Estimation of Genetic Parameters of Reproductive Traits in Holstein Cattle from Southern China

Reproductive performance is a critical determinant of dairy farm productivity and profitability. This study aimed to estimate genetic parameters for key reproductive traits in Holstein cattle from southern China. Phenotypic data comprising 117,514 records from 44,861 cows across eight farms were analyzed. Genetic parameters were estimated separately for heifers and multiparous cows using single-trait animal and repeatability models via the AI-REML module in DMU. Bivariate and multi-trait models were employed to estimate trait correlations within and across parity groups, respectively. The results indicate that heifers generally exhibited superior reproductive efficiency compared to multiparous cows. Heritability estimates ranged from 0.04 to 0.47 in heifers, with age at first service (AFS) being the highest (0.47 ± 0.02), while estimates for multiparous cows were consistently low (0.03-0.14). Strong genetic correlations were observed among functionally related traits, particularly in multiparous cows. For example, high positive genetic correlations were detected between Number of Services (NSc) and Interval from First to Last Service (IFLc, 0.91), NSc and Days Open (DO, 0.87), Interval from First to Last Service (IFLc) and Calving Interval (CI, 0.94), and IFLc and DO (0.96). Multi-trait analysis revealed moderate genetic correlations (0.13-0.85) for the same trait across heifers and different cow parities, with gestation length (GL) showing the strongest correlation (0.99) across all parities. The findings confirm that most reproductive traits in this population, especially in multiparous cows, exhibit low heritability and are strongly influenced by management and environment. This study thereby delivers practical guidance for the genetic improvement of low-heritability traits and for the production management of Holstein cattle in southern China.