Genetic Insights: Balancing Milk Yield, Fat: Protein Ratio and Fertility in Primiparous Cows From Subtropical Regions.

In this study, we aimed to evaluate the genetic association between fertility traits, milk yield and the fat: protein ratio (FPR) on the test day in primiparous Holstein cows. The analysed traits were milk yield (TDMY) and FPR assessed on the test day, as well as the following fertility traits: period from calving to first service (CFS), days open (DO) and calving interval (CI). Genetic parameters were estimated through bivariate analysis, using a random regression model (considering fourth-order Legendre polynomials) and the Bayesian method, with GIBBS2F90 software. Heritability estimates varied between 0.11 and 0.22 for TDMY, between 0.16 and 0.30 for FPR and between 0.03 and 0.05 for the fertility traits. Correlation estimates between TDMY and fertility traits tended to increase from early lactation until approximately day 100, then decreased slightly before continuing to grow until the end of lactation. Genetic correlations between TDMY and FPR were negative throughout lactation, ranging from -0.04 on day 5 to -0.37 in the final third of this period. The genetic correlations between FPR and fertility traits were positive in early lactation and negative in late lactation (except for CFS). These results indicate that TDMY and FPR are heritable and can be used as selection criteria in Holstein cows in Brazil. However, for fertility traits, genetic gains through direct selection may be slow. Additionally, a high level of milk production and FPRs in early lactation negatively impact fertility traits.