Factors affecting length of productive life of Brazilian Holstein cows assessed using survival analysis

Context Length of productive life is defined as the number of days from first calving to the last day of milking records, when the cow is no longer sufficiently productive. Although it has economic implications, length of productive life has not yet been included in Brazilian dairy cattle breeding programs because it depends on other traits such as age at first calving, milk production level and disease occurrence. In addition, its evaluation occurs later in life, and heritability coefficients are low. Aims. The aim of this study was to identify the effects of both age at first calving and accumulated 305-day milk yield in the first lactation on length of productive life in Holstein cows. Methods. Official dairy control data of 31 562 Holstein cows, calved from 2010 to 2014, in 321 herds from Parana state, Brazil, were analysed by using the Kaplan-Meier estimator and Cox regression model. Key results. First calving at <21 months was associated with shortest productive life (1197.01 +/- 232.40 days), whereas first calving at 24 months was associated with the lowest hazard ratio (0.572), suggesting that this is the most suitable age of first calving for a longer productive life. Low-yielding cows (first lactation yield <6392 kg) had the second lowest survival probability (47.20%), the shortest productive life (1150.60 +/- 234.51 days) and the highest voluntary hazard ratio (1.30) and were prematurely culled from the herds. High-yielding cows (first lactation yield >12 024 kg) had lower culling risk at the first lactation. Conclusions. Cows that started their reproductive life similar to 16 months of age (first calving at 24 months) had the longest productive life and an average first lactation milk yield similar to 9000 kg. Implications. Reproductive management practices could affect length of productive life through effects on age at first calving and first lactation milk yield. Most of the Holstein semen used in Brazil comes from countries where longevity has already been included in selection indexes; thus, early identification of cow longevity could be used for culling decisions.