S.K. Johnson*, J.E.
Huston**, D.E. Grum**, and M.L.
Day1**
*Kansas State University Department of Animal Sciences
**The Ohio State University Department of Animal Science
Abstract
The potential of adding gonadotropin-releasing hormone (GnRH) to an MGA-PGF2a based synchrony system to increase the precision of synchronized estrus and augment timed AI was investigated. Yearling heifers were fed melengestrol acetate (MGA) daily for 14 days. Nineteen days after the last feeding of MGA, all heifers were administered prostaglandin F2a (PGF2a) (day 0). Heifers receiving no further treatment served as the untreated controls. In the second treatment, heifers received an injection of GnRH on day _7. Both groups of heifers were artificially inseminated 12 hours after detection of estrus. Heifers in the third treatment received GnRH on day _7 and day 2 and were time bred at the time of the second GnRH injection. In comparison to a MGA-PGF2a synchronization system, addition of GnRH on day _7 did not increase the degree of synchrony of estrus. The addition of two injections of GnRH (day _7 and day 2) facilitated timed artificial insemination (AI); however, pregnancy rate for timed AI was lower than pregnancy rate when AI was performed 12 hours after detection of estrus.
Introduction
The MGA-PGF2a synchronization system (Brown et al., 1988) has been highly effective for facilitating the use of AI in heifers. A modification of the original program that has increased the precision of heat has been to increase the interval between the last feeding of MGA and the injection of PGF2a from 17 to 19 days. Various permutations of timed AI and/or double inseminations have been tried with the MGA-PGF2a system in order to circumvent or minimize the need to observe heifers for estrus and inseminate accordingly. However, these approaches present a relatively high degree of risk since no method to precisely control the timing of ovulation is included in this program. A preliminary report (Wood et al., 1999) with limited numbers of animals, suggested that addition of an injection of GnRH seven days before PGF2a in an MGA- PGF2a system would increase the precision of estrus and to the degree that would facilitate timed AI. The objectives of this study were to determine if addition of one injection of GnRH, one week before PGF2a in the MGA- PGF2 a system, would indeed increase the precision of estrus and furthermore to see if this approach might be used to develop a timed AI protocol in yearling heifers.