Early Pregnancy After Embryo Transfer in Artificial-Cycle and Cycling Recipient Mares and Donor Embryo Yield Across Calendar-Defined Periods: A Five-Year Retrospective Commercial Study.

BACKGROUND

Seasonal reproductive inactivity may limit year-round equine embryo transfer (ET), although artificial-cycle protocols can expand recipient availability.

AIMS/

OBJECTIVE

To compare early pregnancy after ET between artificial-cycle and cycling recipients and donor embryo yield per flush opportunity between calendar-defined pre-breeding and breeding periods.

METHODS

Retrospective records from a commercial ET program in Mexico (2021-2025) were analyzed. Early pregnancy was evaluated in 231 ET procedures using mixed-effects logistic regression with recipient status and year as fixed effects and recipient mare as a random intercept. Donor embryo yield was analyzed using Poisson generalized estimating equations with calendar period and year as fixed effects, donor mare as the clustering variable, and log(number of flush attempts) as an offset.

RESULTS

Early pregnancy was 74.5% (38/51) in artificial-cycle recipients and 65.0% (117/180) in cycling recipients. Recipient status was not associated with early pregnancy (P = 0.244), whereas year was associated with the outcome (P = 0.0019). Donor data comprised 67 donor-year records, 344 flush attempts, and 250 embryos. Embryo yield was 0.667 embryos/flush during the pre-breeding period and 0.777 embryos/flush during the breeding season. The adjusted rate ratio was 0.832 (95% CI, 0.668-1.036; P = 0.100). Within the pre-breeding period, yield was 0.633 embryos/flush in 2021-2023 and 0.701 embryos/flush in 2024-2025 (rate ratio, 1.160; 95% CI, 0.896-1.502; P = 0.259).

CONCLUSIONS

No statistically detectable difference in early pregnancy rate was observed between artificial-cycle and cycling recipients. No statistically detectable difference in donor embryo yield was observed between the calendar-defined periods after adjustment for flush opportunities.