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.