Twelve 380 kg beef steers were fed a 90% concentrate, cracked corn- based diet at 85% of ad libitum. Steers were divided into three daily ionophore treatment groups, Monensin - Tylan (MT), Lasalosid (L) or rotation of MT and L (MTL). Each daily dosage of Monensin and Tylan was 2.7 and 1.1 mg per BW,7S, respectively. Lasolosid additions were given at 3 mg per BW'75 daily. Ruminal samplings were taken on d 0, 2, 5, 16 and 45 after beginning ionophore treatments. A conventional digestibility trial was conducted with nine steers (three per treatment) at day 34 of feeding the ionophore and lasted for six days. Digestible energy (83.7%), urinary losses (3.3%) of total feed energy and the ratio of ME/DE (90%) did not differ across treatments. Methane losses were from 17 to 37% less on d 2 and 5 for MT and MTL treatments but increased quadratically (PO.05) on d 16 and 45 so that production exceeded d 0 levels. Treatment L did not affect methane production. Acetate to propionate and to butyrate ratios were reduced (PO.05) by MT and MTL with no effect for L addition. These data suggest that methane suppression does not persist beyond two weeks of ionophore treatment and a daily rotation of ionophores had no digestive or metabolic advantage over feeding MT daily. Further research is recommended to investigate the proper dose, componation and duration of ionophores supplementation in order to achieve the best results out of the fattening operations.