Performance data on nearly 16.000 head of cattle that were used in trials to document effects of monensin on feedlot cattle were summarized. Cattle fed monensin-containing diets gained 1.6% faster, consumed 6.4% less feed and required 7.5% less feedi00 kg gain than cattle fed control diets. Monensin resulted in the greatest improvement in feed at 2.9 Meal metabolizable energy(ME)kg diet dry matter DM). gain Within the range of monensin concentrations used in the trials that were summarized(31.8 +/- 7.5 mgkg DM), high monensin concentrations did not improve feed gain over that obtained with lower concentrations. Carcass characteristics not significantly influenced by monensin. Responses were of cattle to monensin and implants additive. Energy metabolism data suggested that monensin were improved digestibility of DM,reduced fsting heat production and increased dietary net energy maintenance(NEm) values more than it increased net energy gain(NEg) values. Data showing the response of cattle to monensin when fed dietary protein concentrations or sources of supplemental various N suggested that monensin had a protein sparing effect. Monensin has also been shown to reduce lactic acid production, aid in the control of coccidia and bloat and to be toxic to face and hom fly larva in feces of monensn-fed catte.In pasure trias, monensin improved daily gais When fed to beef cows monensin reduced of feed required to maintain cow weight. amounts