Factors affecting pregnancy rates in acyclic recipient mares prepared with an intravaginal progesterone device

In acyclic embryo recipient mares, artificial cycle protocols are conventionally based on estradiol followed by long acting (LA) progesterone injections. The use of intravaginal devices has been encouraged because it allows a rapid resynchronization of recipients (Rigoleto-Júnior et al., Journal of Equine Veterinary Science 2021; 97:103325) and days of estradiol injections have also been discussed (Silva et al., Journal of Equine Veterinary Science 2021; 103:103669), but further studies are needed to develop more appropriate protocols. The present work aimed to evaluate factors that affect fertility in a commercial ET program. A total of 114 healthy mares (5-15yr-old) were used as embryo recipients and divided into two groups according to ovarian activity: cyclic (n=57) and acyclic (n=57, anestrus or transitional). Day-0 was defined as the day of ovulation in the cyclic group and the day of a 1g intravaginal P4 device (iP4) insertion in the acyclic group. Before iP4 insertion, 17β-estradiol doses varied from 0 to 10mg daily depending on the presence of natural or induced uterine edema that ranged from 3 to 7 days in duration. A one-by-one Pearson's Chi-square test was used to compare pregnancy rates between groups and the effects on pregnancy rate of 1) number of days of 17β-estradiol injections, 2) uterine edema (0-4 score)on D0 in the acyclic group, and 3) day of ET in both groups. Plasma P4 concentrations were evaluated daily by RIA in acyclic (n=10) and cyclic (n=13) mares from D0 to D7. Repeated measure models followed by Tukey's test were used to compare mean plasma P4 within groups. Pregnancy rates did not differ between acyclic (82%, 47/57) and cyclic (75%, 43/57) mares (P=0.5). Uterine edema on Day 0 (P=0.7) and day of ET (P=0.05) did not affect pregnancy rates of acyclic mares. Three days of estradiol treatment in acyclic mares resulted in a lower pregnancy rate (P=0.0425) than four days of injections. For cyclic recipient mares, day of ET affected pregnancy rates (P=0.0001) where D4 (96%, 26/27) showed higher pregnancy rates than D5 (66.7%, 12/18) and D6-7 (41.7%, 5/12), but D5 and D6-7 did not differ. Mean plasma P4 concentrations varied over days for both groups (P>0.05). In cyclic mares, mean plasma P4 increased daily to D5 (plasma P4 = 10ng/mL) followed by a plateau, whereas in the acyclic group, iP4 insertion caused an abrupt rise in plasma P4 concentration within 24h (plasma P4=5.67mg/mL) followed by a gradual daily fall to D7 (plasma P4=2