Field experiments were conducted to quantify functional relationships between giant ragweed density and sweet corn yield and ear traits. A rectangular hyperbolic model was fit to yield loss measured in terms of marketable ear mass, appropriate for the processing industry, and boxes of 50 marketable ears, relevant to the fresh market industry. The initial slope of the hyperbolic yield loss function (I), which describes the linear portion of yield loss as weed density (weeds per square meter) approaches zero, was 119 for loss of ear mass and 97 for loss of boxes of ears. Furthermore, 10 of 12 ear traits including green ear mass, husked ear mass, ear length, filled ear length, ear width, number of kernels per row, number of rows, kernel depth, kernel mass, and kernel moisture content were significantly affected by giant ragweed interference.

Varietal growth habit could play a key role in the competitiveness of crop plants with weeds. The relative competitiveness of two erect, two semierect, and two prostrate cowpea genotypes with a tall-growing (common sunflower) or a low-growing (common purslane) species was evaluated in 12 replacement-series experiments conducted in 2003 and 2004. Growth analysis of the six cowpea genotypes and two weed species were also conducted to investigate the relationship of competitiveness and growth. Cowpea genotype and competitor biomass were used to calculate relative yield total (RYT) and aggressivity indices (AI). RYT and AI means of six cowpea genotypes were compared using ANOVA, and an isotonic regression was used to confirm the relative competitive superiority of the cowpea growth types. All cowpea genotypes had similar RYTs when grown with sunflower or common purslane, indicating that cowpea used the same resources as sunflower or purslane. When grown with sunflower, erect and semierect genotypes had higher AI than prostrate genotypes. When grown with purslane, erect and prostrate genotypes had higher AI than semierect genotypes. The results were confirmed by isotonic regression tests on the respective AI order. Correlation and regression between AI and growth parameters showed that the relative growth rate (RGR), leaf area ratio (LAR), and height growth rate (HGR) explained 92% of the variation of AI when cowpeas were grown with sunflower and that leaf weight ratio (LWR) and biomass explained 82% of the variation of AI when cowpeas were grown with purslane. These experiments indicate that cowpea genotypes differ in their ability to compete with purslane or sunflower. Erect genotypes were the most competitive, suggesting that an erect growth habit may be more effective in suppressing weeds than a semierect or prostrate growth habit. These results are consistent with field experiments on the competitiveness of erect, semierect, and prostrate cowpea genotypes.

Managing weeds in lawns using cultural practices such as mowing, irrigation, and fertilization may be important in integrated management. A field experiment evaluated the impact of cultural factors on weed populations in St. Augustinegrass turf. Irrigation treatments were daily to replace evapotranspiration (“Daily”), weekly to saturate the root zone only when wilted (“Conditional Weekly”), and as needed to saturate the root zone after severe wilt (“Severely Wilted”). Averaged over 3 yr, the Daily, Conditional Weekly, and Severely Wilted irrigation treatments resulted in 30, 6, and 2% dollarweed cover, respectively. Except for dollarweed and mat lippia, the largest cover of other weeds was under Severely Wilted irrigation. Fertilization rates were 0, 14, or 28 g N m−2 yr−1; in 2002, the highest fertilization rate had the smallest cover of weeds other than dollarweed. Mowing heights were 64, 89, or 114 mm; in 2003, the shortest mowing height had the smallest cover and dry weight of weeds other than dollarweed. After 3 yr of cultural management, most plots were excessively weedy, and turfgrass quality for all cultural management-treatment combinations, in the absence of herbicides, was unacceptable.