RESPONSE OF DAIRY-COWS TREATED WITH BOVINE SOMATOTROPIN TO A LUTEOLYTIC DOSE OF PROSTAGLANDIN-F2-ALPHA

The objective of this study was to evaluate the timing of follicular w aves in cows treated with bovine somatotropin (bST) by measuring ovari an responses to a luteolytic dose of PGF(2 alpha) on d 12 of the estro us cycle. Thirty lactating cows (26 Holstein and 4 Guernsey) were assi gned to receive bST (500 mg; n = 18) or saline (control; 1.5 ml; n = 1 2) every 14 d for three injection cycles. On d 12 of a synchronized es trous cycle, cows were injected with PGF(2 alpha) to induce luteolysis . Following PGF(2 alpha), 9 cows ovulated from the dominant follicle d uring the first follicular wave (4 cows treated with bST and 5 control cows), and 14 cows ovulated from the dominant follicle during the sec ond follicular wave (8 cows treated with bST and 6 control cows). Of t he cows that ovulated during the second follicular wave, cows treated with bST had more class 3 follicles (greater than or equal to 10 mm) t han did control cows. Concentrations of estradiol rose earlier after P GF(2 alpha) injection in cows treated with bST than in control cows. T his rise in estradiol was parallel to the development of dominant foll icles. 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 wi th bST expressed estrus. These results are consistent with the hypothe sis 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.