SELECTION IN HEREFORD CATTLE II EXPECTED AND REALIZED RESPONSE

Abstract

Data from 14 inbred lines and 14 linecross groups of Hereford cattled at the San Juan Basin Research Centre, Hesperus, were used to evaluate expected and realized response in birth and weapling fraits and post weaning traits in males and females over a 28-year period. There were large differences in the means and variations of the performance traits among the inbreds and lineerosses, the inbreds showing in breeding depression and greater variability while the linecrosses manifested within-breed heterosis. Except for gain from weaning to 12 months, genetic progress was expected in all traits studied, mainly due to sire selection. Regressions of annual trait means on years indicated positive phenotypic trends in the inbreds for heart girth circumference, adjusted weaning weight (adjusted for inbreeding), weaning score, final weight and feed consumption of males and the yearling weights and gains of females. Changes were negative for other traits. In the linecross group phenotypic trends were positive in all traits except heart girth circumference, weaning age, initial test weight and feed efficiency. Estimated genetic progress per generation due to within line selection was negative in the inbreds but was considerably positive for the linecrosses in most of the traits. As expected, between line selection yielded greater genetic improvement in the inbred than in the linecross population. The dissimilar patterns of response in the two populations are attributed to high rates and levels of inbreeding. Although variable, the actual progress was below prediction in most of the traits studied.