Estimation of risk factors associated with difficult birth in ewes

In eight closely recorded Australian Merino and crossbred sheep flocks, all lamb deaths were examined and the cause of deaths identified if possible. Dystocia was identified as one of the major causes of lamb death and this study examined factors that could be used to identify ewes at high risk of dystocia, either to avoid dystocia or to assist with early intervention where possible. Dystocia was least common in lambs of ~4.8 kg, but there was a higher risk at both lower and higher birthweights. Dystocia with both low and high birthweight was more common in older ewes, ranging from negligible low birthweight dystocia in ewes less than 3 years old at lambing, to 5% in older ewes. Low birthweight dystocia increased with increasing litter size, with 40% dystocia in ewes at least 4 years of age with triplets. In contrast, high birthweight dystocia was not affected by litter size. A previous record of low birthweight dystocia was a risk factor for future low birthweight dystocia, but the same relationship was not observed for high birthweight dystocia. A high lambing ease score (difficult birth) with high birthweight was a risk factor for future high birthweight dystocia, but this was not the case for low birthweight dystocia. These differences between the risk factors for low and high birthweight dystocia suggest that they have different causes. High ewe liveweight and condition score during pregnancy may be additional indicators of the risk of dystocia, particularly for ewes with high liveweight in the first 60 days of pregnancy. For most ewes dystocia was difficult to predict, but there was a small proportion of ewes with a very high risk of dystocia and if these could be identified in advance they could be monitored much more closely than the rest of the flock.