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Selecting for Weight and Rate of Gain in Inbred Lambs

U. S. Department of Agriculture¹, Dubois, ID 83423

Abstract

A total of 389 Rambouillet, 260 Targhee and 150 Columbia inbred ewe lambs, which averaged 28, 26 and 28% inbreeding, respectively, were weighed at birth in April, at weaning in August and at about monthly intervals thereafter through shearing in May. Postweaning gains were calculated for feeding periods from October through December on mature, dry, fall range forage and from January through April on alfalfa-hay pellets. Gains were near zero during the early postweaning period but averaged about .08 kg per day during the later period. Heritabilities tended to increase with time after weaning as did components of additive genetic variation. Both genetic and phenotypic correlations tended to decrease as time increased between observations on correlated variables. Heritabilities of later postweaning gains under a good nutritional environment were two to 10 times those of early postweaning gains under a poor environment. Genetic variation in gains was four to 15 times higher under the better environment. Various selection indexes for maximizing genetic progress in improving weights and gains were developed and compared. Indexes most efficient for selection within the inbred lines contained the same selection criteria, with about the same emphasis per criterion, as those most efficient (when adjusted to a noninbred basis) for selection in the base population from which the lines were developed. However, genetic progress was expected to be generally from 5 to 15% slower from selection within the inbred lines because of reduced genetic variation and resultant reduction in accuracy of indexes. A composite index combining genetic merit in weaning weight and postweaning daily gain was developed to maximize economic return as well as genetic progress in improving weight at a specified age.

¹ Agricultural Research Service, U. S. Sheep Experiment Station in cooperation with the University of Idaho, Moscow.