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1986). The response of store lambs to protein supplementation of a roughagebased diet. ABSTRACTThe response of 35-kg store lambs to diets containing increasing rumen-undegradable nitrogen (UDN) concentrations was measured by live-weight gain and nitrogen (N) balance. A range of UDN concentrations from 2-4 to 7-2 g/kg dietary dry matter was achieved by supplementing a basal diet of NaOH-treated barley straw, tapioca, extracted rapeseed meal and minerals with both formaldehydetreated rapeseed meal and fish meal. Sufficient diet to provide maintenance and a growth rate of 150 g/day was given to the lambs, according to live weight. Increasing levels of UDN promoted a linear increase in live-weight gain and N retention. However, the response to fish meal was greater than that for formaldehyde-treated rapeseed meal. The true digestibility of N of formaldehyde-treated rapeseed meal determined in chickens by ileal analysis was low in comparison to that of untreated rapeseed meal.

Forty-eight, individually-penned lambs (mean live weight 31·4 kg) were given diets with or without fish meal (0 and 90 g intake daily) in sufficient quantities to provide energy for maintenance and either 100, 150 or 200 g daily gain. Nitrogen (N) degradability in the rumen and fractional outflow rates of protein supplements were determined and the diets formulated such that the lambs received about 2·2 and 6·7 g undegradable rumen N daily on all three feeding levels.Live-weight gains were generally greater during the first 3 weeks than during the final 4 weeks of the experiment. Lambs given fish meal had greater gains (P < 0·001) throughout the experiment, but there was a feeding level x protein interaction (P < 0·05) during the first 3 weeks, such that the difference between diets with or without fish meal was greater at the highest feeding level.Retention of N increased with feeding level (P < 0·001) and was greatest when fish meal was given (P < 0·001). There was a feeding level x protein interaction (P < 0·05) for g N retained per 100 g N digested; N retention increased with feeding level only when fish meal was given.