Selection in Beef Cattle II. Selection Response2

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Abstract
Selection response was studied in three lines of Hereford cattle selected for (1) weaning weight (WWL), (2) yearling weight (YWL) or (3) index of yearling weight and muscling score (IXL). Selection response was evaluated by several measures of offspring regression on selection in parents. Average estimated response, expressed in standard deviation units per generation, in the three lines, WWL, YWL and IXL, were: birth weight, 0.22, 0.28 and 0.28; weaning daily gain, 0.20, 0.13 and 0.12; weaning weight, 0.23, 0.17 and 0.15; postweaning daily gain 0.28, 0.42 and 0.33; yearling weight, 0.36, 0.43 and 0.33 and muscling score, –.03, 0.01 and 0.24, respectively. Birth weight response per unit of selection was 0.47 and appeared highly correlated genetically with all performance traits. Expected increase in birth weight could be reduced 30% if all emphasis on growth was directed to postnatal growth rate rather than weaning or yearling weight. Response per unit of selection applied was low for weaning gain or weight (0.10 or 0.12) relative to other traits. Selection for postweaning gain or yearling weight may increase weaning gain or weight more than direct selection for these traits. Response in postweaning gain and yearling weight per unit of selection applied was relatively large (0.37 and 0.48). Correlated responses to selection in the three lines suggest that a wide variety of selection patterns will lead to improvement in all traits even though optimum selection indexes may maximize improvement in particular traits. Copyright © 1974. American Society of Animal Science . Copyright 1974 by American Society of Animal Science.

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