FORAGE QUALITY RESPONSES OF C3 AND C4 PERENNIAL GRASSES TO SHADE

Both C3 and C4 grasses exhibit similar morphological responses to shad e, but they differ in photosynthesis-related responses to shade, such as forage yield and growth rate. Morphological adaptation and reduced photosynthesis under shade may affect forage quality. The objective of this research was to determine the effect of adaptation to shade on f orage quality of C3 and C4 grasses. The C3 species were tall fescue (F estuca arundinacea Schreb.), reed canarygrass (Phalaris arundinacea L. ), and deertongue grass (Panicum clandestinum L.); the C4 species were switchgrass (Panicum virgatum Michx.) and big bluestem (Andropogon ge rardii Vit.). Grasses were established near Ames, IA, and grown under shade cloths which allowed 37 and 70% of ambient sunlight (AS) to pene trate. In addition, the grasses were subjected to a nonshaded (100% AS ) treatment. Leaf blades, nonlaminar herbage, and total herbage sample s were obtained at approximately 21-d increments from late spring to e arly summer for 2 yr. Neutral-detergent fiber (NDF) concentration in l eaf blades was 18% greater in C4 species than in C3 species. Otherwise , few clear differences were observed between C3 and C4 types. Compare d with the 100% AS treatment, NDF concentration in the 37% AS treatmen t was reduced 2 to 3%, whereas N concentration was increased by about 31% in nonlaminar herbage. In vitro digestible dry matter was increase d 3 to 5% by shade. Nitrogen content per unit leaf area was not affect ed by shade, suggesting that nitrogenous compounds were concentrated b y thinning of leaves. Thus, stressful growth conditions that limit pho tosynthate, such as shade, may improve forage quality.