Product Development for Estrous Synchronization : Beef Cattle

Physiological principals underlying synchronization of estrus identified 1) hormones naturally controlling the estrous cycle of cattle, 2) hormones commercially available, 3) mode of action of the hormones commercially available, and 4) when/how they might be used to manage estrus and breeding of beef cattle. The products available are progestogens (melengestrol acetate (MGA) and progesterone delivered via CIDR), prostaglandin F2 alpha (PGF2α and PGF2α analogs), and gonadotropin releasing hormone (GnRH). Progesterone and progestogens block estrus. If delivered for sufficient time, removal of the progestogen block will allow cattle to return to estrus in a pre-determined synchronized interval. PGF2α and PGF2α analogs regress the corpus luteum (CL) when administered during d 6 through 18 of the estrus cycle, resulting in cattle returning to estrus on about d 2 through 6 post-injection. GnRH induces the release of an ovulatory surge of LH resulting in ovulation of a dominant follicle or initiation of a new follicular wave. Cattle estrous synchronization was perceived in the late 1950s and 1960s to meet an unmet need of beef cattle producers who desired to utilize artificial insemination (AI). During the 1950s, frozen bovine semen was developed and AI with progeny tested bulls became recognized as effective to make more rapid genetic progress for milk yield and beef production. In the 1950s and 1960s, a major detriment to AI in beef cattle was the requirement for daily estrus detection and AI over 60-90 d or more. Ulberg et al. (1951) reported progesterone injected daily in cattle would block estrus. Based on this information and understanding of the bovine estrous cycle in 1960, progesterone injected daily or a progesterone analog (progestogen) delivered daily for 18 d was the only potential means for cattle estrous synchronization. Progesterone is not active orally; and at that time, there was no identified delivery method for progesterone that was economically acceptable to the beef industry. PGF2α and GnRH were not known at the time.