Alternative use of Somatic Cells Counts in genetic selection for mastitis resistance: a new estimated breeding value for Italian Holstein breed

Abstract

A new estimated breeding value estimation for mastitis resistance has been implemented in the Italian Holstein dairy genetic evaluation. This breeding value uses indicators derived from test-day somatic cell counts (SCC) in order to predict the breeding value for resistance to mastitis occurrence. The pattern of test-day SCC contains additional information on mastitis resistance beyond its mean value usually used. These breeding values are combined in an “udder health” index, using different weights based on (co)-variance structure among indicators and direct mastitis observations. This new index will be additional to the already used somatic cell score index. The correlation between the two indexes is estimated at 80% meaning that the two are in the same direction, but they differ in terms of ranking individuals. In order to set up the new index, several indicators, derived from test-day somatic cell counts, have been derived. Alternative SCC traits were: mean and standard deviation of somatic cell score within lactation (SCS_t and SD_SCS_t, 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 SCC test-day record was above 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; severity, 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 cell’s peak defined as the number of peaks during lactation (number of times when SCC shows a change from <100,000 to >400,000 mL^-1 on three consecutive test-days). Once indicators traits have been defined, these have been validated on a sample data-set with direct mastitis information. Multiple trait animal model has been applied and genetic parameter for these traits including direct mastitis have been estimated. The highest genetic correlations between mastitis itself and indicators were with SCS_150, SD_SCS_, severity and peak equal to 0.39, 0.44, 0.41 and 0.51 respectively. Selection index methodology was applied to estimate appropriate weights to combined these four traits in the aggregate udder health index. This index has been published for the first time during December 2017 evaluation with mean 100 and standard deviation 5. Initially this index will be published only for national and international bulls. Following steps are to set up the genomic evaluation and, also, to participate to the GMace evaluation.