Test-Day Model for National Genetic Evaluation of Somatic Cell Count in New Zealand

In New Zealand (NZ) herd testing is done for milk, fat, protein and somatic cell count (SCC). Currently, routine genetic evaluation is done for milk, fat and protein yields but not for SCC. A new system has been developed to calculate SCC breeding values (BVs) for the dairy industry. The current evaluation of milk, fat and protein involves combining herd test results to predict a 270-day yield deviation, corrected for test day effects. These yield deviations are then used as the records for genetic evaluation. SCC is measured and reported as a concentration (1000s of cells per ml of milk). Hence, expressing the SCC as an accumulated yield deviation would be inappropriate. Many countries have implemented, or are moving towards, random regression (RR) test-day (TD) models for genetic evaluation of herd test data (de Roos et al., 2003; Liu et al., 2003). The use of a TD model for genetic evaluation of SCC would not require the calculation of an accumulated or average yield as the record for evaluation. Additionally RR TD models have the benefits of improved accuracy of evaluation by simultaneously accounting for the fixed effects and genetic, permanent environmental (PE) and temporary environmental (TE) random effects that affect the trait across time. Because of the benefits of using a RR TD model, we chose to develop such a model for the genetic evaluation of SCC in NZ.