Technical Abstract: One condition for unbiased prediction of random effects is correct specification of the linear model. The difference in preweaning growth of calves nursing two-year-old dams compared with contemporaries nursing older dams is accentuated in the Miles City Line 1 Hereford herd relative to the differences implied in national cattle evaluation. Three analyses of 205-day weight records for 4998 calves were conducted. Fixed effects in analysis 1 were contemporary groups (year, sex of calf subclasses), age of dam (2,3,4,5-10, & 11+ years of age) and linear regressions on inbreeding of calf and dam. Analysis 2 was conducted using the same model after adjustment of the 205-day weight records using factors from the American Hereford Association's Total Performance Records program. Analysis 3 was similar to analysis 2, except age of dam was omitted from the model. Predictions of direct and maternal additive genetic effects on 205-day weight for sires and maternal grandsires were extracted from each analysis. As differences from the 5-10 year old age effect, estimated age of dam effects from analysis 1 were 45, 19, 6, & 19 kg for 2,3,4, & 11+ age of dam classes, respectively. Comparable estimates from analysis 2 were 20, 6, 1, & 14 kg. The rank correlations of expected progeny differences for direct and maternal 205-day weight from analyses 1 & 3 were .97 & .72, respectively. These data clearly indicate bias of industry standard age of dam correction factors in adjusting 205-d weight of Line 1 Hereford cattle raised at Fort Keogh Livestock and Range Research Laboratory. Also shown is potential for biased genetic evaluation when adjustment factors to correct for dif- ferences in age of dam are not appropriate to the targeted population.