Effects of recombinant bovine somatotropin (sometribove) on ovarian function in lactating and nonlactating dairy cows.

Metabolic and ovarian responses of lactating and nonlactating cows to recombinant bST (sometribove) were measured. Eighteen lactating cows (60 to 100 d postpartum) and 6 nonlactating cows (> 400 d postpartum) were injected daily with bST or saline during one of two periods. Concentrations of hormones and metabolites were measured in plasma, and ultrasonography was used to quantify ovarian follicular growth. Concentrations of glucose, insulin, IGF-I, progesterone, and estradiol in plasma were greater in nonlactating cows than in lactating cows. Lactating cows had fewer class 2 (6- to 9-mm) and class 3 (10- to 15-mm) follicles and more class 4 (> 15-mm) follicles than nonlactating cows. Bovine somatotropin increased the numbers of follicles in lactating cows so that the numbers of class 2 and of class 3 follicles were equivalent to those for non-lactating cows. Sizes of the largest follicles were similar for bST-and saline-treated cows. The second largest ovarian follicles, however, were larger in bST-treated cows. Ovarian follicular dynamics were altered by bST and lactation. Bovine somatotropin increased the numbers of follicles (6 to 15 mm) in lactating cows and size of second largest ovarian follicles in both lactating and nonlactating cows. Lactating cows developed dominant follicles that were larger and less estrogenic than those of nonlactating cows.