Evaluation of glyphosate-resistant Glycine max genotypes for competitiveness at recommended seeding rates in wide and narrow rows

Field experiments were conducted in 2002 and 2003 to identify simple plant characteristics breeders could use to select for competitive Glycine max L. Merr. genotypes grown in wide and narrow row widths at recommended seeding rates. Soybean height, width, canopy volume, and visual assessment of Glycine max   groundcover were used to evaluate differences in competitiveness among genotypes. ‘SC00-883’ was highly competitive in both row widths and years, suppressing weed biomass and having lower yield loss. ‘Hartz 6255’, ‘Delta and Pineland 6299’, ‘Asgrow 6202’, and ‘Northrup King S60-E4’ had high yield loss and were ineffective in suppressing weed growth in wide rows. With the exception of ‘Pioneer 96B21’, competitiveness was generally consistent across row widths; meaning genotype competitive rank in wide and narrow rows is often similar. Weed-free yields were not correlated with yield loss or weed biomass, but were correlated (r=0.80r=0.80) with yields in the presence of weeds across row widths and years. Height differences among genotypes never exceeded 6 cm during vegetative growth, and Amaranthus palmeri overtopped all genotypes prior to Glycine max reproductive development, negating any potential height effects on competitiveness. Glycine max groundcover was similar among genotypes in higher population narrow rows but differences among genotypes in lower population wide rows occurred at 7 weeks after Glycine max emergence (WAE) in both years. Yield loss and weed biomass were not correlated with groundcover in wide rows at 7 WAE, nor were they correlated with Glycine max height, width, canopy volume, or groundcover through 7 WAE. Glycine max competitiveness therefore appears linked to more complex or difficult to measure traits, or a combination of traits.