Selection for weaning weight in Merino sheep. 1. Direct response to selection

The heritability of corrected weaning weight has been estimated by dam-offspring and paternal half-sib correlations in a random-bred, medium-wool Peppin Merino flock. All weights were corrected for age and type of birth within each year. The dam-off-spring heritability estimates were 0.28 � 0.10 for ewes and 0.32 � 0.11 for rams. The paternal half-sib estimates were 0.18 � 0.07 for ewes and 0.19 � 0.13 for rams. Responses in flocks selected for high and low weaning weight for four generations (1951 to 1961) were used to calculate realized heritabilities. These were 0.33 � 0.03 and 0.18 � 0.06 for ewes and rams respectively in the high weaning weight flock, and 0.22 � 0.10 and 0.23 � 0.19 for ewes and rams in the low weaning weight flock. The average increases in inbreeding coefficients since selection commenced were 3.97 per cent in the high weaning weight flock, 3.58 per cent in the low weaning weight flock and 0.39 per cent in the randomly selected control flock. The regressions of weaning weight on inbreeding, independent of the effects of selection, ranged from -0.076 to -0.387 with an average of -0.191 lb weaning weight per one per cent inbreeding.