Selection for weaning weight and postweaning gain in Hereford cattle. I. Population structure and selection applied

Irgang, R.; Dillard, E.U.; Tess, M.W.; Robison, O.W.

Journal of Animal Science 60(5): 1133-1141

1985


ISSN/ISBN: 0021-8812
PMID: 4008361
Accession: 001457018

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Abstract
Single-trait selection was practised in 3 lines of Hereford cattle derived from a common base population. Selection was practised on malemale only, within sire families, for increased body weight at weaning in the WW line, for postweaning gain in the PG line, and at random in the control line. Females were culled for age or reproductive failure. Progeny of selected bulls were produced in 2 herds from 1970 to 1981. The data consisted of records on 2467 progeny of 125 sires and 922 dams. The number of generations of selection prior to the 1981 calf crop averaged 1.96, 1.85 and 1.80 for the 3 lines resp. For calves born in 1981, mean cumulative selection differentials were 54.5 kg in the WW line and 37.8 kg in the PG line. Corresponding values in standard deviation units were 2.31 and 1.68 resp. Secondary selection differentials were 25-40% as large as selection differentials for the primary traits. Unintentional selection in the control line in 1981 was 16.2 kg (0.68 sigma ) for body weight at weaning, and 0.2 kg (0.01 sigma ) for postweaning gain. Regressions of cumulative selection differences on year were 4.1 kg (0.17 sigma ) in the WW line and 3.2 kg (0.14 sigma ) in the PG line. Realised selection differentials were approximately 88% of the potential selection differentials in both lines. Inbreeding coefficients of dams and calves in 1981 were 2.0 and 3.5% resp. in the WW line, 2.1 and 3.5% in the PG line, and 2.9 and 5.8% in the control line. .