Studies on dystocia in sheep II: Pelvic measurements of ewes with histories of dystocia and eutocia

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Abstract
Three groups of mature Romney ewes (⪖ 4 years of age), with different histories of lambing performance, were radiographed to obtain measurements of pelvic dimensions. The ewes in Group I had all been assisted to lamb at least once (group mean 2.3), whereas the ewes of Groups II and III had histories of freedom from dystocia. Group II ewes were selected from a commercial flock and the Group III ewes from a closed flock that had been unshepherded at lambing for approximately 40 years. Pelvic inlet area (conjugate diameter × transverse diameter) differed between groups such that Groups II and III had larger pelvic areas than Group I ewes (p<0.01). Size of the pelvic inlet in relation to birth weight of the lambs also differed between groups, and Group III ewes had the largest, and Group I ewes the smallest, relative pelvic inlet area. Significant correlations were recorded between a number of internal and external pelvic measurements, but were not considered high enough to be of use in predicting internal pelvic dimensions from external measurements. Differences were observed in correlations between internal and external measurements of different groups of mature ewes. Similarly, differences in correlations between internal and external pelvic measurements were found in different age groups, when an additional two groups of 2 and 3-year-old ewes were obtained from the Group III flock, and compared with the mature ewes of that flock. It was concluded that an incompatibility in size between the maternal pelvis and the lamb at birth is largely responsible for the need for repeated assistance at birth. Selection for large pelvic-inlet area by using external measurements (as a means of reducing the incidence of dystocia in the flock) was shown to be impracticable. Furthermore, such a policy ignores the importance of birth weight. Selective breeding for freedom from dystocia in both ewes and rams appears to be a more effective way of overcoming the problem of lambing difficulties.