Genetic analysis of test day production in second lactation of British Holstein-Friesian cows

Monthly test day records obtained on 24 345 British Holstein-Friesian cows in their second lactation were considered as multiple traits in genetic analyses of dairy performance. Estimates of genetic parameters of 10 monthly test day and completed lactation records for milk, fat and protein yields were obtained using multivariate restricted maximum likelihood methods fitting a sire model. Heritabilities for monthly test day yields were highest in mid-lactation, ranging from 0.15 to 0.51 for yields of milk fat or protein. Estimates for lactation records for these traits were 0.52, 0.58 and 0.46 respectively. Correlations between test day yields decreased the further apart the test days. Estimates of genetic correlation of test day yields with corresponding lactation yields were highest (0.94 to 0.98) for mid-lactation records. Phenotypic correlations were lower than the corresponding genetic correlations, but both followed the same pattern. These results were consistent with estimates reported for first lactation test day records from the same population. Partial yields derived from threee - monthly test day milk yields were used to provide measures of persistency of milk production, calculated as partial yield for test days 5 to 7 divided by partial yield for test days 2 to 4 (P-2:1) and partial yield for test days 8 to 10 divided by partial yield for test days 2 to 4 (P-3:1) These ratio measures had moderate heritabilities (0.28 and 0.35), positive genetic (0.45 and 0.26) and negative phenotypic (-0.09 and -0.12) correlations with the partial yield during early lactation.