SELECTION INTENSITY AND RESPONSE TO SELECTION FOR YEARLING WEIGHT IN BEEF CATTLE

Abstract

Ten years of selection for yearling weight in two replicate herds of beef Shorthorns has resulted in genetic increases of 4.8 ± 3.1 and 4.1 ± 3.0 kg/yr in males and 3.3 ± 2.7 and 2.3 ± 1.5 kg/yr in females. These increases were measured as deviations from an unselected control line mean and comprise about 40–45% of the absolute increase in yearling weight that took place during the selection program. The accumulation of selection differential was irregular at first but, in the last 6 yr of the program, advanced regularly at rates of 12.1 ± 0.7 and 11.2 ± 0.9 kg/yr for males and 9.1 ± 0.3 and 7.6 ± 0.4 kg/yr for females. Estimates of realized heritability derived from the two herds were.49 ±.12 and.51 ±.16 for males and.50 ±.11 and.29 ±.08 for females. A separate evaluation, based on line crosses within one herd, tended to confirm the heritability estimates.