Two experiments were conducted to evaluate digestion kinetics of alfalfa (Medicago sativa L.) substitution for grass hay in beef cattle. In Exp. 1, forage combinations evaluated in situ consisted of 0% alfalfa-100% big bluestem (Andropogon gerardi Vitman), 25% alfalfa-75% big bluestem, 50% alfalfa-50% big bluestem, and 100% alfalfa-0% big bluestem. Nonlinear regression was used to determine the immediately soluble fraction A, the potentially degradable fraction B, the undegraded fraction C, and the disappearance rate of DM and NDF. Dry matter fraction A increased linearly (P = 0.03), and DM and NDF fraction B decreased linearly (P = 0.01) with increasing alfalfa substitution. Rate of DM and NDF disappearance increased linearly (P </= 0.02) with increasing alfalfa substitution. In Exp. 2, treatments were arranged as a 2 x 2 factorial testing alfalfa substitution [none or 25% (as-fed basis)] to orchardgrass hay (Dactylis glomerata L.) and intake level [restricted to 1% of BW daily (DM basis) or ad libitum]. Nutrient intakes were lowest (P </= 0.05) by steers fed restricted diets, intermediate by steers fed orchardgrass ad libitum, and greatest by steers fed orchardgrass plus alfalfa ad libitum. Intake level and forage source had no effect (P >/= 0.23) on total tract apparent digestibility of all nutrients except CP. Steers fed orchardgrass plus alfalfa had 33% greater (P = 0.01) total tract apparent digestibility for CP than those fed orchardgrass alone. Lag time of DM and NDF disappearance was not affected (P >/= 0.20) by alfalfa supplementation or intake level. Rate of DM and NDF disappearance of orchardgrass was faster (P </= 0.01) in steers fed orchardgrass plus alfalfa, at both restricted and ad libitum levels of feeding, than in animals fed orchardgrass alone. Mean retention times of large and small particles of orchardgrass tended to be shorter (P </= 0.06) when steers consumed ad libitum vs. restricted diets. Small orchardgrass particles tended to have a faster (P = 0.09) rate of passage under ad libitum feeding conditions and with alfalfa addition. Ad libitum intake was associated with a shorter mean retention time of orchardgrass and faster rate of passage of small orchardgrass particles, whereas alfalfa addition increased the rate of passage of small orchardgrass particles and the rate of DM and NDF disappearance.