Effects and interactions of corn and soybean oil supplementation and forage source on feed intake and digestibility by mature and growing cattle were determined. Eight mature beef cattle (571 ± 17 kg initial body weight) were used in two simultaneous 4×4 Latin squares. Cattle in each square consumed long‐stemmed alfalfa (AL; 16% crude protein, 54% neutral detergent fibre and 6.1% acid detergent lignin) or orchardgrass (OR; 11% crude protein, 71% neutral detergent fibre and 9.4% acid detergent lignin) hay ad libitum for 15 d followed by 6 d of restricted consumption (85% of ad libitum). Supplement treatments were Control, ground corn (C; 0.5% body weight), soybean oil (O; 0.125% body weight), or C + O. Total ad libitum dry matter (DM) intake was greater for AL than for OR(P< 0.05) and with than without C (P <0.05), and a corn x soybean oil interaction occurred (P =0.07; 11.8, 14.0, 13.6, 14.2, 8.4, 10.7, 9.3 and 10.3 kg/d); total tract neutral detergent fibre (NDF) digestibility was 66.0, 67.7, 65.8, 68.8, 52.5, 50.6, 55.1 and 59.4% for AL, AL‐C, AL‐O, AL‐C+O, OR, OR‐C, OR‐O, and OR‐C + O, respectively (SE 2.46). Eight Holstein steer calves (83 ± 5 and 131 ± 11 kg initial and final body weight, respectively) were subjected to the same dietary treatments, except for higher levels of C (1.0% body weight) and O (0.25% body weight) and periods with 21 d of ad libitum forage intake. Total DM intake was greater (P =0.06) for AL than for OR, increased (P< 0.05) by C, and decreased (P< 0.05) by O (3.92, 4.17, 3.51, 4.00, 2.53, 2.90, 2.09 and 2.51 kg/d), and total tract NDF digestibility was affected by forage source (P< 0.05) and a corn × soybean oil interaction (P= 0.08; 58.8, 56.9, 60.1, 56.0, 41.9, 44.5, 45.8 and 40.1% for AL, AL‐C, AL‐O, AL‐C + O, OR, OR‐C, OR‐O and OR‐C + O, respectively). In conclusion, effects of supplementation with corn and (or) soybean oil on feed intake and digestibility were similar for AL and OR, which differed in voluntary intake and digestibility, and animal characteristics such as stage of maturity may have influenced effects of soybean oil on feed intake.