Thirty wether lambs (52.5 +/- 1.5 kg) were used in a replicated 3 x 5 factorial treatment arrangement and four periods to determine dietary intake and digestibility of three ground forages with increasing levels of supplemental ground corn. Forages were a mature, low-protein grass hay (LQH; 5.2% CP), an immature, medium-protein grass hay (MQH; 10.2% CP), and an immature, high-protein grass hay (HQH; 14.2% CP). Supplementation treatments were no supplement (NS), a protein supplement (PS), protein plus .25% BW of corn (LC), protein plus .5% BW of corn (MC), and protein plus .75% BW of corn (HC). Crude protein intake (1.4 g/kg BW) from supplements was equalized with soybean meal and corn gluten meal. Lambs were housed in metabolism crates. Supplements were fed at 0700 each morning after orts from the previous day's feeding were removed. Hay was subsequently offered at approximately the previous day's consumption plus 25%. Each of the four periods lasted 21 d. Sixteen days were allowed for diet adaptation. Feces were collected with fecal bags during d 17 to 21. Lambs receiving LQH and PS had higher forage (P < .04) and total DMI (P < .001) than lambs fed LQH and NS. Corn supplementation decreased forage intake (quadratic effect; P = .08), total intake (quadratic effect; P = .08), and increased apparent DM digestibility (linear effect; P < .04) with lambs receiving LQH. Protein supplementation did not affect forage or total DMI (P > .11), and corn supplementation did not affect total DMI (P > .20) with either MQH or HQH.(ABSTRACT TRUNCATED AT 250 WORDS)