Influence of follicular size, plasma progesterone, and estradiol benzoate administration on endometrial edema onset and duration in mares.

Endometrial edema is an ultrasonographic sign of estrus in mares and is positively associated with fertility. This study aimed to evaluate the influence of follicular size, systemic progesterone (P4), and exogenous estradiol benzoate (EB) on the onset and duration of endometrial edema. In Experiment 1, anestrus (n = 4) and diestrus mares (n = 4) received 4 mg of EB intramuscularly; edema scores and plasma P4 were monitored over four days. In a complementary group (n = 5), mares were monitored until ovulation, then administered 4 mg of EB and examined every 12 h for 48 h. In Experiment 2, mares (n = 9) received PGF2α and were categorized based on follicle size (<25 mm or >25 mm) to assess effects on edema onset. In Experiment 3, mares (n = 13) received PGF2α alone or PGF2α plus EB to evaluate combined treatment effects. Anestrus mares (P4 <1 ng/mL) showed maximal edema at 24 h post-EB, resolving by day 4. Diestrus mares did not develop edema. In the OV group, edema peaked at 24 h post-EB but declined significantly by 36 h (p < 0.05), concurrent with a P4 increase from 2.2 ± 0.3 to 3.6 ± 0.6 ng/mL (p < 0.01). Follicle size tended to influence the interval to ovulation (9.4 ± 2.2 vs. 6.0 ± 2.1 days, p = 0.052) and significantly affected the total number of edema days (7 vs. 4.5, p < 0.05). Edema developed despite the presence of only small follicles, and EB at PGF2α did not accelerate onset. Overall, edema dynamics appear to be progesterone-driven, and follicle size may affect estrus duration and time to OV.