Risk factors and population attributable fractions for displaced abomasum in Iranian dairy cattle: a retrospective analysis of field data.

Displaced abomasum (DA) usually occurs in the early postpartum period and can negatively affect production and welfare of dairy cows. The objective of this research was to identify predictors for the occurrence of DA in Iranian Holsteins. Therefore, we analyzed data collected from six commercial dairy farms in Iran (calving between January 2011 and October 2020). We used generalized linear mixed-effects model to identify predictors based on estimates of odds ratios (ORs). Population attributable fractions (PAFs) were then calculated, as these are expected to more accurately prioritize risk factors on population level by considering the association between the risk factor and the its prevalence in the population. Over the 6 herds, the average percentage of DA was 2.5% (range 1.1-3.4% across herds). The results showed that calving during winter increased the odds of DA 1.4 times. Cows with high milk fat-to-protein ratio before the occurrence of disease, abnormal calving (including dystocia, stillbirth, and twinning) and long pregnancy length (≥ 280 d) showed increased odds of DA with ORs of 1.52, 1.31 and 1.30, respectively. Milk yield was also positively associated with DA in next lactation. Among the modifiable predictors of DA, the highest PAF was for the ratio of milk fat-to-protein (11.0%); other PAFs were retained placenta (4.1%), metritis (3.1%), abnormal calving (2.7%), and ketosis (2.7%). The results of the present study can provide managers with the a helpful strategy to reduce the occurrence of DA.