Induction of abortion in mares with equimate: effect on secretion of progesterone, PMSG and reproductive performance.

Thirty-two light-horse mares were confirmed to be pregnant and assigned to one of four treatments: (1) injected with 250 micrograms of Equimate on day 70 and again on day 77 if abortion had not occurred; (2) injected with 250 micrograms of Equimate on day 70 and every 24 hr until abortion occurred (maximum four injections); (3) injected with 250 micrograms of Equimate on day 70 and every 12 hr until abortion (maximum eight injections); and (4) injected with 250 micrograms of Equimate once only on day 35 of gestation. Mares were observed four times daily for incidence of abortion or side effects. Estrual behavior was monitored daily and follicular activity either daily or every third or fourth day until estrus and ovulation. Samples of jugular blood were obtained at 0600 hr day -1 and every 6 hr until the first day of estrus after abortion, or for 2 weeks after abortion if estrus had not occurred, or for 1 week after treatment for mares that had not aborted. A single injection of Equimate terminated pregnancy in all but one mare injected on day 35, but none of the mares given an injection on day 70 and 77 aborted. Multiple injections of Equimate beginning on day 70 of gestation terminated pregnancy in all mares. Fewer (P less than .05) injections were required for abortion of mares injected daily. During the immediate week post-treatment, concentrations of progesterone decreased (P less than .05) in all mares injected with Equimate, but overall concentrations were greater (P less than .05) for mares injected once on day 70 than for those in the other three groups. Equimate did not affect secretion of PMSG in the day 70 group of mares. Estrus and ovulation after abortion were delayed (P less than 0.05) for mares injected daily or twice daily beginning on day 70 compared to those injected once on day 35. Thus, rebreeding of these mares during the same breeding season in which they were aborted would be difficult.