Improved Weed Competitiveness in Soybean: Screening Tactics and Traits of Interest.

The expansion of some organic soybean markets has led some producers to explore alternative weed management systems that utilize a variety of tactics to reduce weed competitiveness. The use of more competitive soybean varieties could be one such tactic in a more diverse weed management program. We grew 27 soybean genotypes chosen for their differing seed and leaf morphology in 2007 and 2008 at Kinston, NC. Experimental design was a strip, split plot design with main plot soybean genotype groupings, weedy and weed free treatments stripped across the block, and soybean genotype subplots. Measurements were made from 2 weeks after emergence (WAE) through 7 WAE to evaluate differences in early and late main stem height, canopy cover estimates at 3 and 5 WAE, canopy light interception at 4 and 6 WAE, soybean biomass at 7 WAE in weedy and weed free plots, and weed biomass associated with a genotype at 7 WAE. A soybean genotype with less weed biomass in the weedy plots was considered a more competitive genotype. Multiple regression analysis was utilized to determine the optimal screening tactics for identifying competitive soybean genotypes. Multiple regression analysis was also utilized to discern relationships of seed size, leaflet length, leaflet width, petiole length, petiolule length and main stem height on soybean genotype competitive ability. Overhead photography at 3 WAE was the best predictor for genotype competitiveness. Multiple regression models showed seed size to be the most significant trait measured in overall genotype competitive ability in both years.