Influence of follicular size on the response of mares to allyl trenbolone given before the onset of the ovulatory season

The progestin, allyl trenbolone, was given orally to mares for 15 days before the beginning of the ovulatory season. At the onset of treatment, the largest follicle was <20 mm, 20-25 mm, or > 30 mm. The progestin did not hasten the onset of the ovulatory season, regardless of the diameter of largest follicle at the onset of treatment. The progestin did not alter the numbers or diameters of follicles when treatment was initiated when the largest follicle was <20 mm. However, initiation of treatment when the largest follicle was 20-25 mm or > 30 mm resulted in suppressed follicular growth. Concentrations of FSH were elevated in all three progestin-treated groups, although the increase was not significant in the group with a large follicle at the onset of treatment. The compound reduced the protracted period of estrus and follicular development which commonly precedes the first ovulation of the breeding season. The number of estrous determinations and inseminations was reduced without an apparent reduction in conception rate.