A deterministic beef cattle computer simulation was used to estimate effect of genetic change in mature size (WMA), maximum daily milk production (PMA) and 60-d conception rate (CP) on costs per 100 kg slaughter weight (COST) and costs per 100 kg weaned weight (WNCOST) both adjusted for cull cow sales. Genetic inputs were defined as peak values for mature cows in an unrestricted environment and ranged from 400 to 800 kg WMA, 6 to 30 kg/d for PMA and 75 to 95% for CP. Two planes of nutrition for wintering cows, high and low and two economic schemes, high. versus low cow herd costs, were simulated. In all four systems, WMA and CP had quadratic effects (R² > .99) on cost over the range simulated. PMA effects were also curvilinear but could not be described by a simple quadratic curve (R² < .63). PMA appears to have a feasible range within which COST and WNCOST are little changed by changes in PMA. PMA levels below the lower end of the range were associated with increased costs due to increased calf death losses. Levels above the upper limit increased costs due to increased lactational stress and lower reproductive rates. The width of this range was reduced by lowering the plane of nutrition for wintering cows. The phenotypic manifestations of these three traits were also investigated, weaning rate (WRATE), weaning weight of an average 7 mos old steer (W117) and mature size (WMA). The results for these traits are similar to the genetic variables. The worth of a phenotypic standard deviation change in each trait (ah²) predict that a change in PMA or W117 is worth the most when outside the feasible range, followed by WMA and then CP or WRATE when COST is the measure. When WNCOST is the economic measure, PMA or W117 is again the most important of the traits but is now followed by CP or WRATE and then WMA. / M.S.