Effect of GnRH administration on follicular development and ovulation during the early luteal phase in subtropical goats

This study aimed to evaluate the ovulatory response to GnRH treatment based on the day of its administration in the first follicular wave of the estrous cycle in goats. We hypothesized that maximum ovulatory response with GnRH treatment is dependent on the day of its administration during the early luteal phase of estrous cycle. Forty-eight goats were presynchronized with a single dose of PGF_2α, and ultrasonography was performed to confirm ovulation (Day 0). Following ovulation, goats (n = 38) were randomly assigned to receive GnRH treatment on Days 2, 4, 6, or 8 of the estrous cycle. Follicular dynamics were assessed every 4 h after GnRH treatment and 37% of goats ovulated within 12.0 ± 1.5 h after treatment. Ovulation rate was significantly higher on Day 4 compared to Day 2 (P = 0.03) and gradually declined by Day 8. Goats with dominant follicles with diameters between 6.1 and 7.0 mm had significantly higher ovulation rate than goats < 6.1 mm (P = 0.01). A strong negative correlation was found between dominant follicle diameter at the time of GnRH administration and anovulation (r_pb = -0.79). The growth rate of dominant follicles after GnRH administration remained higher in Day 4 than Day 8 (P < 0.05). Similarly, the interval to new follicular wave emergence was longer on Day 4 than on Days 2, 6 and 8 (P < 0.05). In conclusion, administering GnRH on Day 4 of the estrous cycle yielded the highest ovulatory response, particularly when the dominant follicle measured between 6.1-7.0 mm during the first follicular wave of estrous cycle.