NEW CLINICAL USES OF GNRH AND ITS ANALOGS IN CATTLE

Gonadotropin-releasing hormone (GnRH) and its analogues cause an acute secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH) such that concentrations in peripheral blood are elevated for a 3-5 h period. GnRH-induced alterations in the function of the corpus luteum (CL) or follicle appear to be indirect through alterations in LH and FSH secretion. Repeated injections of GnRH during diestrus or single injections late in diestrus will cause acute increases in plasma progesterone and a delay in CL regression. Injections or continual administration of GnRH during early phases of CL development appear to augment CL differentiation and alter subsequent CL function. These effects are attributable to induced increases in LH. Injections of GnRH during the estrous cycle will re-synchronize follicle development owing to ovulation or luteinization of the dominant follicle leading to subsequent recruitment and selection of a new dominant follicle during a 7 day period. Injection of GnRH followed by injection of prostaglandin (PGF(2 alpha)) at 6 or 7 days is a system of estrous synchronization in which follicle development and CL regression are both synchronized and fertility at the induced estrus is good. Injection of GnRH during the luteal phase post-insemination (e.g. Days 12-14) or post-embryo transfer, to alter CL and/or follicular function, has not resulted in a consistent increase in pregnancy rates. An overall assessment of studies that injected GnRH at the time of insemination in first service postpartum cows or in repeat breeders is rather disappointing. Considerable variation existed among studies within both types of cow populations relative to significant differences, directions of pregnancy rate change ( + vs. -), and magnitude of pregnancy rate increases. Recent findings indicate that timing of GnRH injections closer to the onset of estrus may be beneficial in increasing the conception rate. Utilization of GnRH in combination with progesterone and PGF results in an acute treatment sequence to program follicular development, ovulation and a subsequent cycle in cows with follicular cysts. The use of GnRH, with or without PGF, as a reproductive management program in the early postpartum period has not shown a clear improvement in subsequent reproductive efficiency. Development of precise systems to control ovarian function and reproductive efficiency with GnRH and other pharmaceutical agents is possible. However, such advancements must be founded on a clear understanding of GnRH-induced physiological effects and ability to capture any advantage by good management of the farm unit.