Selection in Hereford cattle. II. Expected and realized response.

Data from 14 inbred lines and 14 linecross groups of Hereford cattle at the San Juan Basin Research Center, Hesperus, were used to evaluate expected and realized response in birth and weaning traits and postweaning traits in males and females over a 28-yr period. There were large differences in the means and variances of the performance traits among the inbreds and linecrosses, with the inbreds showing inbreeding depression and greater variability among lines, while the linecrosses manifested within-breed heterosis. Except for gain from weaning to 12 mo, in females, 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 at birth, adjusted weaning weight (adjusted for inbreeding), weaning score, final weight, feed consumption and the yearling weights and gains in 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 most of the traits in the inbreds but was considerably positive for the linecrosses for most of the traits. As expected, between-line selection yielded greater genetic improvement in the inbred than in the linecross population. The different 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.