Pretreatment with recombinant bovine somatotropin enhances the superovulatory response to FSH in heifers

One of the primary limiting factors to superovulation and embryo transfer in cattle has been the large variability in response, both between and within animals. It appears that the primary source of this problem is the variability in the population of gonadotropin-responsive follicles present in ovaries at the time of stimulation. We have shown that treatment of heifers with recombinant bovine somatotropin (rbGH) increases the number of small antral follicles (2 to 5 mm) and, therefore, enhances the subsequent superovulatory response to eCG. To investigate further the potential of using this approach to improve superovulatory regimens in cattle, the effect of rbGH pretreatment on the response to pituitary FSH was studied. The estrous cycles of 16 heifers were synchronized using PGF2 alpha. On Day 7 of the synchronized cycle, half of the animals were injected with 320 mg sustained-release formulated rbGH, while the other half received 10 mi saline. Five days later, all heifers were given a decreasing-dose regimen of twice daily injections of oFSH for 4 d, incorporating an injection of PGF2 alpha with the fifth FSH treatment, to induce superovulation. All animals were artificially inseminated twice with semen from the same bull during estrus. Ova/embryos were recovered nonsurgically on Days 6 to 8 of the following estrous cycle, and the ovulation rate assessed on Day 9 by laparoscopy. Using the same animals as described above, the experiment was repeated twice, 3 and 6 mo later,with no laparoscopy in the third experiment. The animals were randomized both between experiments and for the day of ova/embryo collection. Pretreatment of heifers with rbGH significantly (P<0.01) increased the number of ovulations, total number of ova/embryos recovered and the number of transferable embryos. The percentage of transferable embryos was significantly (P<0.05) increased by rbGH pretreatment. In addition, the incidence (2/16) of follicular cysts with a poor ovulatory response (<6 ovulations) for the rbGH-pretreated heifers was significantly lower (P<0.05) when compared with the incidence (7/16) in the control animals. It is concluded that pretreatment with rbGH may provide a useful approach for improving superovulatory response in cattle.