Factors affecting the efficiency of foal production in a commercial oocyte transfer program

Transfer of donor oocytes to the oviducts of inseminated recipient mares (oocyte transfer, OT) presents a valuable method for production of foals from otherwise infertile mares. Little information is available, however, on factors affecting success of OT in a clinical setting. We report the findings over three breeding seasons in a commercial OT program developed at an equine embryo transfer center in Argentina. Overall, 25 mares were enrolled, and 197 follicle aspiration procedures were performed. The average mare age was 23 years. Follicle aspiration was performed with a needle placed through the flank; the oocyte recovery rate per follicle aspirated was 149 of 227 (66%). Induction of donor ovulation with deslorelin + hCG resulted in a significantly higher oocyte recovery rate than did induction with deslorelin alone (75% vs. 58%). There was no significant effect of mare age (17-20, 21-24, or 25-27 years) on oocyte recovery rate. Twelve oocytes were degenerating or lost during handling; transfer of the remaining 137 oocytes resulted in 42 pregnancies (31%) at 14 days. Of these, 32 (23% per transfer) went on to produce a foal or ongoing pregnancy. Transfer of oocytes recovered with a compact cumulus, without donor follicle induction, or less than 20 hours after induction was associated with a significantly reduced pregnancy rate (1/16, 6%), as was use of noncycling, hormone-treated recipients (2/22, 9%). To evaluate management factors affecting pregnancy rate, noncycling, hormone-treated recipients were disregarded, and only procedures using mature (expanded cumulus) oocytes recovered and transferred on the standard schedule (n = 99) were included. Mare age did not significantly affect rates of pregnancy or pregnancy loss. Similar pregnancy rates were obtained using recipients inseminated from 1 to 27 hours before transfer. Counterintuitively, insemination of recipients immediately (1-2 hours) after aspiration of the recipient follicle was associated with a high pregnancy rate (10/12, 83%). There was no significant effect on pregnancy rate of donor induction agent, the time the oocyte was in culture (2-20 hours) before transfer, time from recipient insemination to transfer, or total time from donor induction to transfer (32-45 hours). These findings establish that OT is robust, in that it is effective over a wide variation in timing of the different components involved, and can be successfully developed in a private embryo transfer practice.