Effects of dystocia, retained placenta, and metritis on milk yield in diary cows

The effects of dystocia, retained placenta, and metritis on milk yield were studied in 37,776 Finnish Ayrshire cows that calved during 1993; cows were studied for one lactation. Monthly test day milk yields were treated as repeated measurements within a cow in a mixed models analysis. Index categories were created to relate the time of disease occurrence to the test dates and to capture the short-term effects of diseases on milk yield. Diseases other than dystocia, retained placenta, or metritis were categorized into two groups: diseases occurring within 42 d after calving or diseases occurring later than that. The statistical models for each parity included calving season, stage of lactation, and disease variables as fixed effects. For parities 2, 3, and 4 or higher, the cows were grouped into four categories based on previous lactation milk yield, and the disease effect was studied separately for each yield level. An autoregressive covariance structure was used to model the association among the repeated measurements. Models with a 305-d yield as the outcome were also run; the diseases were treated as binary covariates in these models. Dystocia, retained placenta, and early metritis significantly affected milk yield, as indicated by monthly test day milk yields. Late metritis was not associated with milk loss. The impact of the diseases differed across parities and also across different levels of milk yield. Using 305-d milk yield as the milk measure, no diseases were associated with reduced milk yield.