Relationship between the duration of endometrial edema and reproductive parameters of estrus, diestrus, and fertility in mares: a field study

• Most estrous cycles in mares were characterized as long rather than short • Days of uterine edema was associated with estrus and diestrus reproductive parameters • Higher duration of endometrial edema was observed in pregnant mares • Longer estrous cycles were associated with higher fertility in donors and broodmares The duration of endometrial edema has been positively correlated with uterine receptivity and likelihood of pregnancy. Therefore, this study aimed to evaluate whether there is a relationship between the duration of endometrial edema (DEdema) and other key parameters of the estrous cycle: maximum edema (MEdema), largest follicle diameter (LPF), corpus luteum and uterine tone on day 5 post ovulation (CLD5 and TD5), progesterone concentration (P4) and fertility. A total of 88 cycles from 36 mares were evaluated and divided into Short Estrus (SE), Long Estrus (LE), Inseminated (AI) and Non-Inseminated (N-AI) groups. In addition, AI group was divided into pregnant (P) and non-pregnant (N) mares. From the detection of a dominant follicle and uterine edema ≥1.5, mares were monitored daily until ovulation (D0). Data on dominant follicle diameter and endometrial edema were recorded. On D5, CL size and uterine tone were assessed and blood was collected for P4 measurement. Of all cycles, 30.7% (27/88) were SE and 69.3% (61/88) LE. Correlations were observed between DEdema and the variables: LPF ( P = 0.001; r = 0.32), MEdema ( P = 0.002; r = 0.31), and TD5 ( P = 0.01; r = 0.28). Furthermore, DEdema was associated with a higher probability of a positive pregnancy outcome (OR= 1.45; P = 0.049) and pregnancy rates were higher ( P =0.006) in LE group (67.5%) than SE (16.6%). In conclusion, the duration of endometrial edema was positively associated with key estrous cycle parameters, as well as fertility.