Synchronization of ovulation in beef cows (Bos indicus) using GnRH, PGF2 alpha and estradiol benzoate

The objective of this study was to evaluate protocols for synchronizing ovu lation in beef cattle. In Experiment 1, Nelore cows (Bos indicus) at random stages of the estrous cycle were assigned to 1 of the following treatments : Group GP controls (nonlactating, n=7) received GnRH agonist (Day 0) and P GF2 alpha (Day 7); while Groups GPG (nonlactating, n=8) and GPG-L (lactatin g, n=9) cows were given GnRH (Day 0), PGF2a (Day 7) and GnRH again (Day 8, 30 h after PGF2 alpha). A new follicular wave was observed 1.79+/-0.34 d af ter GnRH in 19/24 cows. After PGF2a, ovulation occurred in 19/24 cows (6/7 GP, 6/8 GPG, 7/9 GPG-L). Most cows (83.3%) exhibited a dominant follicle ju st before PGF2a, and 17/19 ovulatory follicles were from a new follicular w ave. There was a more precise synchrony of ovulation (within 12 h) in cows that received a second dose of GnRH (GPG and GPG-L) than controls (GP, ovul ation within 48 h; P<0.01). In Experiment 2, lactating Nelore cows with a v isible corpus luteum (CL) by ultrasonography were allocated to 2 treatments : Group GPE (n=10) received GnRH agonist (Day 0), PGF2a (Day 7) and estradi ol benzoate (EB; Day 8, 24 h after PGF2 alpha); while Group EPE (n=11), rec eived EB (Day 0), PGF2a (Day 9) and EB (Day 10, 24 h after PGF2a). Emergenc e of a new follicular wave was observed 1.6+/-0.31 d after GnRH (Group GPE) . After EB injection (Day 8) ovulation was observed at 45.38+/-2.03 h in 7/ 10 cows within 12 h. In Group EPE the emergence of a new follicular wave wa s observed later (4.36+/-0.31 d) than in Group GEP (1.6+/-0.31 d; P<0.001). After the second EB injection (Day 10) ovulation was observed at 44.16+/-2 .21 h within 12 (7/11 cows) or 18 h (8/11 cows). All 3 treatments were effe ctive in synchronizing ovulation in beef cows. However, GPE and, particular ly EPE treatments offer a promising alternative to the GPG protocol in time d artificial insemination of beef cattle, due to the low cost of EB compare d with GnRH agonists. (C) 2000 by Elsevier Science Inc.