Ovulatory response to GnRH agonist during early and late fall in mares.

Abstract: This study aimed to assess the ovulatory response of deslorelin acetate during the fall and the response to PGF2α 8 d post-ovulation. One hundred estrous cycles from 22 mares kept in 40° latitude were evaluated. Mares were checked by transrectal ultrasonography until a preovulatory follicle was detected and ovulation induced with deslorelin acetate. Ovulation was confirmed by ultrasonography performed at 24, 36 h post-induction and then repeated at 2-h intervals post-induction. Serum progesterone concentrations and luteal tissue area were determined daily to assess CL function. A dose of PGF2α was administered 8 d post-ovulation and interval to the subsequent ovulation was observed; each mare completed up to five cycles. The effects of local climate on endpoints were analyzed. Cycles were grouped as early (Sept 13, 2020-Oct 31, 2020; n = 55; 22 mares) and late fall (Nov 1, 2020-Dec 31, 2020; n = 45; 20 mares) based on the date of induction. The overall number of cycles with ovulations between 24 and 48 h was 90%. The number of multiple ovulations were similar between early (n = 5) and late (n = 4) fall (P = 0.87). There were no differences in deemed spontaneous ovulations occurring before 24 h between early (n = 6) and late (n = 2) fall (P = 0.29). Two failures to respond to deslorelin by 48 h were recorded in early fall and none in the late fall. The interval from induction to ovulation was similar in early (40.6 ± 0.4 h) and late (41.2 ± 0.5 h) fall (P = 0.55). The percentage of mares ovulating between 36 and 48 h post-deslorelin did not vary between early and late fall (91 vs. 95%, P = 0.21), as did not for ovulation occurring between 38 h and 44 h (62 vs. 60%, P = 0.69). Edema scores varied with time relative to ovulation (P < 0.001) and were lower in late fall (P = 0.01). Progesterone concentrations varied with time (P < 0.001) but did not differ between early and late fall (P = 0.73) and correlated weakly with the luteal area (r = 0.13; P = 0.031). Follicles <35 mm at the PGF2α had a shorter interval to the next ovulation than follicles ≥ 35 mm (9.2 ± 0.5 d vs. 10.6 ± 1.2 d) (P = 0.03). Lower temperature was associated with a smaller follicle size at induction (P = 0.0021) and ovulation (P = 0.009) and lower relative humidity was associated with a larger follicle size at ovulation (P = 0.032). In conclusion, cycling mares displayed a highly efficacious response to deslorelin acetate and apparently normal luteal function during the fall, despite lower edema scores in late fall.