Heritability of Weaning Weight and Staple Length in Range Rambouillet Lambs

Estimates of heritability were calculated for weaning weight and staple length at weaning age from data taken on the breeding flocks of the Western Sheep Breeding Laboratory, Dubois, Idaho. The data consisted of records taken on 2183 lambs born in 1941 and 1942 and on 892 dam-offspring pairs. The methods of estimation were based on the half-sib correlation and the intra-sire regression of offspring on dam, modified to discount the effects of non-random mating systems. Some of the lambs were produced in inbred lines and some by top-crossing inbred sires to noninbred females. The effect of these systems is to increase the half-sib correlation as compared with that expected in noninbred populations. Inbreeding causes a decrease, but top-crossing has no effect upon the intra-sire regression of offspring on dam. Estimates of heritability by the half-sib and by the intra-sire regression methods were reasonably consistent, indicating that the effects of the mating systems had been adequately discounted. The average estimates for heritability of weaning weight and staple length were 0.30±.04 and 0.40±.05, respectively. Since the records had been adjusted for several environmental factors, which accounted for about 50 and 20 percent of the variance in weaning weight and staple length, respectively, it appears that in unadjusted data heritability of weaning weight and staple length may be no more than 0.15 and 0.32, respectively. The maximum gain expected from selecting for either weaning weight or staple length alone is about 0.9 pound or 0.06 cm. per year, respectively. When allowance is made for emphasis on other traits, as is necessary in a properly balanced breeding program, it seems probable that the gains actually made will not be more than ½ or ¼ of the above figures.