Factors affecting the success of oocyte transfer in a clinical program for subfertile mares

Oocyte transfer is a potential method to produce offspring from valuable mares that cannot carry a pregnancy or produce embryos. From 2000 through 2004, 86 mares, 19.2 ± 0.4 yr of age (mean ± S.E.M.), were used as oocyte donors in a clinical program at Colorado State University. Oocytes were collected from 77% (548/710) of preovulatory follicles and during 96% (548/570) of cycles. Oocytes were collected 21.0 ± 0.1 h after administration of hCG to estrous donors and cultured 16.4 ± 0.2 h prior to transfer into recipients' oviducts. At 16 and 50 d after transfer, pregnancies were detected in 201 of 504 (40%) and 159 of 504 (32%) of recipients, respectively, with an embryo-loss rate of 21% (42/201). Pregnancy rates were similar (P > 0.05) for cyclic and noncyclic recipients and for recipients inseminated with cooled, fresh or frozen semen. One or more recipients were detected pregnant at 16 and 50 d, respectively, for 80% (69/86) and 71% (61/86) of donors. More donors <20 than ≥20 yr (mean ages ± S.E.M. of 15.5 ± 0.4 and 23.0 ± 0.3 yr, respectively) tended (P = 0.1) to have one or more pregnant recipients at 50 d (36/45, 80%; 28/45, 62%, respectively). Results of the program confirm that pregnancies can consistently be obtained from older, subfertile mares using oocyte transfer.