MGA and PGF₂α for Estrus Synchronization

Abstract

Our goal in studying estrous cycle control is to develop a low cost system that will elicit a highly fertile, tightly synchronized estrous response in a large percentage of the treated animals.

Melengestrol acetate (MGA), an inexpensive oral progestogen, is currently approved and used to suppress estrus and promote growth in feedlot heifers. Even though MGA effectively synchronizes estrus in cattle, the estrus following MGA treatment is subfertile while the fertility of subsequent estrus periods is normal (Zimbelman et al., 1966; DeBois et al., 1970). This decreased fertility has limited the use of MGA for estrous synchronization.

It is well known that prostaglandin F₂α (PGF₂α) is only effective for estrous synchronization in cows between days 5 and 15 (the luteal phase) of their estrous cycle. Recent research has shown that stage of cycle within this luteal phase further affects both estrous response and the interval to estrus following PGF2a injection, with early cycle animals (days 5-9) having a lower percentage response and showing heat sooner (a shorter interval to estrus) than late cycle animals (King et al., I 982). Another study shows that PGF₂α injected during the late stage of the cycle (days 10-15) may result in a more fertile estrus than if it were injected in the early stage of the cycle (Bearden and Fuquay, 1984). Thus, by manipulating a group of heifers to increase the percentage in the late stage of their estrous cycle when PGF₂α is injected, estrous response and degree of synchrony, as well as fertility would theoretically be improved.