings and young trees are protected from grazing by thorns,

· there was active propagation by landholders in early years,

· it has long distance dispersal mechanisms (domestic stock and floods) allowing uncontrollable spread,

· there is large seed production (up to 175,000 seeds/tree),

· it has long-lived seeds,

· the young plants grow rapidly,

· it is tolerant of grazing, drought, fire and salinity,

· there was a treeless, fire-free habitat to invade,

· the trees are long-lived (30-60 years),

· growth is possible over an extensive climatic range, and

· the useful characteristics of the plant and slow initial spread led to complacency among producers and government authorities.

The lessons learnt from this invasion and many others worldwide suggest that all plant introductions should be screened with great care. This screening should involve observations in the native homeland environment and other areas of introduction including:

· measurements of seed production and longevity,

· monitoring of methods of seed dispersal,

· examination of susceptibility of seedlings and small trees to grazing (e.g. whether thorny or not),

· a bioclimatic and soils analysis to predict potential in new country,

· investigation of the effects of insect predators, plant pathogens and fire on control of the plant in its native environment, and

· investigation of practical methods of chemical and biological control in the case of weediness problems.

There are numerous difficulties with screening of plants in this way. These include:

· unpredictable behaviour after removal from native pathogens, insects and browse animals,

· unpredictable behaviour under new climatic, management and fire regimes (the full genetic potential of a plant is not necessarily expressed in areas in which it is endemic), and

· genetic drift or hybridisation which may change the character of the plant introduced.

Once species are introduced there should be:

· long-term monitoring (problems may not show for 50-100 years), and
· rapid action if weediness appears a problem.

Once a plant has established as a weed it is usually impossible to eliminate it, and action usually involves control of further spread (an expensive and often futile process) and evolution of new management strategies to cope with and minimise the effect of the weed. Biological control is rarely a complete success and should not be relied upon as a last resort. I believe all plant introductions should be presumed weeds until proven otherwise.


Bolton, M.P. and James P.A. (1985) A survey of prickly acacia (Acacia nilotica) in five western Queensland shires. Stock Routes and Rural Lands Protection Board, Brisbane. Internal Report, November 1985.

Bolton, M.P., Carter, J.O. and Dorney, W.J. (1987) Seed production in Acacia nilotica subsp. indica (Berth.) Brenan. In: Proceedings of Weed Seed Biology Workshop, Orange, N.S.W. September 1987, pp. 29-34.

Brenan, J.P.M (1983) Manual on taxonomy of Acacia species: present taxonomy of four species of Acacia (A. albida, A. senegal, A. nilotica, A. tortilis). FAO, Rome, pp. 20-24.

Carter, J.O. and Cowan, D.C. (1988) Phenology of Acacia nilotica subsp. indica (Berth.) Brenan. In: Proceedings 5th Biennial Conference, Australian Rangelands Society, Longreach, Queensland, pp. 9-12.

Carter, J.O., Newman, P., Tindale, P., Cowan, D. and Hodge, P.B. (1990) Complementary grazing of sheep and goats on Acacia nilotica. In: Proceedings 6th Biennial Conference, Australian Rangelands Society, Carnarvon, Western Australia, pp. 271-272.

Ehoche, O.W., Theresa, Y.M., Buvanendran, V. and Adu, I.F. (1983) The nutritive value of tannin treated cottonseed cake for growing lambs. Journal of Animal Production Research 3, 15-25.

Gupta, R.K. (1970) Resource survey of gummiferous acacias in western Rajasthan. Tropical Ecology 11, 148-161.

Harvey, G.J. (1981) Recovery and viability of prickly acacia Acacia nilotica indica seed ingested by sheep and cattle. In: Proceedings, 6th Australian Weeds Conference, Vol. I, pp. 197-201.

Hill, A.F. (1940) Acacia nilotica (L.) Delile. Botanical Museum Leaflets, Harvard University, no. 8, pp. 94-100.

Holm, L.G., Pancho, J.V., Herberger, J.P. and Plucknett, D.L. (1979) A Geographical Atlas of World Weeds. Wiley, New York.

Khan, M.A.W (1970) Phenology of Acacia nilotica and Eucalyptus microtheca at Wad Medani (Sudan). The Indian Forester 96, 226-248.

Kumar, R. (1983) Chemical and biochemical nature of fodder tree tannins. Journal of Agricultural and Food Chemistry 31, 1364-1366.

Kumaresan, A., Mshelia, T.A. and Aliu, Y.O. (1974) Biochemical evaluation of bagaruwa seeds (Acacia nilotica) for use as livestock feed. Animal Feed Science and Technology 11, 45-48.

Lamprey, H.F., Halevy, G. and Makacha, S. (1974) Interactions between Acacia, bruchid seed beetles and large herbivores. East African Wildlife Journal 12, 81-85.

McMeninan, N.P., Beale, I.F. and Murphy, G.M. (1986a) The nutritional evaluation of south-west Queensland pastures. I. The botanical and nutrient content of diets selected by sheep grazing on mitchell grass and mulga/grassland associations. Australian Journal of Agricultural Research 37, 289-302.

McMeniman, N. P., Beale, I F. and Murphy, G.M. (1986b) Nutritional evaluation of southwest Queensland Pastures. II The intake and digestion of organic matter and nitrogen by sheep grazing on mitchell grass and mulga grassland associations. Australian Journal of Agricultural Research 37, 303-314.

McMeniman, N.P., Kondos, A.C. and Beale, I.F. (1986c) Nutritional evaluation of southwest Queensland pastures. III The amino acid composition of pasture plants in a mitchell grass association and their digestibility by grazing sheep. Australian Journal of Agricultural Research 37, 315-322.

Mahgoub, S. (1979) On the subspecies of Acacia nilotica in the Sudan. Sudan Silva 4, 57-62.

Nasroun, T.H. (1979) Pulp and paper making properties of some tropical hardwood species grown in the Sudan. Sudan Silva 4, 22-32.

New, T.R. (1984) A Biology of Acacias. Oxford University Press, Melbourne, 153 pp.

Pande, M.B., Talpada, P.M., Patel, Z.N., Purohit, L.P. and Shukla, P.C. (1982) Note on processed babul feeding to mature Kankrej bullocks. Indian Journal of Animal Science 52, 798-799.

Puri, D.N. and Khybri, M.L. (1975) Economics of Chambal ravine afforestation. Indian Forester 101, 448-451.

Reed, J.D. (1986) Relationships among soluble phenolics, insoluble proanthocyanidins and fibre in East African browse species. Journal of Range Management 39, 5-7.

Reynolds, J.A. and Carter, J.O. (1990) Woody weeds in central western Queensland. In: Proceedings 6th Biennial Conference, Australian Rangelands Society, Carnarvon, Western Australia, pp. 304-306.

Scanlan, J.C. and Burrows, W.H. (1990) Woody overstory impact on herbaceous understorey in Eucalyptus spp. communities in central Queensland. Australian Journal of Ecology 15, 191-197.

Shetty, K.A.B. (1977) Social forestry in Tamil Nadu. Indian Farming 26, 82.

Tanner, J.C., Reed, J.D. and Owen, E. (1990) The nutritive value of fruits (pods with seeds) from four Acacia spp. compared with extracted noug (Guizotia abyssinica) meal as supplements to maize stover for Ethiopian highland sheep. Animal Production 51, 127-133.

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