Culling of dairy cows. Part III. Effects of diseases, pregnancy status andmilk yield on culling in Finnish Ayrshire cows

The effects of 15 diseases, pregnancy status and milk yield on culling were studied in 39727 Finnish Ayrshire cows that calved in 1993 and were follow ed until culling or next calving. Survival analysis, using the Cox proporti onal hazards model, was performed with diseases, pregnancy status and milk yield as time-dependent covariates, Effects of parity, calving season and h erd were also accounted for. Pregnancy status was the single most influential factor affecting culling d ecisions, followed by milk yield. Several diseases also had a significant e ffect on culling, the most influential ones being mastitis, lameness, teat injuries, and milk fever. The effects of all of these factors varied accord ing to the stage of lactation. Milk yield had a significant effect on culling decisions, depending on the stage of lactation. At the beginning of lactation, milk production did not have any effect on culling decisions, but later on, the highest producers w ere at the lowest risk of being culled and the lowest producers had the hig hest risk. Adjusting for milk yield modified the effects of parity, most di seases and also pregnancy status on culling. Effects of parity increased af ter including milk yield in the model, indicating that milk yield and parit y are interrelated in their effects on culling. The effects of pregnancy st atus also increased towards the end of lactation when milk yield was accoun ted for in the model. The effects of mastitis, teat injuries and lameness d ecreased after adjusting for milk production. These diseases lower milk yie ld and thus, part of their effect on culling was mediated through milk prod uction. The effects of anestrus and ovarian cysts were mainly modified by p regnancy status, but not by milk yield. The effects of milk fever on cullin g increased at the beginning of lactation after including milk yield in the model. This suggests that even though cows with milk fever tend to be high er producers, it is the disease as such that triggers the culling decision early in the lactation. The changes in the effects of other diseases after adjusting for milk yield varied, depending on the disease and the stage of lactation. (C) 1999 Elsevier Science B.V. All rights reserved.