Promising tropical grasses and legumes as feed resources in Central Tanzania: VI. Nitrogen balance in growing bulls consuming tropical herbaceous forage legumes

Nitrogen balance was measured in six growing Mpwapwa bulls (mean age 1.5 years, weight 131 kg) fed Cenchrus ciliaris hay and cassava as basal diet (negative control) supplemented with dried leaves from four herbaceous legumes: Macrotyloma axillare cv. Archer (MAA), Stylosanthes scabra cv. Seca (SSS) and Neonotonia wightii cv. Mpwapwa (NWM); or with Fish meal (FM) as positive control in a 6×6 Latin square design. All treatments with protein were formulated to provide the same energy and protein levels of intake. The digestible organic matter intakes were 13.9, 13.6, 14.2, 13.8 and 13.6 g per kg body weight (BW) per day in treatments supplemented with MAA, SSS, FM and NWM respectively and these were similar (P>0.05) but significantly higher (P<0.05) compared to the negative control. The crude protein contents of the DM consumed ranged from 8.6 to 9.5% among the supplemented diets and these were higher than the unsupplemented treatment of only 3.9%. Nitrogen intake, faecal nitrogen output, urinary nitrogen output and nitrogen balance were significantly higher (P<0.001) in the supplemented diets compared with the negative control. For the negative control treatment, nitrogen balance was 0.09 g per kg W0.75 whereas N balance ranged from 0.19–0.32 g per kg W0.75 in the other treatments. The ranking was: FM>SSS>MAS>MAA>NWM but they were not significantly different from each other. Only when calculated as percent N retained, was FM superior to NWM (P<0.05). It is concluded that using leaves from these legumes as protein supplements will have an effect on N balance in growing Mpwapwa bulls comparable to fish meal.