Effects of abortion and other risk factors on conception rate in Iranian dairy herds

In order to estimate costs of abortion, influencing factors on conception rate (CR) need to be identified. Data from 6 commercial herds were used to determine influencing factors on CR in Iranian dairy herds. Insemination observations were assigned to 3-weeks in milk (3-WIM) records from 21 to 294 days after calving (2nd - 14th estrus cycle (3-WIM period)). After eliminating outliers and periods with missing records, the dataset included 200,580 insemination records of 31,830 cows observed between 2004 and 2014. A logistic regression model was developed to evaluate the effects of herd, parity, calving season, cumulative fat corrected milk yield level (CFCML), mastitis in current 3-weeks in milk period (MAS), experience of mastitis in previous 3-WIM periods (EMAS), days in milk for insemination (IDIM) and all significant 2-way interactions on CR. Overall, the CR (total cows pregnant/total number of inseminations) until 294 days in milk was 35.1% ranging from 31.7% to 40.1% across studied herds. The results of logistic regression analysis showed that the CR differs at the herd level. Furthermore, the herd effect significantly interacted with other factors, except EMAS, illustrating that the effects of the investigated factors on CR also differed between herds. The odds of conception decreased with higher parity, MAS incidence, and calving during warm seasons. The CFCML significantly interacted with other factors. Therefore the odds of conception for high or low yielding cows will depend on the herd, parity, IDIM, abortion and EMAS history. There was a general decrease in CR with abortion incidence. However, the probability of conception for aborted cow depended on other parameters in the model. The trend of CR during the lactation was season dependent. For example, for cows calving in winter the CR decreased with lactation stage, while for cows calving in summer the CR increased with lactation stage. Moreover, the predicted probability of conception for different parities, calving seasons, CFCML and EMAS varied across the lactation stages. An experience of mastitis was just an indirect influencing factor because of a significant interaction with IDIM and milk yield level. In conclusion, this study revealed that difference in CR between as well as within studied herds was remarkable. Moreover, calving season (due to for example heat stress in the warmer season) is a more important factor influencing CR than milk production.