Feeding trials were conducted with 428 forages in three forage classes (C3 grasses, legumes, C4 grasses) fed ad libitum to sheep and with 170 forages fed to cattle over a 20-yr period. Of this total, 153 forages were fed concurrently to sheep and cattle. Where the same forages were fed, mean dry matter digestibility (DMD) and dry matter intake (DMI) were lower (P less than .01) for sheep than for cattle, with significant effects of forage class. With the main population of forages, the regression of DMI on DMD was positive and curvilinear (P less than .01) for sheep, with no effect of forage class, and positive and linear (P less than .0001) for cattle, with a significant effect of class. For all forages, correlations between DMI and neutral detergent fiber (NDF) concentration were -.57 for sheep and -.41 for cattle; regressions differed (P less than .05) with class and animal species. Correlations of DMD with NDF and acid detergent fiber (ADF) concentrations were -.45 and -.59 (P less than .01), respectively, for sheep, and -.32 and -.39 (P less than .01) for cattle; again, regressions differed significantly with class and animal species. Within forage class, regressions of DMD and DMI on fiber components generally were linear. Intake of NDF (NDFI) was related to NDF percentage for all forages by a quadratic regression, with significant differences due to animal species and forage class. Results indicate that relationships between DMD, DMI and fiber fractions differ between forage classes and animal species, that C4 grasses are consumed at levels higher than would be expected from their DMD and fiber concentrations and that ruminants increase NDFI in response to higher NDF concentrations in the forage.