132 INDUCTION OF FIRST POSTPARTUM OVULATION AFTER ADMINISTRATION OF GnRH, ESTRADIOL BENZOATE, OR LH ASSOCIATED OR NOT WITH CALF REMOVAL IN NELORE COWS

Abstract

There are reports in the literature indicating that the presence of calf, body condition score, energy balance, number of births (multiparous v. primiparous), and breed are factors that influence the duration of postpartum anoestrus in beef cows. The purpose of the present study was to evaluate, during early postpartum, the time necessary for reestablishment of pituitary LH stocks, determined by the hypothalamic-pituitary axis responsiveness to exogenous administration of GnRH and estradiol benzoate (EB). Multiparous lactating Nelore cows (n = 51) were kept in Brachiaria brizantha pasture. The animals were randomly allocated in 8 groups, according to the hormonal treatment: EB Group (1.0 mg of EB, i.m. Estrogin^®, Farmavet, Sao Paulo, Brazil; n = 7), EB and calf removal (CR) group (EB/CR, calves were removed at the time of birth, n = 4), EB/SUP group (cows were supplemented with a balanced diet, based on cotton meal and ground corn, from the beginning of the experiment until ovulation), GnRH group (100 μg, licereline, i.m. Gestran Plus^®, ARSA SLR, Buenos Aires, Argentina; n = 7), GnRH/CR group (n = 4), GnRH/SUP group (n = 8), LH group (12.5 mg, i.m. Lutropin^® Bioniche, Ontario, Canada; n = 7), and control group (did not receive any hormone to induce ovulation or diet supplementation, n = 5).The hormones were administered weekly, from 7 days postpartum (±5 days) until the occurrence of the first ovulation, which was determined by the presence of CL during weekly ovarian ultrasonography (Aloka 900, Tokyo, Japan, 7.5 MHz probe). The data were analyzed by ANOVA. On average (±SEM) the first ovulation occurred 86.7 ± 0.5 (EB); 19.7 ± 0.6 (EB/CR); 57.8 ± 0.4 (EB/SUP); 44.7 ± 0.5 (GnRH); 25.9 ± 0.6 (GnRH/CR); 32.8 ± 0.4 (GnRB/SUP); 42.4 ± 0.5 (LH); and 59.3 ± 0.5 (control) days postpartum. There were significant differences between groups: EB v. EB/SUP (P < 0.01), EB v. EB/CR (P < 0.01), EB v. control (P < 0.04), EB v. GnRH (P = 0.01), and there was a tendency between groups: GnRH v. GnRH-SUP (P = 0.10) and LH v. control (P = 0.10). The first ovulations were detected from the second week postpartum (animals from Groups GnRH and EB/CR) until the last ultrasound examination on Day 120 postpartum (Group EB). The results indicate that from the second week postpartum there is sufficient LH in the pituitary to induce ovulation after GnRH or EB administration. However, the cows from EB group ovulated later than animals from the other groups, possibly due to the sensitivity of hypothalamus to negative feedback of estrogens, inhibiting the pre-ovulatory LH surge. Additionally, calf removal and food supplementation reduced in 2 to 4 weeks the time of the first postpartum ovulation induced by GnRH or EB.

Fellowship from FAPESP. Supported from FAPESP (Sao Paulo, Brazil).