Risk factors for calving assistance and dystocia in pasture-based Holstein-Friesian heifers and cows in Ireland.

The incidence of and risk factors associated with calving assistance and dystocia in pasture-based dairy herds were determined from analysis of 152,641 records of full-term calvings from Holstein-Friesian dams served by artificial insemination (AI) sires of seven breeds in herds of 20 calvings or more per year over 4 years. The overall incidence of calving assistance and dystocia was 31.1% and 6.8%, respectively. The incidence in primiparae and pluriparae was 40.0% and 28.2% for assistance, and 9.3% and 5.8% for dystocia, respectively. Association analyses were undertaken using generalised estimating equations using a logit link function. The likelihood of calving assistance or dystocia did not change over time but was greatest in autumn and in spring. The likelihood of calving assistance or dystocia was greater for males in primiparae and males sired by Charolais sires. The odds of calving assistance and dystocia were greater in twin calves (OR 2.0 and 2.4; P<0.001) and in dams that had dystocia at the previous calving (OR 1.65 and 2.9; P<0.001). The logit of the probability of calving assistance and dystocia increased linearly per unit increase in sire predicted transmitting ability for direct calving difficulty. The probability of assisted calving, but not dystocia, increased linearly in primiparae as animals calved at a younger age relative to the median age at first calving. Herd size and day of the week of calving were not associated with the odds of dystocia. Stakeholders must focus on identified modifiable risk factors to control the incidence of dystocia in dairy herds.