Previous experiments with early weaned lambs at pasture have indicated the large protein losses which occur in the rumen (prairie grass 3%, ryegrass 22%, white clover 29% and lucerne 42%) (Cruickshank et al. 1985). Growth appeared to be related to duodenal protein supply and these were similar for the grasses and for the legumes (grasses 228 g/d liveweight gain and 0.92 g duodenal NAN/kg W/d, legumes 314 g/d and 1.22 g duodenal NAN/kg W/d). Efficiency of use of absorbed amino acid also appeared to be low (0.43). These results suggested that growth of lambs grazing these high quality pastures could be increased by increasing duodenal protein supply and/or by amino acid supplementation. Because of the large protein losses which occur in the rumen of animals grazing legume pastures in particular there is scope for manipulation of this aspect either through supplementation or plant breeding.

The objective of this experiment was to examine the liveweight gain response of lambs grazing grass or legume pastures to fishmeal supplementation and also the response of lambs grazing legume pastures to two forms of protected amino acid supplements designed to supplement first limiting amino acid(s).