COMPARISON OF SELECTION BY INDEPENDENT CULLING LEVELS FOR BELOW-AVERAGE BIRTH-WEIGHT AND HIGH YEARLING WEIGHT WITH MASS SELECTION FOR HIGH YEARLING WEIGHT IN-LINE-1 HEREFORD CATTLE

Mass selection by independent culling levels (YB subline) for below-av erage birth weight (BWT) and high yearling weight (YWT) was compared w ith single-trait mass selection (YW subline) for high YWT in the inbre d population of Line 1 Hereford cattle at Miles City, Montana. There w ere 4.2 generations of selection in YB and YW. Heritability estimates for the base population derived from multiple-trait REML were .28 and .31 for direct effects and .16 and .06 for maternal effects on BWT and YWT, respectively. Mid-parent cumulative selection differentials for BWT of YB and YW diverged ( -2.9 vs 8.2 kg, respectively), as did the associated genetic trends for direct effects (-.014 kg/yr vs .105 kg/y r, respectively). Mid-parent cumulative selection differential for YWT of YB (102.1 kg) was 64% of that attained in YW (160.7 kg). Likewise, response in YWT of YB (.91 kg/yr) was 61% of response attained in YW (1.5 kg/yr). For BWT and YWT, maternal genetic trends were similar acr oss selection lines. Assistance at parturition of first-parity 2-yr-ol d heifers was consistently less frequent in YB than in YW.