Effect of an Oral Progestin on the Estrous Cycle and Fertility of Mares1

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Abstract
On February 4,14 of 28 seasonally anestrous mares were fed .044 mg per kilogram of body weight of an oral progestin, allyl trenbolone (17α-allyl-estratriene 4-9-11, 17β-ol-3-one) for 12 days. There was no difference (P>.05) between groups for the number of mares exhibiting estrus after treatment or follicular activity during or after treatment. After March 6, 18 mares that had not yet ovulated were used in a second trial. On day 3 of estrus, nine of 18 mares were assigned to be fed allyl trenbolone for 12 days. Estrus ceased in all mares within 3 days of treatment. Duration of post-treatment estrus, interval from end of treatment to ovulation and from estrus to ovulation were shorter (P<.01) for treated mares. Influence of stage of cycle at the onset of treatment was evaluated in a third trial. Mares (N = 25) were assigned to one of five groups: 1) controls artificially inseminated every other day (E/O) during estrus; 2) fed allyl trenbolone for 12 days beginning day 3 of estrus and bred E/O; and groups 3 to 5) fed allyl trenbolone for 12 days beginning day 3 of estrus or days 5 or 10 of diestrus. Mares in groups 3, 4 and 5 were given HCG (3,300 IU) on day 17 and inseminated on days 17 and 19. The interval from treatment to estrus was shorter (P<.05) for mares treated during diestrus vs those treated during estrus. Pregnancy rates were not different (P>.10) among groups after two cycles or between treated and controls after 1 cycle (60 and 60%) or 2 cycles (80 and 60%). In another study, 34 mares were exposed to 16 hr photoperiod. On January 25, 17 of the 34 mares were fed allyl trenbolone for 12 days. One-half of the mares in each group received 3,300 IU of HCG on day 2 of estrus. More (P<.05) treated mares exhibited estrus and ovulated within 12 days after treatment than controls. Pregnancy rates after 1 and 3 cycles were similar (P>.05). Progestin treatment effectively regulated estrual behavior early in the year, and synchronized estrus in cycling mares and mares previously exposed to artificial light. Copyright © 1979. American Society of Animal Science . Copyright 1979 by American Society of Animal Science.