Genetic Relationships of Longevity and Fertility Traits With Somatic Cell Traits Across Lactation Stages in the First Lactation of Japanese Holsteins.

Abstract

This study investigated the genetic correlations of somatic cell traits with longevity and fertility traits across lactation stages in primiparous Holsteins. Particularly, a previously defined trait, ML, derived from the expected milk loss (ML) conditional on test-day somatic cell score (SCS) and differential somatic cell count (DSCC), was compared with SCS. Data consisted of 121,316 test-day records from 36,706 cows, along with records for overall stayability, stayability until second lactation, days to first service, conception rate at first service, and days open. The genetic parameters were estimated using multi-trait linear animal models via Gibbs sampling. Heritability of MLwas generally higher than SCS, especially in mid-to-late lactation. Both traits showed negative genetic correlations with overall stayability, with MLshowing stronger correlations in late lactation, likely related to its greater sensitivity to chronic mastitis. However, genetic correlations between MLand fertility traits were weaker than those observed for SCS, possibly due to the negative relationships between milk yield and fertility. These findings support that selection for low MLcan improve longevity by enhancing mastitis resistance. Further refinement of MLmay be achieved by accounting for the effects of SCS and DSCC on functional traits beyond milk production.