Effect of recombinant bovine somatotropin on estradiol-induced estrous behavior in ovariectomized heifers.

To evaluate whether bST administration could affect the intensity of estrous behavior in the absence of the ovary, 17 ovariectomized heifers were assigned to receive a placebo or 500 mg/14 d of recombinant bST and were induced to display behavioral estrus by administration of 2 mg of estradiol cypionate. Estrous behavior was monitored for the following 60 h with a surveillance camera and a video recorder, and mounting activity was quantified. Blood samples were collected before and after bST administration. Administration of bST numerically elevated plasma insulin and NEFA and caused a significant increase in concentration of insulin-like growth factor-I in plasma. Heifers that received bST started to display estrous behavior later (24.5 +/- 1.1 and 21.9 +/- 1.1 h after estradiol administration for bST and placebo treatments, respectively), displayed estrous behavior of shorter duration (bST, 13.2 +/- 1.2 vs. placebo, 15.8 +/- 1.1 h), and instigated fewer mounting events (bST, 45.5 +/- 5.9 vs. placebo, 62.6 +/- 5.7 mounts) than control heifers, although only the number of mounting events was significantly different between the two groups.