Genetic parameters and evaluations from single- and multiple- trait analysis of dairy cow fertility and milk production

Genetic parameters and breeding values for dairy cow fertility were estimated from 62 443 lactation records. Two-trait analysis of fertility and milk yield was investigated as a method to estimate fertility breeding values when culling or selection based on milk yield in early lactation determines presence or absence of fertility observations in later lactations. Fertility traits were calving interval, intervals from calving to first service, calving to conception and first to last service, conception success to first service and number of services per conception. Milk production traits were 305-day milk, fat and protein yield. For fertility traits, range of estimates of heritability (h2) was 0.012 to 0.028 and of permanent environmental variance (c2) was 0.016 to 0.032. Genetic correlations (rg) among fertility traits were generally high (>0.70). Genetic correlations of fertility with milk production traits were unfavourable (range −0.11 to 0.46). Single and two-trait analyses of fertility were compared using the same data set. The estimates of h2 and c2 were similar for two types of analyses. However, there were differences between estimated breeding values and rankings for the same trait from single versus multi-trait analyses. The range for rank correlation was 0.69–0.83 for all animals in the pedigree and 0.89–0.96 for sires with more than 25 daughters. As single-trait method is biased due to selection on milk yield, a multi-trait evaluation of fertility with milk yield is recommended