Genetic parameters and relevance for heat stress assessment in dairy cattle of 2 udder health traits: Somatic cell score and differential somatic cell count.

Somatic cell count is widely used for large-scale udder health monitoring and remains a proxy for mastitis incidence still used in many genetic evaluation systems. This trait and its log-transformation, SCS, are thus also available to study the effect of heat stress on mammary gland health. Currently, a new trait called differential somatic cell count (DSCC), which represents the percentage of neutrophils and lymphocytes in the total SCC, is increasingly phenotyped simultaneously with SCC. By combining information, SCS and DSCC could more closely reflect the direct trait than SCS alone, providing a better proxy for mastitis incidence including during heat stress events. On this basis, the first objective of this study was to evaluate the interest of DSCC for heat stress assessment with a focus on mammary gland health with SCS as comparison. Additionally, the interest of both traits for genetic evaluation of udder health thermotolerance was explored. Because studies providing basal genetic parameters for DSCC are still rare, they were also estimated in this study. To do so, a random regression model on DIM was performed considering each parity as a different trait. For both SCS and DSCC, similar averaged daily heritability (0.10 to 0.11 for SCS and 0.11 to 0.14 for DSCC) and repeatability (0.61 to 0.64 for SCS and 0.54 to 0.63 for DSCC) were obtained. Moderate averaged daily genetic correlations were also estimated between SCS and DSCC (0.43 to 0.55). From the residuals of the same model, average residual responses with temperature-humidity index (THI) were studied. The results showed that DSCC reaction in mean and in variance with high THI was stronger than SCS. In addition, the reaction with increasing THI seemed to be inconsistent between lactation numbers for SCS conversely to DSCC. In this way, DSCC presented more relevant characteristics than SCS to discriminate thermotolerant and thermosensitive cows for udder health. However, for general heat stress detection, udder health traits seemed not to be the most adapted biomarkers. Low heritability (0.02 to 0.03 for SCS and 0.03 to 0.04 for DSCC) and repeatability (0.12 to 0.18 for SCS and 0.20 to 0.26 for DSCC) values were also obtained for SCS and DSCC newly defined thermotolerance traits.