Genetic evaluation of differential somatic cell count in Italian Holstein cattle

Abstract

Mastitis is a prevalent inflammatory condition affecting udder tissue in dairy cows. It leads to reduced milk production, increased veterinary costs and potential culling of affected animals, impacting both animal welfare and economic outcomes in dairy farming. Recently introduced as a supplementary measure to somatic cell count (SCC), the differential somatic cell count (DSCC) is an innovative indicator for intramammary infection. DSCC quantifies the proportion of polymorphonuclear neutrophils plus lymphocytes (PMN-LYM) within milk somatic cells, providing enhanced insights into udder health status and infection severity. The aim of this study was to estimate genetic parameters and develop a genetic evaluation of DSCC in Italian Holstein. An innovative, new categorical phenotype, named state of infection (SI), was created from each test-day, combining SCC and DSCC records. Values from 1 to 4 were assigned to the different test-day records based on two thresholds related to the parity order: 100,000 SCC and 60% DSCC for first parity cows; 200,000 SCC and 65% DSCC for later parity cows. Observations with both SCC and DSCC below the respective threshold were assigned a value of 1; SCC below and DSCC above were assigned to category 2; both above to category 3; SCC above and DSCC below to category 4. A multiple-trait repeatability linear animal model was applied to the two traits, with year-month-parity-region of recording, herd-parity of recording, parity-age at calving-year-region and parity-days in milk-year-region as fixed effects. Random effects included herd-test-day-parity of recording, herd-year-month-parity of calving, animal additive genetic, and permanent environment. The posterior mean (PM) for heritability was 0.13 for SCS (posterior standard deviation, PSD: 0.01) and 0.09 (0.01) for SI. The genetic correlation between SCS and SI was 0.94, highlighting the strong relationship between the two traits but also their differences. A SNPBLUP model was applied for estimating genomic breeding values (GEBV) using either a reference population composed of bulls or of both bulls and cows (mixed reference population). For the validation of GEBV, a three-year back cutoff date for phenotypes was used: the results highlighted the positive impact of a mixed reference population on dispersion and accuracy. The genetic trend based on bulls’ GEBV indicates that the undergone selection for SCS indirectly improved the population also for SI. In conclusion, this study confirmed the possibility to select for SI in Italian Holstein population and provided the bases for the implementation of a routine genetic evaluation for this innovative udder health trait.