Gonadotropin-Releasing Hormone and Prostaglandin F2α in Early Postpartum Dairy Cow
Abstract
Reproductive events in early postpartum dairy cows have been well-characterized. The interval from parturition to the development of ovarian follicles > 1 cm diameter averages about 15 days and the interval to first ovulation about 20 days (Marion and Gier, 1968; Saiduddin et. al., 1968; Morrow, 1969; Oxenreider and Wagner, 1971, and Callahan et.al., 1971). First estrus usually occurs from 30-72 days after calving (Wis. Res. Bull. 270, 1968) and about 15% of dairy cows manifest abnormal ovarian activity (e.g., ovarian follicular cysts, luteinized ovarian follicles and cystic corpora lutea) during the first two months postpartum.
Fertility of dairy cows during the normal breeding period increases in direct proportion to the number of estrous cycles prior to the beginning of breeding (Thatcher and Wilcox, 1973, and Britt et al., 1974). Maximum lifetime production of milk and offspring for dairy cows occurs when average calving interval is 12 months or less. A 12-month or less calving interval results when breeding begins about 40-60 days post-partum (Britt, 1975). Thus, treatments given to in-duce early postpartum ovulations and to increase the number of estrous cycles prior to breeding may result in increased fertility and shorter calving intervals for dairy cows.
Discovery of a naturally occurring decapeptide which caused release of luteinizing hormone in laboratory species (Schally et al., 1971) led to the availability of gonadotropin releasing hormone (GnRH) which appeared to be potentially useful for inducing LH release and subsequent ovulation in early postpartum cows. Similarily, the discovery that prostaglandin F2α was a potent luteolytic agent (Pharriss and Wyngarden, 1969) led to the availability of another naturally occurring compound considered potentially useful for inducing corpus luteum regression and subsequent estrus and ovulation in diestrous cows. Much of the recent research on these two hormones has been reviewed (Hafs et al., 1974, and Britt, 1975).
The objective of this presentation is to review research conducted: 1) to determine whether gonadotropin-releasing hormone (GnRH) could be used to induce normal estrous cycles and improve fertility in early postpartum dairy cows and 2) to determine whether prostaglandin F2α (PGF2a) could be used to synchronize the time of ovulation in groups of heifers. In addition, a model for control of ovulation with GnRH and PGF2α in dairy cattle is presented.
