A review of methods to synchronize estrus in replacement beef heifers and postpartum cows

ABSTRACT This review considers methods currently available to control estrous cycles of postpartum beef cows and replacement beef heifers. Development of methods to control the estrous cycle of the cow has occurred in six distinct phases. The physiological basis for estrus synchronization followed the discovery that progesterone inhibited preovulatory follicular maturation and ovulation. Regulation of estrous cycles was believed to be associated with control of the corpus luteum, the life span and secretory activity of which are regulated by trophic and lytic mechanisms. Phase I (Progesterone Phase) included efforts to prolong the luteal phase of the estrous cycle or to establish an artificial luteal phase by administering exogenous progestins. Later, progestational agents were combined with estrogens or gonadotropins in Phase II (Progesterone-Estrogen Phase), whereas Phase III (PG Phase) involved prostaglandin F_2α (PG) and its analogs as luteolytic agents. Treatments that combined progestational agents with PG characterized Phase IV (Progestogen-PG Phase). Precise monitoring of ovarian follicles and corpora lutea over time by transrectal ultrasonography expanded our understanding of the bovine estrous cycle and particularly the change that occurs during a follicular wave. We now know (Phase V, GnRH-PG Phase) that precise control of estrous cycles requires the manipulation of both follicular waves and luteal lifespan. This review includes specific discussion of progestins, PG, GnRH, and various combinations of these hormones or their analogs used to more precisely control the interval and timing of estrus following treatment (Phase VI, Progestogen-GnRH-PG Phase). The review also addresses the potential benefits of these treatments in eliciting a response from peripubertal heifers and anestrous cows, and points to the flexibility in matching specific protocols with the particular beef management system involved. Recent advances in the development of methods of artificially inseminating beef cows and heifers at a fixed time with high fertility are discussed, which should potentially result in a dramatic increase in the adoption of AI in beef herds.

Implications

New methods of inducing and synchronizing estrus for postpartum beef cows and replacement beef heifers in which the gonadotropin-releasing hormone-prostaglandin F_2α protocol is preceded by a progestin offer significant potential to more effectively synchronize estrus with resulting high fertility. These new protocols may provide significant opportunity to enhance results from fixed-time artificial insemination and offer the beef cattle industry the prospect of expanding the use of artificial insemination.

Key Words: Artificial Insemination, Beef Cattle, Estrous Cycle, Synchronization

© 2003, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2003. 81(E. Suppl. 2):E166-E177