Estimates of Genetic Parameters for Milk Production Traits and Lactation Persistency in Holstein Cows Using Multi-Trait Models

This study was conducted on productive data (n=3399) collected from 1800 Holstein cows that calved between 2009 and 2020. The present study aimed to investigate the fixed effects of parity, calving season, and calving year on milk production traits, and to estimate the relevant genetic parameters. These traits encompass total milk yield (TMY), standardized 305-day milk yield (305d-MY), lactation length (LL), peak yield (PY), lactation persistency (LP), total fat yield (TFY), and total protein yield (TPY). Genetic parameter estimation was conducted using multivariate mixed models utilizing average information restricted maximum likelihood (REML) procedures via the MTDFREML program algorithm. Parity significantly affected LP (P<0.05), with cows in higher parities (≥5) exhibiting lower persistency compared to those in lower parities (1st to 4th). The traits studied displayed moderate heritability and high repeatability values, with heritability estimates ranging from 0.26 to 0.37 and repeatability estimates from 0.55 to 0.72. 305d-MY, LP, and TPY exhibited the highest repeatability values. Moderate genetic correlations (0.37 to 0.40) were found between LP and productive traits (TMY, 305d-MY, LL, and TPY), while LP showed slight correlations with TFY and PY (rg= 0.11 and 0.15, respectively). In summary, the study suggests that LP's moderate heritability and higher repeatability values facilitate genetic improvement through selection, advocating for the evaluation of a cow's persistency based on its first lactation. Additionally, the positive phenotypic and genetic correlations between lactation persistency and milk yield imply that selecting for increased milk yield could enhance lactation persistency.