COMPARISON OF PHENOTYPIC VARIATION WITHIN PATERNAL HALF SIB FAMILIES FOR WEANING WEIGHT IN A PUREBRED AND A SYNTHETIC BEEF CATTLE POPULATION

Records of 1487 adjusted weaning weights expressed as deviations from year means in a multibreed Beef Synthetic and a purebred Hereford population raised and managed together under the same selection program from 1968 to 1978 were used to examine the behavior of the phenotypic variation within sire families in the two populations and to further test the conventional common error variance assumption imposed on a sire model. The results indicated that on the average there was no difference in the phenotypic variation within sire families between the purebred and the synthetic populations. Within-sire standard deviations in the two populations were also normally and similarly distributed. Within-sire variances were not heterogeneous. The results suggested that the conventional common error variance assumption imposed on a sire model would be a reasonable approximation applicable to the two populations of different origins. Key words: Beef cattle, variation among half-sibs, weaning weight