INDUÇÃO DE DUPLA OVULAÇÃO EM FÊMEAS NELORE x ANGUS (BOS INDICUS x BOS TAURUS).

Abstract

Birth and weaning rates are recognized as the main indicators for measuring reproductive efficiency in beef cattle. The number of calves born can be increased by protocols for inducing twin pregnancies. To verify the effect of equine chorionic gonadotropin (eCG) on induction of double ovulation, 78 multiparous Nelore x Angus (Bos indicus x Bos taurus) cow were submitted to E2+P4-based ovulation synchronization protocol, with different doses of eCG (300 vs 1,000 vs 1,200 IU) administered on different days of the protocol (Progesterone device for 8 days + eCG at D6 vs Progesterone device for 9 days + eCG at D7). There was no effect of the eCG dose on follicular diameter nor in ovulation time of the dominant and subordinate follicles. The number of ovulations was higher in 1,000 and 1,200 IU treatments in cows treated with eCG at the D6 of the protocol (group CIDR 8), whereas in those receiving eCG at D7 there was no differences in the number of ovulations. The highest percentage of double and multiple ovulations was found in treatments eCG 1,000 and eCG 1,200 respectively in the CIDR 8 protocol. Both in the CIDR 8 and in the CIDR 9 protocols there was a significant effect of the eCG dose on the number of fetuses at 40 days after TAI. Pregnancy rates declined in the same proportion as the dose increases in the eCG 300 [76.9% (20/26)], eCG 1,000 [57.7% (15/26)] and eCG 1,200 [38.5% (10 / 26)] treatments, respectively; P = 0.02).. Finally, we conclude that the use of equine chorionic gonadotrophin (eCG) at doses of 1,000 and 1,200 IU increases double ovulations but reduces pregnancy rates and increases the multiple ovulations in Bos taurus x Bos indicus cows submitted to TAI Therefore, it is not recommended as a method to increase the efficiency of beef cattle production.