Follicular function in lactating dairy cows treated with sustained-release bovine somatotropin

The objective of this study was to determine the long-term, short-term, and residual effects of bovine somatotropin (bST) on ovarian function. Twenty-six Holstein cows were assigned to one of four groups and given either bST (500 mg) or saline for seven injection cycles (14 d between injections). Groups were 1) seven injection cycles of saline (control), 2) seven injection cycles of bST, 3) three injection cycles of bST followed by four injection cycles of saline, and 4) three injection cycles of saline followed by four injection cycles of bST. Beginning in the middle of the fourth injection cycle, an estrous cycle was synchronized for study. Serum concentrations of bST and insulin-like growth factor I were lowest during saline injections and increased after bST injections. Growth of the first wave dominant follicle was similar; however, start of the second follicular wave was approximately 48 h earlier in those cows treated with bST (groups 2, 3, and 4). Cows treated with somatotropin had a similar pattern of follicular growth, but tended to have more class 1 (3 to 5 mm) follicles and had more class 2 (6 to 9 mm) follicles than did the control cows. Peak concentrations of serum FSH were lower and occurred earlier for the cows treated with bST. There were no differences in the long-term or short-term effects of bST on ovarian function. Furthermore, cows that were switched from bST to saline (group 3) showed a residual effect of bST for up to 3 wk.