The effect of veterinary-treated clinical mastitis and pregnancy status on culling in Swedish dairy cows

The interaction of the effects of pregnancy status and veterinary-treated clinical mastitis on culling in Swedish dairy cattle was analyzed with survival analysis. The data were from 978,780 cows with first calvings between 1988 and 1996. Four breeds (Swedish Red and White (SRB), Swedish Friesian (SLB), Swedish Polled Breed and Jersey) were included in the analysis, together with the SRB x SLB crossbreds. Length of productive life was defined as the number of days between first calving and culling or censoring (end of data collection). The model (Weibull proportional hazard) included the interaction of parity by pregnancy status by veterinary-treated clinical mastitis, peak test-day milk-yield deviation within herd-year-parity, age at first calving, year by season, region, breed, herd production level, and the random effect of herd. The effects of pregnancy status and veterinary-treated clinical mastitis were modeled as time-dependent covariates. The lactation was divided into five stages during which a veterinary-treated clinical mastitis and culling might occur and in which the pregnancy status was assumed to be known and culling could occur. Open cows had a pronounced effect on culling: they had a very high risk of being culled in all lactations, and it was even higher if they were treated for mastitis in early lactation. For pregnant cows, the later they got pregnant during the lactation, the greater their risk to be culled. The risk associated with cases of veterinary-treated clinical mastitis remained important throughout the lactation.