63 EVALUATION OF HYPOTHALAMIC–PITUITARY RESPONSIVENESS DURING THE POSTPARTUM NELLORE COWS

Abstract

The presence of calf, body condition score, energy balance, number of births (multiparous vs primiparous) and breed are factors that influence the duration of postpartum anoestrus in beef cows. The objective of the present study was to evaluate, during early postpartum, the time of re-establishment of pituitary LH stocks, measured by the hypothalamic-pituitary-axis responsiveness to exogenous administration of gonadotropin-releasing hormone (GnRH) or oestradiol benzoate (EB). Multiparous lactating Nellore cows (Bos indicus, n = 65) were randomly allocated into 6 groups, according to the hormonal treatment: EB group (1.0 mg of EB, IM, n = 7), GnRH group (50 μg of lecireline, IM, n = 16). The EB-supplemented (SUP; n = 9) and GnRH-SUP (n = 16) groups received the same treatments specified above and were SUP with a balanced diet, based on cotton meal and ground corn (3.5 kg cow^–1 per day). Additionally, animals from EB-calf removed (CR; n = 4) and GnRH-CR (n = 13) groups received the same treatments of EB and GnRH group, respectively and had their CR shortly after parturition. 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 corpus luteum during ovarian ultrasonography performed weekly. Blood samples were collected just before and 2 h (GnRH groups) or 18 h (EB groups) after hormone administration, in order to determine LH concentration by radioimmunoassay. Data were analysed by ANOVA. Mean values in days (± standard error of the means) for the first postpartum LH surge were EB (73.0 ± 5.2); EB-CR (16.7 ± 5.8); EB-SUP (41.7 ± 6.7); GnRH (32.3 ± 3.0); GnRH-CR (11.0 ± 3.5); GnRH-SUP (15.6 ± 2.8). There were significant differences (P < 0.05) between groups EB vs EB-CR; EB vs EB-SUP; BE vs GnRH; GnRH vs GnRH-CR; GnRH vs GnRH-SUP and a tendency between EB-SUP vs EB-CR (P < 0.10). 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 the EB group ovulated later than animals from the other groups, possibly due to the sensitivity of the hypothalamus to negative feedback of estrogens, inhibiting the pre-ovulatory LH surge. Additionally, CR and food SUP reduced in 2 to 4 weeks the time of the first postpartum LH surge induced by GnRH or EB in Nellore cows.

A fellowship was received from FAPESP. Support was received from FAPESP.