Effect of variability in response to superovulation on donor cow selection differentials in nucleus breeding schemes.

The effectiveness of nucleus breeding schemes based on multiple ovulation and embryo transfer in practice is highly dependent upon the achieved reproductive rate. This study quantifies the effects of donor cow variability in response to superovulation (random and phenotypically correlated with milk yield) and failure rates (proportion of selected cows producing no transferable embryos) on the mean and variance of realized female selection differentials as affected by herd size, average embryo yield, and the coefficient of variation for embryo yield. Results show that differences in failure rates (0 to 40%) reduced realized female selection differentials up to 64%. The existence of a negative phenotypic correlation (-.3) between donor cow embryo yield and milk yield caused smaller reductions (up to 19%). Variability in realized female selection differentials is largest for small nucleus units and should not be neglected when planning the establishment of a nucleus breeding unit. The use of multiple flushes rather than a single flush per cow to produce the same average number of transferable embryos can reduce the variability in realized selection differentials but increases generation interval. Variability in response to superovulation will also affect inbreeding rates and the average accuracy of selection. Simulation studies are needed to quantify the total effect of variability in response to superovulation on the genetic responses possible in nucleus breeding schemes relying on multiple ovulation and embryo transfer.