92 INDUCTION OF FIRST POSTPARTUM OVULATION, AFTER ADMINISTRATION OF GONADOTROPIN-RELEASING HORMONE OR ESTRADIOL BENZOATE, IN NELLORE COWS

Abstract

There are reports in the literature indicating that the presence of calf, body condition score, number of births (multiparous v. primiparous), and breed are factors that influence the duration of postpartum anestrus in beef cows. The purpose of the present study was to evaluate, during early postpartum, the time of the reestablishment of LH stocks, measured by the hypothalamic-pituitary axis responsiveness to exogenous administration of gonadotropin-releasing hormone (GnRH) or estradiol benzoate (EB). Primiparous lactating Nellore cows (n = 38, body condition score 2.5 to 3.5, on a 0 to 5 scale) were kept in a Brachiaria brizantha pasture. The animals were randomly allocated into 2 groups, according to hormone treatment: EB group (1 mg EB, i.m., Estrogin^®, Farmavet, Sao Paulo, Brazil; n = 20) and GnRH group (100 μg, licerelina, i.m, Gestran Plus^®, ARSA S.L.R., Buenos Aires, Argentina; n = 18). In each group, half of the animals were supplemented with a balanced diet based on cotton meal and ground corn. The drugs were administered from 7 days postpartum (±4 days), at intervals of 7 days, until the occurrence of the first ovulation that was observed by weekly ultrasonography (US, Aloka 900, Tokyo, Japan; 7.5-MHz transrectal probe). The data were analyzed by ANOVA (Proc GLM, SAS Institute, Cary, NC). Within the GnRH and EB groups, the first ovulation occurred 15 and 33 days postpartum, respectively, whereas the group averages (± SD) occurred earlier in cows treated with GnRH compared with those treated with EB (43.0 ± 3.8. v. 75.1 ± 8.7 days postpartum, respectively; P < 0.01). In the EB group, 4 animals did not ovulate until Day 140 postpartum. There was no interaction of treatment (GnRH, EB) × nutrition (supplemented, nonsupplemented). When data from the EB and GnRH groups were combined, there was a significant difference between supplemented and nonsupplemented animals (44.4 ± 6.2 v. 68.9 ± 7.3 days, respectively; P < 0.02). The results indicate that GnRH induces ovulation from Day 15 postpartum, suggesting the presence of sufficient LH in the pituitary to induce the first ovulation at this time. However, EB administration did not induce ovulation in this period, possibly because of the sensitivity of the hypothalamus to negative feedback of estrogens, inhibiting the preovulatory LH surge. These results also indicate that nutritional supplementation anticipates the first postpartum ovulation induced by GnRH or EB.

Support by FAPESP(Sao Paulo, Brazil); fellowship from FAPESP^A; fellowship from CAPES^B.