The relative competitive ability of common lambsquarters and giant foxtail in mixed weed–corn communities was characterized in 1998 and 1999 using empirical models that described late-season weed biomass on the basis of weed density, early-season relative leaf area, or early-season relative shoot volume. Competition coefficients estimated from weed density were inconsistent between years because they indicated that giant foxtail was more competitive than common lambsquarters in 1998 but that common lambsquarters was more competitive than giant foxtail in 1999. In contrast, the competition coefficients based on relative leaf area and relative volume were consistent between years. Competition coefficients estimated from relative leaf area indicated that giant foxtail was more competitive than common lambsquarters in each year. Competition coefficients estimated from weed relative volume indicated that the relative competitive ability of each weed species was similar in each year. Weed relative competitive abilities were characterized further by describing the mechanisms of competition related to shoot height and width growth. Giant foxtail was taller than common lambsquarters shortly after emergence each year, but plasticity of common lambsquarters growth resulted in reduced height differential between the weed species over time. Even so, giant foxtail was taller than common lambsquarters at physiological maturity each year. Coefficients that described the ability of each weed species to crowd neighbors indicated that giant foxtail shoot width was affected more by increased common lambsquarters density and proportion than was common lambsquarters shoot width by giant foxtail. The greater ability of common lambsquarters to crowd neighbors relative to giant foxtail was attributed to the greater leaf area density (LAD) of common lambsquarters compared with that of giant foxtail. Although characterization of shoot height, width, LAD, and biomass elucidated in part the mechanisms of competition between these species, models that accounted for differences in early-season relative plant size were consistent between years, indicating that giant foxtail was equally or more competitive than common lambsquarters in corn.

Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; giant foxtail, Setaria faberi Herrm. SETFA; corn, Zea mays L. ‘Dekalb DK493SR’.