REGULATION OF FOLLICULAR ACTIVITY AND OVULATION IN EWES BY EXOGENOUS PROGESTAGEN

The success of estrus synchronization programs using progestagen spong es, particularly for fixed-time AI, varies considerably. In view of th e recent evidence in cattle that exogenous progestins alter follicular dynamics, it may be that the stage of the estrous cycle at which the synchronization protocol is begun affects the synchrony of ovulation. The goal of this study was to evaluate the effect of medroxyprogestero ne acetate (MAP) intravaginal sponges on follicular dynamics, luteal f unction and interval to ovulation when inserted at 3 stages of the est rous cycle. Sponges were inserted for 12 d beginning on either Day 0, 6.or 12 (n=5) following ovulation. Ovarian activity was monitored usin g real-time ultrasound imaging during the treatment and the post-treat ment estrous cycles. Information from the post-treatment cycle was use d as a baseline to compare with the treatment cycle. Most ewes (79%) i n the post-treatment cycle exhibited 3 follicular waves in an estrous cycle of 16 d, with the second wave follicles having smaller diameter (P<0.001). Treatment with MAP increased the number of follicular waves from 3 to 4 or 5 when sponges were inserted on Days 6 and 12, respect ively. Size of the largest follicle was smaller (P>0.01) in waves in t he early and middle of the 12-d MAP treatment period when compared wit h the last 4 days. This effect was most pronounced when endogenous pro gesterone concentrations were elevated concurrently with the presence of the sponge. Persistence of the ovulatory follicle was increased (P< 0.001) when sponges were inserted on Day 12, the only treatment where these follicles were under the influence of MAP in the absence of func tional corpora lutea. Follicles were regressing at sponge removal in t he Day 6 treatment, which resulted in a delay in emergence of ovulator y follicles, the LH surge and ovulation (P<0.08) in relation to Day 0 and Day 12. Treatment with MAP sponges does not adequately synchronize estrus and ovulation among cyclic ewes due to the different follicula r patterns that result depending on the stage of cycle at the time of sponge insertion. (C) 1998 by Elsevier Science Inc.