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Leaf growth, dry matter distribution , and carbohydrate content of four tall fescue (Festuca arundinacea Schreb.) genotypes selected for high and low net C02 exchange (NCE) and forage yield were studied in controlled environments t o evaluate the leaf NCE-yield relationship. High yield genotypes produced greater amounts of all plant parts and carbohydrate than the low yield genotypes. Within each yield level the high NCE genotypes produced more dry matter and carbohxdrate. Leaf growth of all genotypes was linear throughout light and dark periods, but rates were 50% greater for high yield genotypes, RGR was similar for all genotypes. NAR and RLaGR were greater for high yield genotypes while LAR was greater for low yield genotypes. Yield related factors such as dry matter distribution and leaf growth tended t o overcome NCE differences.