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.

Nomenclature: Common purslane, Portulaca oleracea L. POROL; common sunflower, Helianthus annuus L. HELAN; cowpea, Vigna unguiculata (L.) Walp. ‘California Blackeye 5’, ‘Iron-Clay’, ‘Speckle Purple Hull’, ‘IT89KD-288’, ‘UCR 730’, and ‘UCR 779’ VIGSI.