Influence of Hormonal Induction of Estrus And/or Ovulation on Embryo Transfer Success

The recipient mare is one of the most important elements in an equine embryo transfer program. Mare selection at the time of transfer is based on a number of established criteria such as uterine and cervical tone, absence of endometrial oedema after ovulation and the presence and morphology of the corpus luteum. However, recent studies suggest that other factors, such as estrus duration, have a favorable influence on pregnancy rates. The aim of the study was to determine if the pregnancy rate in recipient mares was influenced by estrous length, induction of estrus with prostaglandin F2alpha and/or induction of ovulation with hCG. Retrospective data from a commercial embryo transfer program in Northern Germany, was analyzed. A total of 1051 embryos were transferred into 880 recipient mares (2-24 years of age) during 2021. Recipient mares were evaluated ultrasonographically daily, from first signs of estrus until ovulation was detected. For the analysis of estrus length, mares were classified according to the number of days that edema was detected ultrasonographically into the following groups: short (1-3 days), medium (4-6 days) and long (7-9 days) estrus. Estrus was induced in 852 (81.06%) and ovulation was induced in 676 (64.31%) cycles. Overall pregnancy rate at day15 post-ovulation was 76.21% (801/1051). Spontaneous estrus had a lower pregnancy rate (70.85%) compared to estrus induced by prostaglandins (77.46%) (p=0.052). Nonetheless, there were no significant differences in embryo transfer success between spontaneous (79.2%) and hCG-induced (74.55%) ovulation (p=0.096). The duration of estrus had no influence on pregnancy rates when evaluating the 1051 embryo transfers. However, when evaluating each estrus length group individually, in the medium and long estrus groups there was a significant effect of ovulation induction on pregnancy rates (p=0.007, p=0.042 medium and long estrus, respectively), suggesting that hCG-induction reduced the success rate vs spontaneous ovulations (71.99% vs 84.05%) in medium length estrus, while increasing pregnancy rate in the long length group (80.85% vs 60.71%). These data suggest that the importance of estrus length on successful embryo transfer rate may be associated with factors such as estrogen exposure and ovulation. In long estrus cycles, ovulation induction may be beneficial in preventing lack of ovulation or estrogen overstimulation. However, a minimum duration of estrogen exposure is necessary to optimize gestation rates. Further studies are needed to evaluate the possible causes of these effects.