Effects of clinical mastitis on milk yield in dairy cows

The effect of clinical mastitis on milk yield was studied in 24,276 Finnish Ayrshire cows that calved in 1993 and were followed for one lactation (i.e ., until culling or the next calving). Cows that had only mastitis, but no other diseases, and cows that had no diseases (healthy cows) during the lac tation were included in the study. Monthly test day milk yields were treate d as repeated measurements within an animal in a mixed model analysis. Mast itis index categories were created to relate the timing of mastitis to the test day milk measures. Statistical models (a separate model for each parit y) included fixed effects of calving season, stage of lactation, and mastit is index. An autoregressive correlation structure was used to model the ass ociation among the repeated measurements. The effect of mastitis occurring at different periods during the lactation was studied. The daily loss durin g the first 2 wk after the occurrence of mastitis varied from 1.0 to 2.5 kg , and the total loss over the entire lactation varied from 110 to 552 kg an d depended on parity and the time of mastitis occurrence. Regardless of the time of occurrence during the lactation, mastitis had a long-lasting effec t on milk yield; cows with mastitis did not reach their premastitis milk yi elds during the remainder of the lactation after onset of the disease.