A mathematical model was developed to simulate a cow-calf-feedlot production system with either weaning and(or) feedlot end points. The biological processes of maintenance, growth, lactation, estrus, and parturition, with their related energy consumptions were mathematically described and integrated in a computer pro g r a m based upon a network approach to modeling. Input prices, output prices, and interest rates were chosen to reflect the economics of east- central U.S. beef production. With both the economic and physiological processes modeled, the effects of culling based on cow age and of selection favoring increased wea n i n g and yearling weights were examined. The results show that selection for adjusted 205-d weight in replacement heifers and 365-d weight in bulls is an effective strategy when the purpose is to increase profit. Selection favoring high weight at a given age had a significant effect on profit at both weaning and feedlot end points. Culling cows at progressively younger ages was shown to be an unprofitable practice: culling cows at younger ages tended to decrease weaning rates and changed the product ratio toward more cows and fewer calves. Given the disparity between the value p er kilog r a m of cull cows and slaughter heifers, culling based on cow age decreased the average value per kilogram of beef produced and d e creased profit at both weaning and feedlot end points.