121 Ovulatory response to gonadotropin-releasing hormone relative to day and diameter of dominant follicle during the first follicular wave in Beetal goats

Abstract

In Beetal goats under subtropical conditions, ovulatory response of first-wave dominant follicle based on diameter and day of oestrous cycle is unknown to date. The objective of this study was to evaluate the ovulatory response to gonadotropin-releasing hormone (GnRH) relative to day and diameter of dominant follicle during the first follicular wave in Beetal goats. Fifty goats were synchronized using a single dose of prostaglandin F_2α (75 µg; D+ cloprostenol, Fatro, Italy), and scanned every 6 hours for ovulation using a 7.5-MHz trans-rectal transducer (Honda HS: 1500, Japan). Following ovulation (Day 0), goats (n = 40; 10/day) were given GnRH (25 µg lecirelin, Fatro, Italy) on Days 2, 4, 6, and 8 of oestrous cycle. Follicular dynamics was monitored daily from D 0 until GnRH, and every 4 hours after GnRH until ovulation. Ovulation frequency relative to day and dominant follicular diameter was analysed by binary logistic regression, whereas diameter of dominant follicle at time of GnRH, ovulation time, time of wave emergence and growth rate of dominant follicle following GnRH were analysed by one-way ANOVA and independent sample t-test (SPSS 20, IBM Corp., Armonk, NY, USA). Eighty percent of the goats ovulated within 74.7 ± 1.3 h after prostaglandin F_2α. Overall, 37% (14/38) of the goats ovulated following GnRH within 12.0 ± 1.5 h. Ovulations (%) following GnRH were higher (P = 0.03) on Day 4 than 2 (60 v. 10%; i.e. 14 times higher than Day 2) and decreased gradually from Day 4 to 8 (P > 0.05). Similarly, ovulations following GnRH were higher (P = 0.00) in first-wave dominant follicles having diameter 6.1-7.0 mm than those having less than 6.1 mm (88 v. 23%; i.e. 24 times higher than <6.1 mm). Ovulations (%) decreased as the diameter of dominant follicle increased from 6.1-7.0 mm (P > 0.05). Diameters of ovulatory follicles at time of GnRH between Day 2, 4, 6, and 8 were not different (P > 0.05). Regardless of day of GnRH treatment, diameter of ovulatory v. anovulatory follicles differed (6.44 ± 0.03 v. 5.1 ± 0.4 mm; P = 0.00). Overall, growth rate of dominant follicles following GnRH was 0.2 ± 0.11 mm and did not differ between ovulatory and anovulatory goats (0.29 ± 0.13 v. 0.13 ± 0.15 mm; P > 0.05). Interval to the emergence of a new wave following GnRH was prolonged for Day 4 compared to Day 2, 6, and 8 (i.e. 22 v. 8, 11, and 12 h; respectively, P < 0.05). In conclusion, GnRH treatment at Day 4 resulted in maximum ovulations as the first-wave dominant follicle reached a size of 6.1 to 7.0 mm.