GENETIC-RELATIONSHIP BETWEEN DAIRY LACTATION PERSISTENCY AND YIELD

Phenotypic and genetic parameters were estimated for three criteria of persistency of lactation production of milk, fat, and protein yield. The criteria included the ratio of yield in last to first trimester of lactation, the ratio of the maximum divided by the mean of test-day production, and the standard deviation of test-day records. Data comprised 15 756 first-lactation yields from north-western Germany with at least 8 recorded test days. Heritability estimates were in the range 0.10-0.15 for milk yields and slightly lower for far and protein. Heritabilities varied only little among the three criteria and across various models, including or neglecting covariates for lactation yield or length of lactation. For a bivariate analysis of persistency and milk yield, the latter was either defined in the traditional way as 305-day lactation records, or a test-day model was applied considering test-day records directly in the form of a repeatability model. The results showed that genetic correlations between milk yield and persistency were favourable for selection on yield for the two ratios (r(g) = 0.50/-0.50) and unfavourable for the standard deviation (r(g) = 0.30), although correlations among criteria of persistency were favourable. No apparent differences existed between using 305-day records or test-day records. For the latter definition of yield, this could also be confirmed when only test-day records from mid-lactation were used in the test-day model.