Impact of donor mare and stallion identity on the likelihood of pregnancy after transfer of in vitro produced embryos

Abstract Background The success of in vitro embryo production (IVEP) is influenced by donor mare and stallion. Objectives To determine whether donor mare and stallion influence the pregnancy rate after transfer of in vitro produced (IVP) blastocysts and to identify factors influencing the likelihood of obtaining one or more pregnancies from a single IVEP session. Study Design Retrospective clinical study. Methods The influence of the following variables on pregnancy outcome was assessed in 2292 transfers of IVP blastocysts: speed of in vitro embryo development, embryo grade after thawing, age of the donor mare, blastocyst rate, age and breed of the recipient mare, year of embryo production, month and year of embryo transfer. Finally, the variability in pregnancy rate between individual mares and stallions, and the effect of mare‐stallion combination was investigated. Results Overall, 85% of successful IVEP sessions resulted in at least one pregnancy. When a single blastocyst was produced, 78% of sessions yielded a pregnancy; this increased to 91% and 98% when two or at least three blastocysts were produced, respectively. Slow in vitro embryo development, poor embryo grade after thawing, advanced donor mare age, and performing embryo transfer late in the season had a negative impact on the pregnancy rate. The variation in pregnancy rates between most mares and stallions was low (~20%). For some mares, the likelihood of pregnancy varied by as much as 50% depending on the stallion used. Main Limitations Due to the limited sample size, variation in pregnancy rates between donors was assessed descriptively. Conclusions IVEP is a very efficient way to obtain pregnancies. The likelihood of pregnancy after transfer of an IVP embryo has improved over time, and individual mare and stallion identity seems to have a low impact. However, for some underperforming mares, results differ markedly depending on the stallion used for intracytoplasmic sperm injection.