ALTERNATIVE GONADOTROPINS FOR SUPEROVULATION IN CATTLE

Superovulation requires the administration of a gonadotrophin preparation rich in or mimicking the effect of follicle stimulating hormone (FSH). In addition, it is necessary that gonadotrophins be available long enough for the follicle to continue growing to enable final maturation of the oocyte, which will ensure normal fertilization and embryo development. Several gonadotrophins have been used to increase the ovulation rate in beef and dairy cattle, either in conjunction with progestogen or prostaglandin F2-alpha treatments to regulate the estrous cycle. Ovulation rates of about 15 are frequently recorded; however, fewer than 10 embryos may be recovered, with about six embryos suitable for transfer. Pregnant mare serum gonadotrophin (PMSG) has been used and associated with high numbers of unovulated follicles, because of its long half-life in blood. The use of anti-PMSG has resulted in a decrease in the number of large follicles at the time of embryo recovery. This decrease has not always been accompanied by a corresponding increase in the number of transferrable embryos. FSH, with its shorter half-life, if given frequently, results in equivalent ovulation rates but generally gives a 'better' ovarian response in terms of fewer follicles and a higher number of transferable embryos. The use of other preparations, such as human menopausal gonadotrophin (hMG), have resulted in variable ovulatory responses. Progestogen treatment, to regulate the cycle, will facilitate superovulation without reference to the stage of the estrous cycle; however, in general, responses have not matched those obtained with prostaglandin F2-alpha at mid-cycle. One of the major problems with superovulation is the large variability in the ovulatory response, with a relatively high proportion of animals failing to produce any transferable embryos. A better understanding of the endocrine and paracrine factors controlling the process of follicle selection, growth and atresia is required in order to develop more effective systems of superovulation.