Systematic error in genetic evaluation of miles city line 1 hereford cattle resulting from preadjustment for age of dam.

Differences in preweaning growth of calves nursing 2- and 3-yr-old dams compared with contemporaries nursing older dams are accentuated in the Miles City Line 1 Hereford herd relative to age-of-dam (AOD) effects implied by preadjustment of 205-d weight in national cattle evaluation. Mixed-model analyses of 205-d weight that fit random individual direct effects and maternal genetic and permanent environmental effects on 4,998 calves were conducted to 1) determine the magnitude of residual AOD effects after preadjustment (PA) using industry-standard procedures and 2) compare changes in genetic predictions resulting from either PA or simultaneous adjustment (SA) for AOD. Expressed as differences from the 5- to 10-yr-old age effect, simultaneously estimated AOD effects were 45 +/- 1, 19 +/- 1, 6 +/- 1, and 19 +/- 3 kg for 2, 3, 4, and 11+ AOD classes, respectively. Comparable estimates of residual AOD effects after PA were 20 +/- 1, 6 +/- 1, 1 +/- 1, and 14 +/- 3 kg. Rank correlations of direct (BVd) and maternal (BVm) breeding values (BV) for 205-d weight from the analysis using PA with BV predicted using SA for AOD were .98 and .77, respectively. Estimated genetic trends were also affected by the method of accounting for AOD effects. One hundred fifty replicate simulations of 205-d weights with pedigree, fixed effect, and variance-covariance structures corresponding to the experimental population were used to establish correlations (r) of predicted BV with underlying true values. The r of predicted BVd with true values were reduced less than .02 by PA compared to SA in accounting for AOD. However, r of predicted BVm with true values were reduced more than .13 by PA compared to SA in accounting for AOD. These data indicate potential for systematic error in genetic evaluations that apply standard adjustments for AOD to 205-d weight.