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Although it has not been documented in published studies, embryo transfer (ET) practitioners have suggested that embryos from Jersey (JE) cattle do not survive freezing as well as embryos from other dairy breeds such as Holsteins (HO). The present study represents a retrospective analysis of pregnancy rates achieved following transfer of fresh and frozen embryos from Jersey and Holstein donors. In addition, a retrospective comparison was made of two different embryo-freezing protocols for each breed of cattle. Embryos were collected nonsurgically 7 to 7.5 days post-estrus from superovulated donors on 57 Holstein and 27 Jersey dairy farms over a 15-year period. Fresh and frozen-thawed embryos were transferred nonsurgically into cows and heifers following either natural or prostaglandin-induced estrus. Embryos were frozen either in 10% glycerol (Gly) or 1.5M ethylene glycol (EG) in 0.25mL straws. Following equilibration, straws were seeded at −6 to −7°C and temperature was maintained for 10 min and then decreased at 0.6°Cper min. Straws were plunged into liquid nitrogen at −32 to −35°C. At thawing, straws were held in the air for 7s and then submerged in 29°C water for 15s. Embryos frozen in EG were transferred immediately following thawing. Embryos frozen in Gly were rehydrated in a standard 3-step Gly-sucrose system prior to being transferred. Pregnancy diagnosis was performed at Days 40 to 90 of gestation. As seen in the Table 1, pregnancy rates were similar for fresh embryos from both HO and JE cattle. Also, there were no differences in pregnancy rates between recipients that received embryos frozen in Gly or EG within donors of either breed. However, JE embryos frozen in either Gly or EG resulted in lower pregnancy rates than did HO embryos frozen in Gly or EG. Embryo stage at freezing was tracked for EG but not Gly embryos. There were no differences in pregnancy rates among morulae, early blastocysts or mid-blastocysts for either HO or JE embryos frozen in EG. The differences in embryo survival may be due to different lipid composition of embryos of the two breeds. Perhaps a more efficacious freezing protocol can be developed for cryopreservation of JE embryos. In conclusion, pregnancy rates with cryopreserved HO embryos were higher than with JE embryos.Table 1

120PREGNANCY RATES RESULTING FROM TRANSFER OF FRESH AND FROZEN HOLSTEIN AND JERSEY EMBRYOS