Effect of health disorders on the hazard of culling on the first or second lactation in Iranian dairy herds.

We conducted a longitudinal retrospective study in order to assess the effect of health disorders (HDs) on culling in the 1st or 2nd lactation cows, in Iranian dairy herds. In total, 7067 first- and second-parity Holstein cows, from 32 Iranian dairy herds, which calved from March 2007 to March 2008, were followed until the next calving or culling. Parametric survival models with time-dependent covariates were used to capture the effect of HDs with different duration and recurrence episodes on the risk of culling. Mastitis, locomotor disorders, ovarian cysts, abortion, diarrhea, rumen disorders, displaced abomasum and respiratory disorders were associated with an increased risk of culling. Hazard ratios and 95% confidence intervals were 2.78 (2.31; 3.35), 3.4 (2.79; 4.13) and 1.62 (1.15; 2.78) for mastitis, locomotor disorders and ovarian cysts as common HDs, respectively. Parity and milk-yield were identified as confounder and effect modifier, respectively. The risk of culling in the presence of uterine infection or traumatic reticulo-peritonitis increased with decreasing milk yield.