Genetic and phenotypic parameters for subclinical mastitis occurrence in pasture-based Girolando cows from a research herd in Southeast Brazil

<h2>Abstract</h2> Subclinical mastitis is still one of the most important health issues in dairy herds, leading to substantial economic losses due to reduced milk yield, treatment costs, and premature culling. Understanding the genetic basis of this disorder is essential in supporting selection strategies to enhance udder health and herd productivity. The aim of this study was to estimate the genetic and phenotypic parameters for subclinical mastitis occurrence using information from 748 lactations of 279 pasture-based Girolando cows from the Federal Institute of Triângulo Mineiro (IFTM)–Uberaba research herd, calved between 2012 and 2024. Variance components were obtained using a Bayesian approach under a complete animal model. The heritability estimates (posterior medians [highest posterior density interval]) for subclinical mastitis were 0.14 [0.09–0.20] and 0.15 [0.10–0.20] for the right (RF) and left (LF) front quarters, and 0.13 [0.08–0.18] and 0.15 [0.09–0.20] for the right (RR) and left (LR) rear quarters, respectively. The repeatabilities were 0.41 [0.37–0.45] and 0.35 [0.31–0.39] for RF and LF, and 0.35 [0.32–0.39] and 0.41 [0.37–0.45] for the RR and LR quarters, respectively. The low to moderate heritabilities indicate that subclinical mastitis is highly influenced by environmental factors, and the moderate repeatabilities estimates indicate that these traits will benefit from a more frequent data collection. The results suggest that the genetic progress for subclinical mastitis in pasture-based Girolando cows will be slow. Genetic and phenotypic correlations among quarters were also estimated using a multiple-trait model. Genetic correlations were consistently high, exceeding 0.80 for all quarter combinations, indicating a largely shared genetic background controlling susceptibility to subclinical mastitis across udder quarters. In contrast, phenotypic correlations were moderate ranging from 0.50 [0.47–0.54] between RF and LR, and 0.59 [0.56–0.62] between RF and LF, suggesting that environmental and management-related factors contribute more substantially to differences in mastitis occurrence among quarters than genetic effects. Overall, these results suggest that selection for reduced subclinical mastitis in one quarter is expected to result in favorable correlated responses in the others. However, given the limited dataset size, further studies using larger multiherd populations in tropical production systems, and genomic information, are necessary to validate these correlations and to investigate potential breed composition differences in udder and teat morphology that can affect mastitis susceptibility.