Effect of Recombinant Bovine Somatotropin on the Superovulatory Response to Pregnant Mare Serum Gonadotropin in Heifers

Abstract

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 I. 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-17 beta, growth hormone (GH), and insulin-like growth factor-I (IGF-I).(ABSTRACT TRUNCATED AT 250 WORDS)