EFFECT OF RECOMBINANT BOVINE SOMATOTROPIN ON THE SUPEROVULATORY RESPONSE TO PREGNANT MARE SERUM GONADOTROPIN IN HEIFERS

Treatment of mature heifers with recombinant bovine somatotropin (BST) increases the number of ovarian follicles of 2-5 mm in diameter. This study was carried out to investigate whether the increase in the number of small follicles induced by BST treatment can enhance the superovulatory response. In the first experiment, 24 heifers were assigned, in a randomized block design, to four treatment groups: 1) control; 2) BST; 3) pregnant mare serum gonadotropin (PMSG); 4) BST + PMSG. On Day 7 of the estrous cycle, animals in groups 2 and 4 received injections of 320 mg BST (in a sustained release formulation), while heifers in groups 1 and 3 received 10 ml saline. Five days later heifers in groups 3 and 4 were treated with a single dose of 2000 IU PMSG to induce superovulation. Embryos were recovered nonsurgically on Days 6-8 of the following cycle, and all heifers underwent laparoscopy on Day 9 to assess ovulation rate. In a second experiment, 24 heifers were assigned randomly to four treatment groups: 1) PMSG (1000 IU); 2) BST (320 mg) + PMSG (1000 IU); 3) PMSG (2000 IU); 4) BST (320 mg) + PMSG (2000 IU), and then treated as for experiment 1. In both experiments, all heifers were scanned daily using real-time ultrasound from the day before BST injection until the day of ovulation to monitor dynamics of ovarian follicular growth and development, and blood samples were collected daily throughout the experimental period for measurement of peripheral progesterone, estradiol-17beta, growth hormone (GH), and insulin-like growth factor-I (IGF-I). In the first experiment, ovulation rate was significantly (p < 0.01) higher in group 4 (23.2 +/- 3.4) than in group 3 (12.5 +/- 4.1), while it was not different between group 1 (1.0 +/- 0.0) and group 2 (1.0 +/- 0.0). Three heifers from group 3 and 1 from group 4 had a poor response (defined as less-than-or-equal-to 3 ovulations and/or development of either follicular cysts or cystic ovaries). The total number of egg collected was significantly greater (p < 0.01) in group 4 (7.4 +/- 1.6) when compared to group 3 (3.8 +/- 1.2), while the number of transferable eggs did not differ. In the second experiment, ovulation rates for groups 1, 2, 3, and 4 were 2.5 +/- o.6, 4.5 +/- 0.7, 10.6 +/- 2.7, and 17.2 +/- 2.8, respectively (p < 0.01). All BST-treated heifers showed a good response to PMSG, while 4 heifers from groups 1 and 3 had a poor response. Numbers of eggs collected for groups 1, 2, 3, and 4 were 1.3 +/- 0.3, 2.1 +/- 0.4, 4.1 +/- 1.3, and 7.1 +/- 1.4, respectively; and numbers of transferable eggs were 1.3 +/- 0.3, 2.1 +/- 0.4, 2.9 +/- 1.0, and 3.6 +/- 1.1, respectively. In both experiments, after BST treatment heifers had significantly higher peripheral GH and IGF I concentrations and numbers of small follicles. Peripheral concentrations of estradiol-17beta after PMSG injection, progesterone concentrations from Day 2 of the estrous cycle after superovulation until the end of the experiment, and populations of follicles > 10 mm and 5-10 mm on the day of estrus in the heifers pretreated with BST were significantly higher than those in heifers treated with PMSG alone. In conclusion, pretreatment with BST enhanced the superovulatory response to PMSG treatment in heifers, and seemed to reduce the variability in the responses among individual animals.