PREGNANCY INCIDENCE IN HIGH PRODUCING DAIRY-COWS TREATED WITH RECOMBINANT BOVINE SOMATOTROPIN

Reproductive performance in cows exposed for two lactations to recombi nant bST was measured using unconditional logistic regression. Intramu scular administration of 17.2, 51.6, and 86 mg of recombinant bST/d pe r cow started at d 70 postpartum and ended at dry-off or 305 d postpar tum. Performance was measured as the cumulative incidence of cows beco ming pregnant by 305 d postpartum. Within each parity group, cows trea ted with bST had. higher total milk yield. During the first study lact ation, the log odds of a cow becoming pregnant decreased linearly as d ose increased, following adjustment for serum cholesterol, blood urea N, average daily milk yield, peak milk yield, net energy balance, and body condition score. However, differences in pregnancy incidence betw een treated and untreated cows were statistically significant only at the higher dose categories, the 51.6 and 86.0 mg of recombinant bST pe r cow. During the second study lactation, pregnancy rates were similar to those in the first lactation; rates reduced as recombinant bST dos e increased. The singular exception was for cows exposed to recombinan t bST during the previous lactation but not during the second. In the latter group, previously treated cows had higher pregnancy incidence t han did the controls.