A field study was employed to identify irradiance level effects on plant morphology, growth, and gaseous and energy exchange of forage grasses, and relate these to forage nutritive value. Five grass species were grown under three irradiance regimes during two spring periods. Three C(,3) entries included deertongue grass (PCL) (Panicum clandestinum L.), reed canarygrass (RCG) (Phalaris arundinacaea L.), and tall fescue (TFS) (Festuca arundinacaea Schreb.) and two C(,4) entries included switchgrass (SWT) (Panicum virgatum Michx.) and big bluestem (BBS) (Andropogon gerardi Vit.). Polypropylene cloths limited irradiance to 37 and 70% of available irradiance (AI) in addition to an unshaded 100% AI treatment. Three harvests were obtained for each of the 2 years of this study;The species generally responded in a parallel fashion to irradiance level. Over all species, there was a 32% increase in specific-leaf weight between the high and low irradiance treatments, while there was a 24 and 12% decrease in leaf-area ratio and leaf-to-stem ratio, respectively. Dry matter yield responded positively to irradiance level with the C(,4) species response being almost twice that of the C(,3) species;Gaseous and energy exchange was measured for SWT and RCG. Carbon exchange rate (CER) for RCG averaged 431, 488, and 533 ng CO(,2) m('-2) s('-1) for the 37, 70, and 100% AI treatments, respectively, while for SWT the CER averaged 518, 775, and 805 ng CO(,2) m('-2) s('-1), respectively. Leaf tempeature (LT) and energy efflux responded positively to irradiance level and in a parallel manner for the two species. Stomatal resistance (r(,s)) did not respond to irradiance level; however, the lack of response may have been because of a lack of response of photosynthetic potential to irradiance level. The results suggest for SWT, however, that r(,s) modified LT through control of transpiration rate;Nitrogen concentraton responded negatively and neutral-detergent fiber and lignin concentration per unit of cell wall responded positively to irradiance level. These responses led to a general negative response of in vitro digestible dry matter (IVDDM) concentration to irradiance level with average IVDDM concentration in the total herbage of 593, 586 and 579 g kg('-1) over all species for the 37, 70 and 100% AI treatments, respectively.