150 CONCEPTION RATES OF FRESH AND FROZEN IN VIVO-PRODUCED MORULAE AND BLASTOCYSTS IN LACTATING DAIRY RECIPIENT COWS.
ABSTRACT The aim of this study was to evaluate conception rates of fresh or frozen embryos in the morula and blastocyst stages of development when transferred to lactating Holstein cows. During a 365-day period, 213 Day-7 embryos were produced by superovulation in lactating Holstein cows or nulliparous Holstein heifers and transferred to lactating Holstein cows synchronized with the Ovsynch protocol at the University of Wisconsin-Madison. After collection, embryos were evaluated and classified according to IETS standards. Subsequently, grades 1 and 2 embryos were conventionally frozen in ethylene glycol or transferred fresh to recipients on Day 7 of the cycle in the uterine horn ipsilateral to the corpus luteum. When there were more recipients than fresh embryos available, frozen embryos were thawed and transferred. Pregnancy diagnosis was performed on Days 30, 60, and 120 by ultrasound evaluations. Moreover, calving data were recorded. Data were analysed using generalized linear models and results are presented as least squares means±standard errors. The average conception rates between 30 and 280 days of fresh (n=45) and frozen (n=168) embryos were similar (38.7±7.3 and 29.4±3.3%, respectively; P>0.10). There was an interaction between treatment and embryo developmental stage (P<0.10). The average conception rate was higher in fresh blastocysts (54.2±12.8%) than fresh morulae (25.2±5.1%) and cryopreserved embryos (P<0.05). Moreover, there was no difference between conception rates of cryopreserved morulae and blastocysts (26.8±3.6 and 32.1±4.8%, respectively; P>0.10). Embryo quality seems not to have influenced the conception rates of Holstein embryos (34.9±3.3 and 32.9±6.9% of grades 1 and 2, respectively; P>0.10). We concluded, based on the results of this study, that cryopreservation of in vivo-produced embryos in Holstein donors did not substantially affect embryo viability and that the developmental stage of frozen embryos did not influence conception rates in lactating dairy recipients.