Effects of inbreeding and estimation of genetic progress upon preweaning traits in beef cattle

Data on 1571 Angus, 1737 Herefords and 1313 Shorthorns from the BCRS, Front Royal, Va. from 1951 through 1969 (excluding 1960) were used to study the effect of inbreeding on birth and weaning weights, ADG birth to weaning and type score at weaning, and estimate genetic progress from selection in each line of each breed. Inbreeding effects were linear and quadratic regressions of the traits mentioned on inbreeding of the calf and of the dam, each sex separately, after adjustment for years (using repeat matings) and age of dam (using type and selection lines of all breeds). Breed weaning weight regression coefficients ranged from -0.073 to -1.333 lbs. per 1% increase in inbreeding of the calf, significant (P<.01) for Angus and Hereford bulls and heifers, and the Shorthorn heifers; the effect was stronger in heifers than in bulls. For inbreeding of the dam, Hereford and Shorthorn bulls were depressed more than heifers, and vice versa in Angus. Most regressions of birth weight on inbreeding of calf were negative (significant [P<.01] for Hereford bulls and heifers and Shorthorn bulls). Type score regressions on inbreeding of calf were negligible in Shorthorns, but significant in Angus bulls (P<.05) and Hereford heifers (P<.01); those on inbreeding of dam were significant (P<.01) for Shorthorn bulls and heifers and Angus heifers. These effects calculated within line showed great dispersion, and the breed trends disappeared. Phenotypic change, calculated as regression of annual means of each trait (sex unweighted) on years, showed sizable differences between the lines. For example, the range of weaning weight estimates was 1.68 to 2.36; -2.05 to 5.60 and 3.92 to 3.70 pounds per year in Angus, Hereford and Shorthorn. Excepting one Hereford line, the selection lines showed a consistent and positive annual change; the inbred lines showed smaller changes that were negative in three out of four Angus and Hereford lines and positive in all Shorthorn lines. Environmental weaning weight regressions on years (using repeat matings) were negative for Angus and Shorthorns and positive for Herefords; the type score regressions were positive for all three breeds. Estimates of genetic progress, i.e., subtract the environmental change from the phenotypic change, indicate wide Variation and definite genetic improvement in most of the lines. Weaning weight ranges in the selection lines for Angus, Hereford and Shorthorn, respectively, were 2.89 to 3.57, -2.65 to 4.99 and 4.30 to 4.52 pounds; ranges in inbred lines were -0.52 to 1.75, -6.26 to 0.47 and 1.20 to 3.76 pounds. When estimates of genetic progress were adjusted for inbreeding of the calf and of the dam, they were positive in all lines except H-2. Results suggest that effects of inbreeding calculated from several divergent genetic populations (lines) do not accurately estimate the effects for individual lines. The combination of inbreeding with selection can yield positive improvement of growth.