Anti-nutritive factors

Harden and Zolfaghari (1988) reported that green and ripe Prosopis pods were used as food for both humans and animals. Gohl (1981), however, stated that green pods were bitter, with little feed value and that only ripe pods were relished by stock. Gohl went on to state that stock poisoning had occurred when P. juliflora pods were eaten after being exposed to rain. NAS (1979) noted that cattle fed on an exclusive diet of pods of P. pallida and P. glandulosa became malnourished and developed jaw and tongue troubles which caused difficulty in chewing and resulted in weight losses. The anti-nutritive factors in pods are not well understood but it would seem prudent to balance a pod-based diet with other feedstuffs where possible.

There is some evidence that certain parts of P. africana contain the alkaloids prosopine and prosopinine (Booth and Wickens, 1988), although it is not certain that these compounds have an antinutritive effect on ruminants. Dalziel (1948) reported that the bark of P. africana trees in east Sudan contained 14-16% tannin, capable of giving a reddish brown colour to both leather and cloth. Tannin is recognized as an anti-nutritive factor when fed to ruminants at concentrations above about 5% (McLeod, 1974).

Tannins are also present in the leaves of certain Prosopis spp. including P. cineraria, P. juliflora and P. glandulosa and there are a number of reports of the negative effects of tannins on DM and crude protein digestibility (Bhandari et al., 1979; Kumar, 1983; Upadhyaya, 1985; Lyon et al., 1988 and Holechek et al., 1990). Kumar (1983) observed that low crude protein digestibility of P. cineraria had been attributed to the presence of high levels of tannins. The low in vitro digestibility of crude protein (32.8% for a crude protein content of 13.7%) appeared to be due to the presence of condensed tannins, estimated with a vanillin-HCl assay to be 160 mg/g, with a protein precipitating capacity of 10.6%. Further reports of low digestibility values for crude protein (average 30.2%) were attributed to the high content of tannins and structural carbohydrates in the leaves. Lyon et al., (1988) negatively correlated in vitro digestibility with phenolic content of various species of Prosopis. Livestock deaths have also been linked with consumption of Prosopis leaves with a high tannin content (Kumar, 1983).

Thus, especially with P. cineraria, it is clear that Prosopis, one of the most abundant and important top-feeds of desert regions, is both poorly digestible and occasionally causes stock losses. There are a number of possible explanations for these observations. Firstly, the levels of tannins in the leaves of Prosopis vary considerably. Sehgal (1984) reported tannin levels of 2.18%, measured by the Folin-Denis test, while in an examination of 15 individual trees, Joshi et al., (1985) found levels ranging from 10.6 to 25.3%. The latter authors did not record the analytical procedures used to determine the tannin contents and it may be that their data are not directly comparable with those of Sehgal (1984). Nevertheless, they serve to demonstrate extreme variability between the foliage from individual trees. Newly emerging leaves had a higher tannin content than older ones, showing greater phytochemical protection for the structures which are of most importance in plant survival. Secondly, environment and stress on the plant are thought to play a part in tannin concentration, although Vaithiyanathan and Singh (1989), while noting seasonal changes in the tannin contents of top-feeds in the arid region, failed to relate this to temperature variation. Lastly, Prosopis is not necessarily suitable as a sole feed for livestock. Some of the in vivo digestibility trials reported in the literature have used Prosopis as the sole feed for small ruminants, but in most on-farm situations, it is likely that animals would self select, or that the fodder would be fed in combination with other feeds. This would dilute the toxic principle and mitigate the more extreme effects.