In 1300 donor cows, total embryos increased from 8.5 to 15.3, (P less than 0.001), the mean number of transferable embryos from 3.1 to 6.5 (P = 0.067) and pregnancies increased from 1.3 to 3.3 (P = 0.584) as the day of collection increased from six to 7.5. Most of the embryos recovered on day 6, 7.5 and 8 were morulae, early blastocysts and late blastocysts respectively. Morulae formed the majority of the embryos collected on days 6 and 6.5. Pregnancy rates in early and late blastocysts were highest on days 7 (54.4 and 60.2 per cent) and 7.5 (53.6 and 53.1 per cent, P = 0.009 and 0.004). There were significant differences in pregnancy rates between embryo stages on days 7 and 7.5 (P less than 0.001), embryo grades on days 7, 7.5 and 8 (P less than 0.001), and within embryo grades 1 (P = 0.015) and 3 and 4 (pooled, P = 0.009). On all days combined there were significant differences in pregnancy rates between both embryo stages (P = 0.007) and embryo grades (P less than 0.001). It appears that the concept of embryo fitness may be applied to blastocysts but not to morulae.
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