STUDIES OF FSH-P INDUCED FOLLICULAR-GROWTH IN COWS

Because cow ovaries do not contain a dominant follicle before Day 3 of the estrous cycle, we hypothesized that gonadotropin treatment early in the estrous cycle would induce growth of multiple follicles and cou ld be used to induce superovulation. In Experiment 1, when 16 cows wer e treated with FSH-P beginning on Day 2 of the estrous cycle and were slaughtered on Day 5, all cows responded to gonadotropin treatment by exhibiting a large number (approximate to 19) of estrogen-active folli cles greater than or equal to 6 mm. In Experiment 2, in response to FS H-P treatment from Day 2 to Day 7, and fenprostalene treatment on Day 6, 11 of 15 cows exhibited estrus and had a mean ovulation rate of 23. 7 +/- 1.5. In Experiment 3, an FSH-P treatment regimen identical to th at used in Experiment 2 was administered to cows beginning either on D ay 2 (Day-2 cows; n=14) or Day 10 (Day-10 cows; n=11) of the estrous c ycle. Twelve of 14 Day-2 cows and all Day-10 cows exhibited estrus aft er fenprostalene treatment. Day-2 cows exhibited 34.3 +/- 7.0 ovulatio ns, which was less (P < 0.05) than that exhibited by Day-10 cows (48.3 +/- 4.4). However, the proportion of embryos recovered per corpus lut eum was about 2-fold greater (P < 0.05) for Day-2 cows than for Day-10 cows (0.49 +/- 0.08 vs 0.27 +/- 0.06). These data indicate that begin ning gonadotropin treatment early in the estrous cycle, when a dominan t follicle is not present, provides an efficacious means to induce gro wth of multiple follicles and superovulation in cows. However, when FS H was administered for 6 d, beginning the treatment on Day 10 also res ulted in a consistent and efficacious response.