Genetics of alternative somatic cell count traits in Italian Holsteins

Mastitis is one of the most costly diseases in dairy herds. Alternative somatic cell count (SCC, cells mL-1) traits have been proposed to improve udder health. The aim of this study was to estimate genetic parameters of alternative SCC traits in first-parity Italian Holstein cows. The original dataset was edited to include records between 5 and 305 days in milk (DIM). Contemporary groups were defined as cows calving in the same herd-year-season (HYS) and HYS with less than 5 animals were removed. After edits, 21,671 cows from 152 herds were available for statistical analysis. Alternative SCC traits were: mean and standard deviation of somatic cell score [(SCS, 3 + log2(SCC/100,000)] within lactation (SCSt and SD_SCSt, respectively), between 5 and 150 DIM (SCS150 and SD_SCS150, respectively), and between 151 and 305 DIM (SCS305 and SD_SCS305, respectively); infection, a dichotomous trait indicating that at least one test-day record had SCC greater than 100,000 cells mL-1 within lactation; severity, the ratio between the number of test-days with SCC greater than 100,000 cells mL-1 and the total number of test-days; severity2, the ratio between the number of test-days with SCC greater than 400,000 cells mL-1 and the total number of test-days; and subclinical mastitis (SCM), identified as two consecutive test-days with SCC below 100,000 cells mL-1 followed by a test-day greater than 400,000 cells mL-1. Means of SCSt, SCS150, SCS305, infection, severity and severity2 were 3.48, 3.12, 3.44, 0.83, 0.40 and 0.14, respectively. Heritabilities of and genetic correlations between the aforementioned traits were estimated using univariate and bivariate animal models, respectively, considering HYS, age of the cow at calving, and number of lactation test-days as fixed effects, and additive genetic animal and residual as random factors. The pedigree included 73,009 animals (6 generations). Heritability estimates were 0.11 for severity, 0.08 for SCSt, 0.06 for SCS150 and SCS305, and 0.02 for SD_SCSt, SD_SCS150, SD_SCS305, infection and SCM. The strongest genetic correlations (0.94 to 0.99) were between SCS traits (SCSt, SCS150 and SCS305) and infection, severity and severity2. Although heritability estimates were generally low, exploitable genetic variation exists for SCC traits. Combination of such traits into an udder health index with appropriate emphasis may enhance genetic gain in resistance to mastitis of Italian Holstein population.