Abomasal casein infusion and exogenous somatotropin enhance nitrogen utilization by growing lambs

Growing Dorset wether lambs (23 kg initial body weight) were used to determine whether the magnitude of nitrogen retention response to daily administration of exogenous somatotropin is limited by post-ruminal amino acid availability in growing ruminants. Eight lambs surgically fitted with abomasal cannulae were fed a total mixed ration of 85% of ad libitum intake. All lambs received a continuous abomasal infusion of 2 L of water or casein and twice daily subcutaneous injections of 0 or 100 micrograms recombinant bovine somatotropin (rbST)/kg body wt for 15 d per treatment in a 2 x 2 single reversal design. The casein solution was infused at a rate (4 to 5 g nitrogen/d) to achieve 25% of nitrogen intake observed with ad libitum feeding prior to initiation of treatments. Each lamb received all four treatments. Nitrogen balance was determined on d 8 to 14 of each treatment. Casein infusion increased nitrogen balance 43.4% (P less than 0.001), and rbST increased nitrogen balance 33.5% (P less than 0.001), without significant interaction (P less than 0.88). Combined effects of casein and rbST were additive, resulting in an 89% increase in nitrogen balance when compared with water plus excipient treatment. Results suggest that the quantity or composition of absorbed amino acids, or both, limit nitrogen retention by growing lambs, and that rbST increases the efficiency of utilization of absorbed amino acids for protein deposition.