REARING PERFORMANCE OF MERINO, SA MUTTON MERINO AND DORMER EWES IN RELATION TO LIVE MASS, REPRODUCTION AND PELVIC DIMENSIONS

Live mass, reproduction and pelvic dimensions (determined mainly by ra diographic pelvimetry at more than 4 years of age) were obtained for e wes of the Langgewens Merino flock (n=102), ewes of the Tygerhoek Meri no flock (n=434) and ewes of the Elsenburg SA Mutton Merino (n=130) an d Dormer flocks (n=130). Ewes were classified as ''good'' or poor'' ac cording to their rearing ability (ewes rearing/ewes lambed - Er/El) an d rearing performance (lambs reared/lambs born - Lr/Lb) averaged over at least 4 lambings in 4 to 5 lambing opportunities. Good ewes tended to be lighter than poor ewes at 18 months (Merino) or weaning (SA Mutt on Merino and Dormer). This tendency was also reflected in live mass a t the time of radiography, being significant (P less-than-or-equal-to 0,01) in Tygerhoek Merinos. Good and poor ewes were similar with regar d to fecundity (lambs born/ewes lambed) and lambing percentage (lambs born/ewes joined), but differed (P less-than-or-equal-to 0,01) markedl y with regard to rearing percentage (lambs reared/ewes joined), Er/El and Lr/Lb. Poor ewes constituted 6,9 to 21,7% of the available ewes in the respective flocks, and were involved in 38,9 to 56,9% of all case s of rearing failure. Good ewes had smaller (P less-than-or-equal-to 0 ,07) transverse pelvic diameters than poor ewes in the Langgewens Meri no and Elsenburg Dormer flocks, while rearing performance was independ ent of transverse diameter in the other two flocks. Good ewes generall y had larger conjugate pelvic diameters than poor ewes, significantly (P less-than-or-equal-to 0,05) so in the case of the 2 Merino flocks. The area of the pelvic inlet of good ewes was larger (P less-than-or-e qual-to 0,05) than that of poor ewes in the Tygerhoek Merino flock and the Elsenburg SA Mutton Merino flock. Phenotypic correlations of conj ugate diameter and pelvic area with Er/El and Lr/Lb were, however, too low (r < 0,20) for accurate predictions. Pooled repeatability estimat es (+/- SE's) for Er/El and Lr/Lb (obtained from the regression of sub sequent performance on records at 2 and 3 years of age) amounted to 0, 06 +/- 0,01 and 0,08 +/- 0,03 respectively. It was concluded that conj ugate diameter and the area of the pelvic inlet were positively relate d to flock rearing performance, despite the fact that rearing failure can also be caused by numerous other factors.