noted that bST increased (approximately 50 percent) the incremental efficiency of protein deposition. Total energy balance and total heat production were not altered by ST, indicating that gross efficiency of utilization of ME for gain was not changed (Eisemann et al., 1986b).

A small increase in net energy requirement is probable for maintenance in bST-treated cattle because significant increases in weights of the liver, kidneys, skeletal muscle, and bone have been observed, although heat production data do not support this. Increased efficiency of use of absorbed amino acids for protein gain may have been responsible for the significant increases in nitrogen retention or protein gain observed in these and other studies, the magnitude of which is less than that observed in growing and finishing pigs treated with similar doses of ST.

Direct comparisons of several protein intake levels at one or more doses of ST in cattle have not been reported. It would seem possible, and has been suggested (Crooker et al., 1990), that amino acid availability may have limited the response to exogenous ST in some studies. Evidence to support this is found in the demonstrated additive effects of abomasal casein infusion and daily bST administration on nitrogen retention in growing steers (Houseknecht et al., 1992) and lambs (Beermann et al., 1991) and in observed additive effects of fishmeal and ovine ST (oST) on feed conversion efficiency and hind leg muscle weights in growing lambs (Beermann et al., 1990).

The Net Carbohydrate and Protein System model developed by Fox and co-workers (Fox et al., 1992; Russell et al., 1992; Sniffen et al., 1992) was used to compare nutrient requirements of control and ST-treated steers in which nitrogen retention was increased 32.6 percent in the absence of abomasal casein infusion and increased an additional 44.6 percent with abomasal casein infusion (Fox et al., 1990a). After input of body weight, frame size, sex, condition, and breed designations, frame size was adjusted for bST-treated cattle to bring the predicted gain to equal observed gain. Predicted microbial protein production and amino acids