Univariate analyses fail to account for covariance among phenomorphological traits implicated in crop competitive ability. A more complete analysis of cultivar–weed interactions would reduce a number of important traits to a few underlying principal factors responsible for sweet corn competitiveness. Twenty-three commercial sweet corn hybrids from nine seed companies were grown in the presence and absence of wild-proso millet to (1) quantify the extent to which phenomorphological traits vary in sweet corn, (2) identify underlying principal factors that describe variation in crop canopy development, and (3) determine functional relationships between crop canopy factors and competitive ability. A principal component factor analysis revealed that 7 of the 18 weed-free crop traits measured at silking loaded highly (0.65 to 0.90) into the first factor, including plant height, shoot biomass, per plant leaf area, leaf area index, and intercepted light, as well as thermal time from emergence to silking and emergence to maturity. All seven traits were highly correlated (0.38 to 0.93) and were interpreted as a “late canopy and maturity” factor. Another five traits formed two additional principal factors that were interpreted as an early “seedling quality” factor (e.g., kernel mass, seedling vigor, and height at two-leaf stage) and a mid-season “canopy closure” factor (e.g., leaf area index and intercepted photosynthetically active radiation at six-leaf stage). Relationships between principal factors and competitive abilities were quantified using least-squares linear regression. Cultivars with greater loadings in the late canopy and maturity and canopy closure factors were more competitive with wild-proso millet. In contrast, crop competitive ability declined with cultivars that loaded highly into the seedling quality factor. The analyses showed that sweet corn's ability to endure weed interference and suppress weed fitness relates uniquely to three underlying principal factors that capture crop canopy development around emergence and near canopy closure and during the reproductive phase.

Nomenclature: Wild-proso millet, Panicum miliaceum L.; sweet corn, Zea mays L. ‘ACX1413BC’, ‘Beyond’, ‘Cahill’, ‘Code128’, ‘Code3’, ‘Code39’, ‘Coho’, ‘DMC2184’, ‘Dynamo’, ‘El Toro’, ‘EX 8716622’, ‘Harvest Gold’, ‘Incredible’, ‘Legacy’, ‘Luscious’, ‘Mystic’, ‘Precious Gem’, ‘Quickie’, ‘Rocker’, ‘SCH7006RR’, ‘Spirit’, ‘Spring Treat’, and ‘Sugar Buns’.