Embryo transfer is now common in the equine breeding industry, but limited data are available evaluating pregnancy outcome. The objectives of this retrospective study were to 1) compare live foal rates in mares carrying their own pregnancy with live foal rates following embryo transfer and 2) evaluate the effect of mare age on pregnancy outcome. Medical records for mares bred during the years 2004-2006 to carry their own pregnancies or bred to donate an unfrozen embryo to a young (≤15 years old) recipient mare were analyzed. Mares were included if they were confirmed pregnant at 16 days after ovulation and pregnancy outcome data were available. Pregnancy outcome data were compared by Fisher’s exact test (2-tailed) and PROC GLIMMIX of SAS (SAS Institute Inc., Cary, NC, USA). A live healthy foal was born from 116 of 134 (86.6%) mares carrying their own foal and 144 of 171 (84.2%) embryo transfer recipients (P > 0.1). The live foal rate for "carry own" mares ≤15 years of age, 89/100 (89.0%), was not different (P > 0.1) from those >15 years of age, 27/34 (79.4%). Recipients of embryos from mares >15 years old had a live foal rate of 41/48 (85.4%), which was similar to the live foal rate of embryos from mares ≤15 years of age, 103/123 (83.7%) (P > 0.1). A significant linear trend (P < 0.05) for an increase in pregnancy loss with increased age was noted when data for both groups were combined (Table 1). In conclusion, no significant difference was noted in pregnancy outcome between embryo transfer recipients and mares carrying their own pregnancies. Table 1.Pregnancy loss rates for “carry own” or ET donor mares of various ages Supported in part by the American Quarter Horse Association (Amarillo, TX, USA).

Personnel involved in the equine embryo transfer industry need to be able to properly identify and evaluate embryos. Accurate assessment of developmental stage, quality, and size of embryos is critical. It is also important to be able to differentiate between an embryo, an unfertilized oocyte (UFO), and non-embryonic structures that may be recovered during an embryo flush procedure. The objectives are to review developmental stages of equine embryos and assignment of embryo quality score or grade. 2. Materials and Methods Initial detection of an embryo in the search dish should be followed by a systematic evaluation of the embryo. In most instances, the evaluation will not take a long period of time. However, in some cases, a decision as to whether or not a structure is an embryo, an unfertilized oocyte, or a non-embryonic structure may take several minutes. It is recommended that the evaluation consist of developmental stage, quality score (grade), and size. A good-quality microscope, with an eyepiece micrometer for measurement of embryo diameter, is essential for proper embryo evaluation. Embryos are typically washed through three or more drops of either flush medium or holding medium before evaluation. The initial flush fluid in the search dish may contain a significant amount of cellular debris. The goal is to transfer the embryo from that initial medium and sequentially remove as much debris as possible during the wash procedure. At the conclusion of the wash steps, the embryo is maintained in holding medium and evaluated before transfer, shipment to another facility for transfer, or cryopreservation. 3. Results