FIELD TRIAL TO COMPARE PREGNANCY RATES OF BOVINE EMBRYO CRYOPRESERVATION METHODS - VITRIFICATION AND ONE-STEP DILUTION VERSUS SLOW FREEZINGAND 3-STEP DILUTION

We designed and conducted a field trial to obtain accurate pregnancy r ates of Day 7 bovine embryos after vitrification in PB1 containing 6.5 M glycerol and 6% BSA (w/v) and one-step dilution in 1 M sucrose comp ared with controlled slow freezing in 1.5 M glycerol and three-step di lution. Embryos were collected from superovulated donor cows, and Grad e 1 and 2 morulae and blastocysts were randomly assigned to each cryop reservation treatment group. Dutch farmers were solicited to participa te in the field trial by an advertisement that offered cryopreserved e mbryos at subsidized cost. Within a period of 11 wk, one of six techni cians visited 150 farms. Standard nonsurgical methods were used to tra nsfer a total of 728 cryopreserved embryos. Recipient cows, mainly mul tiparous and of various breeds, the so-called ''bottom-end'' of the na tional herd, received embryos either 6, 7 or 8 d after standing estrus during natural estrous cycles. We compiled a database on 22 factors t hat may influence establishment of pregnancy in order to check randomi zation of each factor over cryopreservation treatment groups and embry o transfer technicians and to perform the statistical tests. Overall p regnancy rates were 44.5% (n=393) for vitrified embryos and 45.1% (n=3 35) for slowly frozen embryos. Pregnancy rates were not significantly different (ANOVA, P=0.79 or Chi-square analysis, P=0.88). The register ed data confirm that all factors were randomly distributed over cryopr eservation methods and technicians. Technician was not a significant s ource of variation in pregnancy rate (analysis of variance, P=0.79). A lthough three technicians performed better with the one-step procedure :and the other three performed better using the three-step method, the interaction between the technician and cryopreservation method was no t significant (Tukey's test for nonadditivity, P=0.13). Our results in dicate that 1) vitrification and one-step dilution can be successfully used in the field without significant reduction in the pregnancy rate and 2) the pregnancy rate obtained using the ''bottom-end'' of the he rd is satisfactory for practical application. (C) 1997 by Elsevier Sci ence Inc.