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 p roduction, and the standard deviation of test-day records. Data compri sed 15 756 first-lactation yields from north-western Germany with at l east 8 recorded test days. Heritability estimates were in the range 0. 10-0.15 for milk yields and slightly lower for far and protein. Herita bilities varied only little among the three criteria and across variou s models, including or neglecting covariates for lactation yield or le ngth of lactation. For a bivariate analysis of persistency and milk yi eld, the latter was either defined in the traditional way as 305-day l actation records, or a test-day model was applied considering test-day records directly in the form of a repeatability model. The results sh owed 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), althoug h correlations among criteria of persistency were favourable. No appar ent differences existed between using 305-day records or test-day reco rds. For the latter definition of yield, this could also be confirmed when only test-day records from mid-lactation were used in the test-da y model.