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 com bination (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. T here were two levels of rbST (0; rbST) and two levels of the estrogenic gro wth 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 i ntervals. The animals were fed for ad libitum consumption a diet with a met abolizable 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 fa ts, which were weighed separately. The 12th rib cut was saved for analysis; Average daily gain and feed conversion efficiency were increased by rbST t reatment by 9% (P < .005) and 10% (P < .016), respectively. There was no si gnificant 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 th e rbST treatment. The plasma concentrations of GH, insulin, and thyroxin we re increased by rbST treatment (P < .001, P < .01, and P < .03, respectivel y). The concentration of IGF-I was not affected. Synovex had no effect on p lasma hormone concentration. Plasma essential and nonessential amino acid c oncentrations were reduced by 14 and 9%, respectively, when rbST was inject ed. Concentrations of cholesterol and fatty acids in muscle and subcutaneou s fat were not affected (P > .072) by the rbST treatment. Synovex increased the monounsaturated fatty acids (MUFA), and the combinaton of Syn with rbS T reduced polyunsaturated fatty acid (PUFA) concentration in the longissimu s muscle (at the 12th rib). The reduced muscle fat content of the rbST-trea ted animals was associated with a trend toward an increase in polyunsaturat ed fatty acids.