Effect of season on pregnancy rates after embryo transfer in cyclic, synchronized and selected horse recipients

o maximize the reproductive potential of donor mares, horse owners often request embryo transfer (ET) out of the regular breeding season. At the latitude of this study (43° 43’ Lat. North), mares become cyclic around April and stop between October and December. We retrospectively analyzed 5 years of results from our commercial ET service. Pregnancy rates after ET were compared from the 15th of February to the 15th of April, from the 16th of April to the 30th of September, and from the 1st of October to the end of November, respectively. Day 8 embryos of excellent or good quality (grades 1 or 2) (n=252), which were fresh (n=80) or transported cooled for 3-6 hours in a Styrofoam box at 5°C (n=172), were transferred in 2-16 year old, healthy, cyclic, synchronized (day 4-8 after ovulation) and selected recipients graded “acceptable” following clinical evaluation performed the day of ET (Panzani et al. Journal of Equine Veterinary Science 2016, 37:17-23). Recipients were maintained in dry lots, fed with hay ad libitum and 2 to 3 kg of mixed grain per day and body conditions scores (BCS) were evaluated monthly for the study (Henneke et al. Equine Veterinary Journal. 1983, 15:371-372). Differences in recipient ages and BCS for ET period of the year were evaluated by ANOVA GLM. Pregnancy rates at 14 and 40 days were analyzed using Fisher’s exact test with Bonferroni Correction. No differences in pregnancy rates were found between fresh (69% at 14 days and 59% at 40 days) and cooled/transported embryos (72% at 14 days and 65% at 40 days) (P>0.05). Differences at 14 and 40-day pregnancy rates were observed in different times of the year (Table 1). Embryo transfers performed during early Spring and, specially, in Autumn could result in lower pregnancy rates compared to the natural breeding season at this latitude.