The association between cumulative milk yield, days open, and days to first breeding in New York Holstein cows

The associations among milk yield, days open, and days to first breeding were studied in 15,320 Holstein cows, calving between June 1990 and November 1993, in 26 New York herds. Survival analysis, incorporating the Cox proportional hazards model, was used to quantify the effect of milk yield and other covariables on days open and days to first breeding. Sensitivity analysis was also performed. Sixty-day cumulative individual milk yield, calving season, parity, and herd were included in all models. Several diseases and their lactational risk also were studied as potential confounders: retained placenta (5%), milk fever (1%), ketosis (3%), metritis (6%), abomasal disorder (5%), mastitis (5%), and cystic ovary (3%). Cows yielding milk in the highest quintile had a slightly lower conception rate than did cows yielding milk in the lowest quintile. Conception rate varied with parity and season. Retained placenta, metritis, and cystic ovary lowered the conception rate. High yielding cows were more likely to be inseminated than were low yielding cows. Older cows and those with metritis, mastitis, and cystic ovary had lower rates of insemination. These results indicate that conception and insemination might be influenced by factors related to management (e.g., culling) and to the cow (e.g., disease history) but that increased milk yield plays a very minor role.