GnRH34 com ou sem cipionato de estradiol na IATF de vacas Bos indicus

Abstract

The synchrony between ovulation artificial insemination (AI) is one of the main factors to improve pregnancy rates in hormonal protocols for the use of fixed-time artificial insemination (FTAI). Aiming to improve this synchrony, two experiments were conducted to evaluate the effects of GnRH treatment on the fertility of postpartum Nelore cows treated with a protocol based on estradiol/progesterone (E2/P4) for FTAI. Experiment 1 was carried out to compare the effect of GnRH administered 34 h after progesterone alone (GnRH34); and associated with estradiol cypionate (CE-GnRH34) at the time of ovulation, in the production of progesterone and vascularization of the corpus luteum. Postpartum Nelore cows (n = 26) were treated with 2 mg of estradiol benzoate (EB) and IPD containing 1 g of P4. After 8 days, the IPD were removed, and all cows were treated with 150 mg of d-cloprostenol (prostaglandin F2 alpha analogue) and 300 IU of equine chorionic gonadotropin (eCG), in addition to having the base of the tail painted for estrus detection. At this time, they were randomly distributed to receive: 1) 0.9% saline solution i.m. (GnRH34 group, n = 13) or 2) 0.6 mg i.m. of EC (EC-GnRH34 group, n =13). On day 9 (17:00), all cows received a GnRH analogue (10.5 mg of buserelin acetate) i.m. The status of the dominant follicle was monitored by transrectal ultrasonography at 12-hour intervals, from the removal of the IPD until ovulation. The time of ovulation and the proportion of cows that ovulated were similar (P > 0.05) between the groups, and there were no differences regarding CL characteristics and P4 production. Experiment 2 was conducted to evaluate the efficacy of the GnRH34 and EC-GnRH34 protocols on the fertility of postpartum Nelore cows. In addition, these treatments were compared with the EC-GnRH48 protocol, in which the cows received EC at the removal of the IPD, while those that did not show estrus, received GnRH at FTAI. The cows (n = 981) underwent the same protocol described in Experiment 1. Thus, in this experiment, the groups consisted of GnRH34 (n = 322), EC-GnRH34 (n = 335) and EC-GnRH48 (n = 324). A higher (P < 0.05) rate of estrus expression was observed in cows treated with EC after removal of the IPD (EC-GnRH34: 69%, EC-GnRH48: 64.8%) than in cows in the GnRH34 group (45.6%). The P/AI in the EC-GnRH34 group (64.2%; 215/335) tended (P = 0.1) to be higher than in the GnRH34 group (58%; 187/322), this trend was not observed between these groups and the EC-GnR48 (59.9%; 194/324). Although no differences in ovulation synchrony were detected between groups, P/AI in cows treated with CE and GnRH 34 h after IVD removal tended to be higher than in cows treated with GnRH alone34. Therefore, given that P/AI did not differ between the CE-GnRH34 and CE-GnRH48 groups, our results suggest that the most cost-effective protocol consists of administering CE at the time of IVD removal followed by GnRH treatment at the time IATF in cows that did not show estrus.