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Single-leaf CO2 assimilation rate under saturating light (CA) varies as a function of leaf nitrogen content per unit leaf area (NL). Measured CA for many crop species also declines with developmental stage. Because these relationships may have strong implications for crop–weed competition, a field experiment was conducted to quantify corn and velvetleaf CA as influenced by leaf NL and stage of development. Crop and weed CA were measured on the most recent fully expanded leaves of plants grown in four nitrogen (N) application treatments. Both corn and velvetleaf CA increased with increasing NL, up to about 1.5 g N m-2. Corn and velvetleaf NL values were rarely less than 0.75 g N m-2, indicating that both species may restrict leaf growth in order to maintain constant NL under conditions of limited soil N. Corn CA declined to half its maximum by physiological maturity, whereas velvetleaf CA only declined 18%. Although velvetleaf CA was considerably lower than that of corn, the difference decreased as the growing season progressed. Because corn leaf production is complete by anthesis and velvetleaf continues vegetative production throughout its life cycle, velvetleaf will produce relatively greater quantities of biomass late in the season, which may increase competition for light.