Effects of Soybean Seed Size on Weed Competition

Abstract

Organic soybean [Glycine max (L.) Merr.] producers must rely on various, nonherbicidal tactics for weed management. Increased soybean seed size may be one method to increase the competitiveness of the soybean canopy. Soybean varieties Hutcheson, NC‐Roy, and NC‐Raleigh were separated into four or five seed size classes. Seed sizes ranged from 5.15 to 6.75 mm diam. This range of seed size resulted in seed weights of 10 to 20 g per 100 seed. Each seed size class was grown in weedy and weed‐free conditions at Kinston, NC during 2007 and 2008 and at Plymouth, NC during 2008. Redroot pigweed (Amaranthus retroflexus L.) overseeding was done to create uniform weed densities. Increasing soybean seed size resulted in greater soybean canopy coverage with the greatest effect at 3 weeks after emergence (WAE) (P < 0.001, P = 0.003, and P < 0.001 at Kinston 2007, Kinston 2008, and Plymouth 2008, respectively) compared with 5 WAE (P = 0.02, P = 0.07, and P = 0.02 at Kinston 2007, Kinston 2008, and Plymouth 2008, respectively). Plants from larger seed sizes had more soybean biomass in all sites when grown with weeds, but in only two sites under weed‐free conditions. Larger seed size improved soybean competitiveness by increasing petiole length and plant height. In the two sites with higher weed population densities, planting larger soybean seed reduced weed biomass at 7 WAE. Results suggest that use of larger planting seed may improve weed control in organic soybean production.