Impact of asynchronous ovulations on the expression of sex-dependent growth rate in bovine preimplantation embryos

Abstract

In cattle, male embryos have a faster growth rate than female embryos, and this results in alteration of the normal 1:1 sex ratio in embryos divided into three developmental groups. The fastest developed one-third are predominantly males, the slowest one-third predominantly females, and in the intermediate one-third no alteration of the sex ratio is seen. However, the deviations of the sex ratios are only 15-20% from random. These findings are compatible with the assumption that, in superovulated cows, ovulations follow a normal distribution and that, at the time of sampling at Day 7, male and female embryos differ with regard to development by 1 or 2 h. Because of this it is unlikely that larger changes in the sex ratios can be expected.