Models for Genetic Analysis of Dystocia and Calf Mortality

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Abstract
Calvings of 106,750 Israeli Holstein heifers were analyzed for dystocia and calf mortality, which were scored dichotomously. Models fitted included herd-year-season, sex of calf, calving age, calving month, sire of cow, sire of calf, and groups of sire of cow and calf. Herd-year-season, sire of cow and of calf, and residuals were random with diagonal variance-covariance matrices. Traits were analyzed with and without a herd-year-season effect, and dystocia was analyzed separately for male and female calvings and with both sexes combined. Variance components were estimated by REML for linear models and by the counterpart of REML for threshsold models. Heritability estimates were two to four times larger in threshold than in linear models, but correlations between corresponding sire evaluation were all greater than .9. Correlations between evaluations computed with and without herd-year-season effects were 9. for sire of calf evaluations for dystocia and greater than .97 for all other evaluations. Correlations between sire evaluations computed separately for male and female calvings were betweent .3 and .7. Thus, calving difficulty is expressed differently in male and female calvings. Genetic trends were unfavorable for dystocia but favorable for calf mortality. Phenotypic trends were curvilinear with maxima in 1982 and 1983 for calf mortiality and dystocia.