The main factor of reduced pregnancy rates in post foaling mares is time of embryo transfer after parturition

Abstract

Post foaling mares can be used as embryo transfer recipients. Higher pregnancy loss and lower pregnancy rates have been reported in recipient mares used at the foal heat versus open recipient mares. The aim of this study was to analyze the influence of post foaling day of transfer, post ovulation day of transfer, month of the year of embryo transfer, mare age and embryo quality over embryo loss and pregnancy rates in postpartum recipients. Retrospective data from three breeding seasons (2020-2022) were analyze at a breeding facility in Montemorelos N.L. Mexico (n=493 embryo transfers). Mares that did not foal the previous breeding season were considered open recipients (n = 383, 7.3 ± 3.4 years old) while post foaling mares were those that foaled in the same transfer season (n=110, 7.2 ± 3.2). Postpartum recipients were divided in two groups considering the post foaling time at transfer (<120 days and>120 days). Embryo donors were flushed on Day 7-9 post ovulation and recovered embryos transferred into recipients that were 3-8 days post ovulation. Ultrasonographic pregnancy diagnosis was undertaken at days 12, 25, 45 and 60. A nonparametric Chi-Square X2 test was performed using SPSS version 16.0 (SPSS, INC., Chicago, IL). Mares hada significant higherpregnancy rate (85.37%) than postpartum mares (67.27%) (P<0.0001). Although mares transferred >120 days post foaling had a pregnancy rate similar to open mares (80%), and higher than those transferred <120 days postpartum (66.6%), no significant differences were found between these two postpartum mares, probably due to the low number of mares on the >120 postpartum days group (25 mares). In contrast with previous reports, we have no differences in embryo losses (p>0.05) between open (9.78%) and postpartum mares (6.75%), and embryo losses were more related to embryo quality than mare status. Age, post ovulation day of transfer and month of the year of embryo transfer, had no influence on pregnancy rate in any of the groups (p>0.05). It is possible that the time from foaling to transfer is one of the main factors explaining reduced pregnancy rates in post-partum mares. This idea is substantiated by our data which show that similar pregnancy rates are obtained when transferring embryos into open or mares >120 days post-foaling. This study reveals an opportunity to optimize the use of post-foaling mares as recipients by considering the appropriate time to transfer embryos after foaling.