Acacia senegal (gum arabic)

There are four varieties of A. senegal. These differ in the presence or absence of hair on the axis of the flower spike, colour of the axis, shape of pod tips, number of pinnae pairs, trunk branched or not branched from the base and shape of the crown, as well as their geographical distribution.

The germination of seed is slow and young plants usually have to contend with competition from grass and browsing stock. Those which survive will begin to yield gum at three to four years of age. The best yields in West Africa come from trees 12 to 15 years old and up to 20 cm diameter. In Kordofan trees aged four years are opened for tapping, which continues to the age of 20 or older, but elsewhere in Sudan six to 18 years is usual. The best yields come from trees in areas with an annual rainfall of only 250 to 300 mm. Trees die at an age of 25 to 30 years, by which time they will have succumbed to borers and termites.

SEPASAL records 196 local names for this plant, and other common names include: tur, tulh, harheyr (Jibbali), temmar (Dhofari arabic), Sudan gum, Kordofan gum (English), gommier, gommier vrai, gomme blonde, gomme blanche (French), khor (Punjab), kumta (Rajputana), Senegal gum, Somali gum (in the trade industry).

In the food industry gum arabic is used as a flavour fixative and emulsifier, to prevent crystallisation of sugar in confectionery, as a stabiliser in frozen dairy products, for its viscosity and adhesive properties in bakery products, and as a foam stabiliser and clouding agent in beer.

In pharmaceuticals, it is used as a stabiliser for emulsions, a binder and coating for tablets, and as an ingredient in cough drops and syrups. A soothing and softening agent, gum arabic is extensively employed in folk medicines. Among many other uses, it is used internally for coughs, diarrhoea, dysentery, haemorrhage, and externally to cover inflamed areas.

Gum arabic is used in cosmetics as an adhesive for face 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, sensitiser for lithographic plates, coating for special papers, sizing agent for cloth to give body to certain fabrics, and coating to prevent metal corrosion. Gum arabic is also used in the manufacture of matches and ceramic pottery.

Acacia senegal wood is locally valued for fuel wood and charcoal, although biomass yield per unit of land area is not sufficient to plant it for this. In construction the wood is used locally for poles and fence-posts, the light-coloured wood for tool handles and dark heartwood for weaver's shuttles. Strong ropes are made from the bark fibres of the long surface roots. Where the trees are large (for example near the River Niger) they are cut into planks at least 12 cm thick for making canoes for hunting hippopotamuses. The wood is hard and heavy and takes a beautiful polish, with the sapwood being yellowish white and the heartwood nearly black and irregular. The wood is made into throwing-sticks which, in contrast to the Australian boomerang, can be made to fly straight and used for hunting and pageantry.

The dried and preserved seeds of A. senegal are used by people as vegetables. The leaves and pods are browsed by sheep, goats, camels, impala and giraffe. The seeds are sometimes eaten as a vegetable in India.

The flowers are a good source of honey, an important source of nutrition and income generation to support rural livelihoods in marginal lands.

Acacia senegal can help prevent desertification, through sand dune stabilisation and by acting as a wind break.

In agroforestry the seed should be harvested before pods have dried for easy collection and to avoid insect attack. Seed is easily extracted by hand. Freshly extracted seed should immediately be dusted with an insecticide. Seed will remain viable for three to four years if kept in opaque, airtight containers. There are 10,000 to 30,000 seeds per kg. Fresh seed requires no pre-treatment if sown immediately after harvest. Seed collected in previous seasons, however, requires pre-treatment to break seed dormancy. Soaking seed in water for 12 to 24 hours gives good results and is simple to apply. Seeds can also be nicked (a small cut made into them).

Acacia senegal is raised in plant nurseries in polyethylene pots, two to four seeds per pot, thinned to one seedling after four to six weeks. Direct seeding (five to eight seeds in 30 x 30 x 30 cm pits or larger) can also be used. Strict protection from fire and livestock grazing, and efficient control of weed competition during at least the first two years is important for seedling survival. Minimum spacing for block planting is 4 x 4 m. At 10 x 10 m spacing, agricultural intercropping is possible, for example, inter-planting with millet, beans, or groundnuts.

When trees are near well watered locations, they grow taller but they do not produce any gum. The tree has to be under stress, diseased, ill-nourished and cut off from water before it will produce gum.