Modification of follicular dynamics by exogenous FSH and progesterone, andthe induction of ovulation using hCG in postpartum beef cows

Follicular growth and ovulation in response to FSH, progesterone and hCG we re evaluated in postpartum beef cows. In Experiment 1, on Day 21 post partu m, cows received an injection of either saline (control; n=6), FSH (200 mg; n=6), or a PRID (n=5) for 10 d. Both FSH and PRID prolonged maintenance of a dominant follicle (15.5+/-1.16 and 14.4+/-1.29 d, respectively, vs 8.4+/ -1.22 d in control; P<0.01), and increased the maximum diameter of the domi nant follicle (14.0+/-0.91 and 16.4+/-1.01 mm, respectively, vs 10.9+/-0.95 mm in control; P<0.05). The PRID-maintained dominant follicle ovulated in 60% of cows, followed by normal estrous cycles (vs 0% in control; P=0.01), whereas the dominant follicle ovulated in 33% of FSH-treated cows (P=0.08). The PRID regimen shortened the interval to first ovulation preceding a nor mal cycle and continued cyclicity (44+/-4.1 vs 60+/-4.4 d in control; P=0.0 2). In Experiment 2, on Day 21 post partum, cows received either saline (co ntrol), saline+/-PRID, or FSH+PRID (n=16/group). Sixty hours after PRID wit hdrawal, cows received either saline or hCG (1,500 IU, n=8/treatment). The FSH+PRID regimen increased the number of large (>10 mm in diameter) follicl es (3.6+/-0.43 vs 1.9+/-0.39 in control; P=0.005). Both PRID and FSH+PRID p rolonged maintenance of the largest follicle (11.0+/-0.82 and 11.2+/-0.91 d , respectively, vs 8.7+/-0.81 d in control; P<0.05). The PRID-maintained do minant follicle ovulated in 50% of cows, followed by normal estrous cycles. The FSH+PRID-maintained largest follicle had become atretic at PRID withdr awal and was anovulatory. The FSH+PRID+hCG regimen increased the incidence of ovulation preceding a cycle of normal duration and continued cyclicity ( 100 vs 50% in PRID; P=0.03), and reduced the interval to first ovulation pr eceding a cycle of normal duration and continued cyclicity (38+/-6.5 vs 58/-6.3 d in control; P=0.04). The area under the progesterone curve during t he induced cycle was reduced after (PRID+/-FSH)+hCG than after PRID+/-FSH ( P=0.002). These results indicate that PRID alone or with FSH/hCG has the po tential to modify the dominant follicle and initiate cyclicity in postpartu m beef cows. (C) 1999 by Elsevier Science Inc.