FORAGE QUALITY RESPONSES OF C3 AND C4 PERENNIAL GRASSES TO SHADE

Both C3 and C4 grasses exhibit similar morphological responses to shade, 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 forage quality of C3 and C4 grasses. The C3 species were tall fescue (Festuca 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 gerardii Vit.). Grasses were established near Ames, IA, and grown under shade cloths which allowed 37 and 70% of ambient sunlight (AS) to penetrate. In addition, the grasses were subjected to a nonshaded (100% AS) treatment. Leaf blades, nonlaminar herbage, and total herbage samples were obtained at approximately 21-d increments from late spring to early summer for 2 yr. Neutral-detergent fiber (NDF) concentration in leaf blades was 18% greater in C4 species than in C3 species. Otherwise, few clear differences were observed between C3 and C4 types. Compared with the 100% AS treatment, NDF concentration in the 37% AS treatment was reduced 2 to 3%, whereas N concentration was increased by about 31% in nonlaminar herbage. In vitro digestible dry matter was increased 3 to 5% by shade. Nitrogen content per unit leaf area was not affected by shade, suggesting that nitrogenous compounds were concentrated by thinning of leaves. Thus, stressful growth conditions that limit photosynthate, such as shade, may improve forage quality.