PSIV-16 - Factors affecting the LH release and ovulatory response to a GnRH treatment in lactating dairy cows on Day 7 of the estrous cycle

Abstract: This study evaluated factors affecting LH release and ovulation in response to a treatment with a conventional dose of gonadorelin (GnRH), in lactating dairy cows on Day 7 of the estrous cycle. Factors evaluated were: GnRH preparation (ready-to-use vs. lyophilized), body condition score (BCS), parity, days in milk (DIM), milk yield and dominant follicle (DF) diameter, at the time of GnRH treatment. Seven d after an artificial insemination, preceded by a synchronization protocol, lactating Holstein cows were evaluated by ultrasound and those with a corpus luteum (CL) and a DF ≥10 mm were randomly assigned to receive 100 µg GnRH ready-to-use solution (Fertagyl; MSD, Brazil) or lyophilized (Profertil; J.A. Animal Health, Brazil) diluted just before its administration. Overall, 106 cows (BCS = 3.1±0.1; DIM = 96.7±4.2; 39.7±0.7 kg of milk/d) were assigned to GnRH treatments. Ovulation was determined by ultrasound examinations 2 d after GnRH treatment, by disappearance of the DF, and confirmed 5 d later by the presence of an accessory CL. In a subset of cows (n = 33), blood samples were collected just before the GnRH treatment, and 2 and 4 h later, to evaluate circulating P4 and LH concentrations. Data were analyzed by GLIMMIX (SAS 9.4). Treatment effect and its interaction were included in the analyses of all other factors. Differences were declared when P≤0.05 and tendency when 0.05<P≤0.10. Mean P4 concentration at the time of GnRH treatment (2.1±0.1 ng/mL) was not affected by any factor evaluated, except for a tendency between multiparous and primiparous (2.3±0.2 vs. 1.9±0.2 ng/mL). There was no difference on LH peak (1.0±0.2 vs. 1.2±0.2 ng/mL) or ovulation (66.0% [35/53] vs. 69.8% [37/53]), for Fertagyl and Profertil, respectively. However, an interaction was observed between treatment and parity. Primiparous had greater ovulation when treated with Profertil compared to Fertagyl (95.2 [20/21] vs. 65.0% [13/20]), whereas no difference was detected in multiparous (53.1 [17/32] vs. 66.7% [22/33]). Nevertheless, neither parity nor interaction effects were observed on LH peak. Regardless of GnRH treatment, the LH peak was affected by BCS (1-5 scale), as cows with BCS ≤3.0 had greater LH peak than cows with BCS >3.0 (1.4±0.2 vs 0.7±0.1 ng/mL). Moreover, cows with DF ≤12 mm had greater LH peak than cows with DF >12 mm (1.3±0.2 vs. 0.9±0.1 ng/mL). However, these factors did not affect ovulation. Neither milk yield nor DIM affected LH peak or ovulation. In conclusion, although no differences were detected on LH peak, primiparous cows on Day 7 of the estrous cycle had greater ovulation when treated with Profertil compared to Fertagyl. Moreover, regardless of treatment, greater BCS and larger DF ( >12 mm) at the time of GnRH treatment were associated with lower LH peak, however, without impairing ovulation.