Competition and fecundity of giant ragweed in corn

A field study was conducted to determine the effects of giant ragweed emergence time and population density on corn grain yield, giant ragweed seed production, and giant ragweed predispersal seed losses. When weeds and crop emerged concurrently, hyperbolic regression of percent corn yield loss on giant ragweed population densities of 1.7, 6.9, and 13.8 weeds per 10 m2 gave a predicted loss rate of 13.6% for the first weed per 10 m2 in the linear response range at low densities and a maximum yield loss of 90% at high weed densities. Crop yield loss response to weed density was linear when giant ragweed emerged 4 wk after corn, and the regression coefficient indicated a yield loss rate of 1% per unit increase in weed density. A larger proportion of the variation in corn yield loss was explained by weed density (r2 = 0.99) than by weed biomass (r2 = 0.81). There was a positive linear relationship between giant ragweed seed production and weed density at each weed emergence time. When giant ragweed emerged with corn, regression equations for 1997 and 1998 gave a predicted seed rain of 146 and 238 seeds m−2 per unit increase in weed density, respectively. In both years when giant ragweed emerged 4 wk after corn, predicted seed rain was 16 seeds m−2 per unit increase in weed density. Viability of total giant ragweed seed was 56 and 38% in 1997 and 1998, respectively, and was not affected by weed emergence time or weed density. Feeding by insect larvae accounted for 13 to 19% of giant ragweed seed viability losses. Granivorous insects infesting giant ragweed seed were identified as a fruit fly (Diptera: Tephritidae), two weevils (Coleoptera: Curculionidae), and a moth (Lepidoptera: Gelechiidae).