The effects of long-term administration of recombinant bovine somatotropin (Posilac) and Synovex on performance, plasma hormone and amino acid concentration, and muscle and subcutaneous fat fatty acid composition in Holstein-Friesian bull calves.

The effect of recombinant somatotropin (rbST), Synovex (Syn), and their combination (rbST+Syn) on intact male calves was examined in an experiment that lasted an average of 238 d. Holstein-Friesian bull calves were allotted to one of four subtreatments (n = 14/treatment) in a factorial arrangement. There were two levels of rbST (0; rbST) and two levels of the estrogenic growth promoter Synovex (0; Syn). The rbST was administered once every 2 wk as injections of 500 mg of Posilac. Synovex (C and S) was implanted at 90-d intervals. The animals were fed for ad libitum consumption a diet with a metabolizable energy concentration of 11.7 MJ/kg DM and 15% crude protein. The hot carcasses were weighed after the removal of kidney, pelvic, and cod fats, which were weighed separately. The 12th rib cut was saved for analysis. Average daily gain and feed conversion efficiency were increased by rbST treatment by 9% (P < .005) and 10% (P < .016), respectively. There was no significant effect of Syn treatment, nor was there a rbST x Syn interaction. The proportion of the fat of the large depots in the carcass was reduced by 34% (P < .0001) and in the longissimus muscle by 32% (P < .16) owing to the rbST treatment. The plasma concentrations of GH, insulin, and thyroxin were increased by rbST treatment (P < .001, P < .01, and P < .03, respectively). The concentration of IGF-I was not affected. Synovex had no effect on plasma hormone concentration. Plasma essential and nonessential amino acid concentrations were reduced by 14 and 9%, respectively, when rbST was injected. Concentrations of cholesterol and fatty acids in muscle and subcutaneous fat were not affected (P > .072) by the rbST treatment. Synovex increased the monounsaturated fatty acids (MUFA), and the combinaton of Syn with rbST reduced polyunsaturated fatty acid (PUFA) concentration in the longissimus muscle (at the 12th rib). The reduced muscle fat content of the rbST-treated animals was associated with a trend toward an increase in polyunsaturated fatty acids.