Joint estimation of genetic parameters for test-day somatic cell count and mastitis in the United Kingdom

Genetic parameters were estimated in a joint analysis of log(e)-transformed somatic cell count (TSCC) with either mastitis as a binary trait (MAS) or the number of mastitis cases (NMAS) in Holstein-Friesian cows for the first 3 lactations using a random regression model. In addition, a multi-trait analysis of MAS and NMAS was also implemented. There were 67,175, 30,617, and 16,366 cows with records for TSCC, MAS, and NMAS in lactations 1, 2, and 3, respectively. The frequency of MAS was 14, 20, and 25% in lactations 1, 2, and 3 respectively. The model for TSCC included herd-test-day, age at calving and month of calving, fixed lactation curves nested with calving year groups, and random regressions with Legendre polynomials of order 2 for animal and permanent environmental effects. The model for MAS and NMAS included fixed herd-year-season, age at calving and month of calving, and random animal and permanent environmental effects. All analyses were carried out using Gibbs sampling. Estimates of mean daily heritability averaged over a 305-d lactation were 0.11, 0.14, and 0.15 for TSCC for lactations 1, 2, and 3, respectively. Corresponding heritability estimates for MAS were 0.05, 0.07, and 0.09. The heritabilities for NMAS were similar at 0.06, 0.07, and 0.12, respectively, for lactations 1, 2, and 3. The genetic correlations between lactations 1 and 2, 1 and 3, and 2 and 3 were 0.75, 0.64, and 0.92 for computed 305-d lactation TSCC; 0.55, 0.48, and 0.89 for MAS; and 0.62, 0.42, and 0.85 for NMAS, respectively. The genetic correlations between MAS and TSCC were positive and generally moderate to high. The genetic correlations between computed 305-d lactation TSCC and MAS were 0.53, 0.61, and 0.68 in lactations 1, 2, and 3, respectively. Similar corresponding genetic correlations were obtained between computed 305-d lactation TSCC and NMAS in the respective parities. Mastitis as a binary trait and NMAS in the same lactation were very highly correlated and were genetically the same trait. It is intended that the new parameters will be used in setting up a national evaluation system for the joint analysis of TSCC and MAS.