Genetic parameters for test-day somatic cell count at different lactation stages of Finnish dairy cattle

Test-day data comprising of test-day somatic cell count (SCC) records of 31,395 Finnish Ayrshire and of 42,552 Holstein–Friesian cows were used to estimate genetic and environmental parameters for SCC in the first and second lactation. A multitrait approach, together with a continuous covariance function, was used to derive parameters. Variance components were estimated with a sire model. In Ayrshire cows, heritability of the SCC was highest at the beginning of the lactation (0.12 for the first and 0.09 for the second lactation). The lowest value was estimated in mid-lactation, after the peak milk production (0.07 for the first and 0.06 for the second lactation). For Holstein–Friesians, heritability estimates increased from the beginning of lactation and peaked in mid-lactation (0.10 for the first and 0.08 for the second lactation). For both breeds, genetic correlations between different test days varied from 0.29 to 0.89, and phenotypic correlations from 0.08 to 0.61. Generally, in both breeds and lactations, genetic and phenotypic correlations between the SCC on different test days depended on how far apart the test days were, and also on lactation stage. The heritability estimates and correlations suggest that animals will have differences on breeding values of SCC also within lactation. The presented estimation procedure was also found to be feasible to analyse large test-day data and to give accurate (co)variance components.