Effect of feed intake on metabolizable protein supply in Dorper × thin-tailed Han crossbred lambs

•Metabolizable protein (MP) supply was measured in vivo in Dorper crossbred lambs at three levels of feed intake.•MP supplies for lambs with ADGs of 300, 150, and 0 g/day were 102, 91, and 75 g/day, respectively.•MP supply was predicted from intake of organic matter (R2 = 0.89) or crude protein (R2 = 0.89).•The MP requirement for Dorper crossbred lambs with an ADG of 300 g was lower than that for British or American evaluation systems.

We investigated the metabolizable protein (MP) supply in lambs at different levels of feed intake. Twelve Dorper × thin-tailed Han crossbred ram lambs (41.3 ± 2.8 kg body weight) fitted with ruminal and duodenal cannulae were randomly assigned to one of three levels (n = 4 lambs each) of dry matter intake: ad libitum (AL) intake and 70% or 50% of AL intake. Digesta flow was measured using a dual-marker system (Yb and Co). A lower duodenal flow of rumen undegraded nitrogen (RUN) was measured for the 50% AL group (P <  0.05) compared with the other two groups. For lambs of the AL group, the ratio of microbial N/duodenal non-ammonia nitrogen (NAN) was lower (P < 0.05), and the ratio of RUN/duodenal NAN was higher (P <  0.05) compared with the other two groups. The ratio of RUN/N intake was higher for the 70% AL and 50% AL groups compared with that for the AL group (P < 0.05). Apparent post-ruminal N digestibility increased with decreasing feed intake (P < 0.05). A linear correlation was established to predict MP supply (g/day) from the intake of organic matter (kg/day) or crude protein (g/day): MP = 0.036 (±0.004) × organic matter intake + 50.47 (±4.43), R2 = 0.89; MP = 0.27 (±0.033) × crude protein intake + 49.88 (±4.93), R2 = 0.87. The current results provide preliminary data of MP requirements for growth of Dorper crossbred lambs.