Phenotypic and genetic relationships between milk somatic cell counts and clinical mastitis in the French Holstein cattle population

Abstract

This paper summarises three previously published studies on the relationships between somatic cell counts (SCC) and clinical mastitis in French Holstein dairy cows. These studies were based on clinical mastitisdata collected by milk recording technicians one ach test-day for two-and-a-half years in WesternFrance. The first two studies focused on the putativerisk of clinical mastitis associated with very low SCC.The first one analysed the risk of first clinical mastitis occurrence according to the SCC level on the first test day by survival analysis. The second one analysed the risk of clinical mastitis in second lactation by logistic regression according to SCC criteria measured in first lactation. The results were consistent and showed that the lower the SCC level, the lower the risk of clinical mastitis, without any intermediate optimum. In the third analysis, genetic parameters were estimated. The heritability estimate of SCC (0.17) was higher than that of clinical mastitis (0,02) and the genetic correlation estimate between these traits was high (0.72).Thus, the results suggested that selection for lower SCClevel is relevant and should increase resistance to both clinical and subclinical mastitis.