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 functio n. Twenty-six Holstein cows were assigned to one of four groups and gi ven either bST (500 mg) or saline for seven injection cycles (14 d bet ween injections). Groups were 1) seven injection cycles of saline (con trol), 2) seven injection cycles of bST, 3) three injection cycles of bST followed by four injection cycles of saline, and 4) three injectio n cycles of saline followed by four injection cycles of bST. Beginning in the middle of the fourth injection cycle, an estrous cycle was syn chronized for study. Serum concentrations of bST and insulin-like grow th factor I were lowest during saline injections and increased after b ST injections. Growth of the first wave dominant follicle was similar; however, start of the second follicular wave was approximately 48 h e arlier in those cows treated with bST (groups 2, 3, and 4). Cows treat ed with somatotropin had a similar pattern of follicular growth, but t ended 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 o f 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 .