ANALYSIS OF FACTORS AFFECTING SUPEROVULATORY RESPONSES IN RUMINANTS

Data on ovulation rate and numbers of ova and transferable embryos rec overed from superovulated cattle and sheep were analysed using general ized linear models, quasi-likelihood, restricted maximum likelihood (R EML) and generalized linear mixed models (GLMMS). The data pertained t o the operation of nucleus breeding schemes in cattle and the commerci al application of embryo transfer in sheep. Results of the analyses sh owed that generalized linear models involving Poisson and Binomial dis tributions were inappropriate because of over-dispersion, and that ana lyses using quasi-likelihood to model negative binomial and beta-binom ial distributions were more suitable. Factors identified as important in determining the results in cattle were the number of previous super ovulations (a higher proportion of transferable embryos were obtained in the initial flush compared to subsequent recoveries in two out of t hree sets of data), the donor (significant in all analyses with repeat ed recoveries) and its mate (significant in some analyses). In sheep, the use of pFSH or hMG for superovulation increased embryo yields abov e those obtained with PMSG + GnRH. Analyses of a further data set for sheep showed the effect of breed was ambiguous. The effect of donors a nd their mates were treated as random effects in analyses involving RE ML and GLMMS. Results showed that the repeatability of the number of t ransferable embryos produced per donor ranged between 0.13 and 0.23 in three sets of data and was significant in all cases. In these analyse s the variance among mates was not significantly different from zero. The results of analyses were used to develop a random generator to sim ulate the numbers of ova and embryos recovered from a cow following su perovulation. By sampling from negative binomial distributions where t he scale factor used for each cow was a normally distributed deviate, distributions were obtained which had the same mean, variance and repe atability as those observed.