Response of Dairy Cows Treated with Bovine Somatotropin to a Luteolytic Dose of Prostaglandin F2α

Abstract

The objective of this study was to evaluate the timing of follicular waves in cows treated with bovine somatotropin (bST) by measuring ovarian responses to a luteolytic dose of PGF2α on d 12 of the estrous cycle. Thirty lactating cows (26 Holstein and 4 Guernsey) were assigned to receive bST (500mg; n=18) or saline (control; 1.5ml; n=12) every 14 d for three injection cycles. On d 12 of a synchronized estrous cycle, cows were injected with PGF2α to induce luteolysis. Following PGF2α, 9 cows ovulated from the dominant follicle during the first follicular wave (4 cows treated with bST and 5 control cows), and 14 cows ovulated from the dominant follicle during the second follicular wave (8 cows treated with bST and 6 control cows). Of the cows that ovulated during the second follicular wave, cows treated with bST had more class 3 follicles (≥ 10mm) than did control cows. Concentrations of estradiol rose earlier after PGF2α injection in cows treated with bST than in control cows. This rise in estradiol was parallel to the development of dominant follicles. Serum concentrations of FSH were decreased in cows treated with bST. During the first and second estruses, equivalent numbers of cows treated with bST and control cows ovulated, but fewer cows treated with bST expressed estrus. These results are consistent with the hypothesis that cows treated with bST have reduced FSH, a faster turnover of dominant follicles, and differences in the timing of follicular waves. Treatment of cows with bST also increased the incidence of undetected estrus.