Genetic parameter estimates and genetic trends for reproductive traits of Holstein dairy cattle in China

Reproductive efficiency is critical to the profitability and productivity of dairy farms. However, reproductive traits have often been overlooked in favor of enhancing milk production, resulting in unintended genetic antagonism. This study focused on estimating genetic parameters and genetic trends for economically important reproductive traits which include conception rate (CR), days open (DO), age at first calving (AFC), number of services per conception (NS), and calving interval (CInt), using 33,203 records of Holstein cattle that were under intensive management in Shandong Province, China. We employed univariate (heritability estimate) and multivariate (correlation) analyses that integrated phenotypes with the pedigree data spanning 5 generations. The results demonstrated low heritability estimates for reproductive traits, ranging from 0.043 to 0.159, highlighting a limited additive genetic influence but suggesting the possibility of genetic improvement through genomic selection. Among the traits evaluated, AFC demonstrated the highest heritability (0.159) in cows, whereas the lowest values (0.043 and 0.044) were observed for NS and CR, respectively. Strong genetic correlations were observed among the reproductive traits. Conception rate showed a strong and antagonistic genetic correlation with NS (-0.79). Phenotypic correlations reflected the same pattern, with CR and NS exhibiting strong negative correlations (-0.91). The analysis of EBV over time revealed gradual genetic advancements in CInt, DO, and CR, although the progress was inconsistent. These gradual and inconsistent gains suggest that the longstanding emphasis on milk traits may have compromised efforts to improve reproductive performance. Therefore, more focused and sustained genetic selection efforts targeting reproductive characteristics are essential to achieve substantial genetic gains. Future breeding programs that integrate these parameters could enhance fertility traits in Holstein cows in China.