CORRELATED RESPONSE IN BIRTH WEIGHT, GROWTH RATE AND CARCASS MERIT UNDER SINGLE-TRAIT SELECTION FOR YEARLING WEIGHT IN BEEF SHORTHORN CATTLE

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Abstract
Data for male calves from the most recent 5 yr of a long-term (11 yr) selection experiment with beef Shorthorn cattle were analyzed to calculate the direct and correlated response of intensive selection for yearling weight on growth and carcass characteristics. Least squares estimates of line differences (select vs. control) for the final year indicated cumulative selection responses [Formula: see text] of 41.5 and 46.2 kg in yearling weight for the two herds. Associated with this selection response in the two herds were increases in birth weight (3.6 and 3.0 kg; [Formula: see text]) in weaning weight (16.2 and 7.8 kg; [Formula: see text] and NS) and daily gain from birth to 10 mo of age (104 and 118; [Formula: see text]). The indirect effects on carcass merit were higher percentage of bone (0.6 and 0.7; [Formula: see text]), and a reduced lean/bone ratio (−0.19 and −0.11; [Formula: see text] and NS). Other differences were not statistically meaningful. The heritabilities for growth characters calculated as son/sire regressions were: birth wt 0.30 ± 0.13; weaning wt 0.34 ± 0.22; yearling wt 0.47 ± 0.23; weaning wt/yearling 0.38 ± 0.21; and daily gain 0.22 ± 0.16. For the most important carcass merit measurements the heritabilities, calculated as half-sib correlations, were: % lean 0.03 ± 0.21; % bone 0.86 ± 0.29; % fat (ext.) 0.01 ± 0.21; % fat (int.) 0.19 ± 0.23; lean/bone 0.72 ± 0.28; and hindquarter wt/forequarter wt 0.24 ± 0.24.

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