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Investigations into ovine pelvic size and its relationship to repetitive rearing failure require accurate estimation of pelvic dimensions in live animals. Radiographic pelvimetry was used to estimate pelvic dimensions of 84 Merino, 21 Dormer and 20 S A Mutton Merino ewes. Transverse diameter and conjugate diameter were estimated; the area of the pelvic inlet was defined as the product of transverse and conjugate diameters. Dissected measurements obtained from all ewes after slaughter were regressed on estimated dimensions to obtain prediction equations for the correction of radiographic estimates. Prediction equations for the estimation of dissected pelvic dimensions from radiographs were accurate (r greater than or equal to 0.87), irrespective of dimension estimated or breed. Derived equations for estimation of dissected pelvic dimensions for the 3 breeds were not statistically different. It was concluded that pelvic dimensions of ewes could accurately be predicted by radiographic pelvimetry.

Lamb mortality to weaning age was compared in Merino flocks selectively bred from maternal ancestors with high or low lifetime rearing ability. The High Efficiency (HE) flock comprised the descendants of ewes that had reared lambs on 3 or all occasions from 4 rearing opportunities. The Low Efficiency (LE) flock comprised descendants of ewes that had failed to rear any progeny on 2, 3 or all occasions from 4 rearing opportunities. During the first 3 years of testing the response to selection the mean mortality to weaning among HE single lambs was 19.5% of lambs born compared with 32.3% in the LE flock (p less than 0.001). Mean mortality of twin lambs in the HE flock was 21.5% compared to 36.3% in the LE flock (p less than 0.01). In the HE flock significantly fewer single lambs died during and within 3h of birth and significantly fewer twins died during birth and shortly thereafter, and between 3h and 2 days of age. The viability of LE singles and twins may have been lower overall as the mortality rate in most periods was higher than that of their HE counterparts. The results highlighted the major impact of intrinsic maternal and, or foetal defects on lamb survival. It was concluded that rearing ability was heritable, and that selection for rearing ability offers excellent prospects for alleviating the hitherto intractable problem of high perinatal lamb mortality.