Recipients' pregnancy rate was affected by season but not by the temperature-humidity index (THI) in an equine commercial ET programme in Southern Europe.

This study retrospectively analysed the effects of different environmental factors such as the photoperiod, temperature/humidity index of 19 years of a commercial embryo transfer programme performed in central Italy. From 340 donors of different breeds and aged between 2 and 25 years, 576 embryos of an excellent or good quality were obtained by uterine flushing on days 7-8 after ovulation. The embryos were transferred to 259 recipients, aged between 2 and 16 years, and belonging to different breeds. Both the donors and the recipients were employed for several cycles/years. At the time of transfer, the recipients were in diestrus, from day 4 to day 8 after ovulation, and were classified as excellent or good based on clinical examination findings. Out of the total number of embryos transferred, 437 pregnancies were obtained at 14 days (75.9%), with 48/437 (10.1%) embryonic losses at 40 days; with no significant differences among years. Significant differences (p < .05) were observed between 14-day pregnancy rates for transfers performed during the spring transition and breeding seasons (77.1%) compared to those performed during the autumn transitional season (57.9%). No effect of temperature/humidity index was found on pregnancy rates or pregnancy losses after the transfer to the recipients (p > .05). Embryo transfer during autumn was less successful than the transfers performed during the spring transition and the breeding season.