Day of embryo collection, quality and pregnancy rates in cattle.

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.