The leaves and stems of Acacia nilotica are readily browsed by sheep, cattle, goats and camelids. Trees can be lopped for foliage during dry periods to provide supplemental feeding to livestock (Carter, 1994). In the drier areas of Sub-Saharan Africa, for instance, the pods ripen and fall well into the dry season when there are few alternative protein sources. They may be collected to be brought back to the village to feed the livestock or sold in fodder markets. In some regions, the animals are taken to the trees and the pods are consumed as they fall down naturally or are knocked down by herders (Mlambo et al., 2008; Tanner et al., 1990). Different ruminant species do not eat the same plant parts: in Djibouti, it was observed that camels preferred the non-lignified tips of branches once the leaves had been eaten by sheep and goats. Camels ate the pods whole while goats discarded the seeds and ate only the pod husks. Young female goats were fed almost exclusively with fallen inflorescences at the end of the flowering period (Audru et al., 1993).
Foliage
Information regarding digestibility values for Acacia nilotica leaves is scant but in vivo DM digestibility of 55% (Barbind et al., 1994) and 66% (in sacco 48 h, Cheema et al., 2011) have been reported. The latter value should result in a ME value under 10 MJ/kg DM. Acacia nilotica leaves included at up to 20 % in goat diets had a detrimental effect of nutrient digestibility, N absorption and retention and rumen bacteria (Sotohy et al., 1997). In a comparison with four other Acacia species (Acacia karroo, Acacia tortilis, Acacia sieberiana, Acacia rhemniana), young dried leaves of Acacia nilotica were found to be among the most palatable with an intake of about 200 g/d in sheep and goats (Mokoboki et al., 2011). The relationship between tannins and palatabilty is not simple: in a comparison between browse fodder of Acacia nilotica, Acacia seyal and Sesbania sesban as a protein supplement to a tef straw-based diet in sheep and goats, acceptability was similar for the three browses even though the Acacia species contained more condensed tannins than Sesbania (Ebong, 1995).
Pods
In some areas, such as South Africa, Acacia nilotica pods have been reported to be favoured by both cattle and goats during dry periods (Chepape et al., 2011). However, while the pods supply protein and energy, their high tannin content is detrimental to their digestibility, degradability and palatability (Ngwa et al., 2002; Mlambo et al., 2008, Rubanza et al., 2003a). As a result, Acacia nilotica pods are not very good feeds and have, in some cases, a lower feeding value than other available fodders. In sheep fed maize stover supplemented with acacia pods, lower growth rates were reported with Acacia nilotica than with Acacia tortilis and Faidherbia albida and it was considered to be a less suitable protein source than those species (Tanner et al., 1990). Acacia nilotica pods were found to be able to meet the maintenance requirements of sheep but they were of low digestibility and phosphorus supplementation was required (Chellapandian et al., 2003). The pods are not palatable and using them as a supplement has resulted in a high refusal rate (Ncube et al., 1994).
Several methods have been proposed to alleviate the negative effects of tannins in Acacia nilotica-based diets for ruminants. The use of tanniniferous browse fodder such as Acacia nilotica could be optimized through feeding a mixture of supplements with readily available nitrogen to dilute the tannin antinutritional activity (Rubanza et al., 2003a). Sun-drying may also help to reduce tannin content (Rubanza et al., 2003a; Ebong, 1995). Soaking pods with polytethylene glycol (PEG) was shown to increase significantly in vitro gas production and 95-h organic matter degradability (from 69 to 79%) but this treatment is too costly for smallholder farmers (Mlambo et al., 2001). While alkaline treatment with NaOH or NH3 can be dangerous, one economical and readily available source of alkaline material is wood ash solution: pods crushed and soaked in a wood ash solution showed a decrease in the concentration of soluble tannins (Sikosana, 2006).
