ARS | Publication request: Effects of Soybean Seed Size on Weed Competition

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 26, 2010
Publication Date: January 1, 2011
Citation: Place, G.T., Reberg-Horton, S.C., Carter Jr, T.E., Smith, A.N. 2011. Effects of Soybean Seed Size on Weed Competition. Agronomy Journal. 103:175-181.

Interpretive Summary: Weed control is the biggest obstacle for farmers involved in organic soybean production. One answer to this problem is to produce a bigger soybean canopy that covers the ground sooner and shades out weeds better. Increased size of soybean planting seed may be one way to produce a larger canopy in the first weeks after emergence. A rapid way to get large seed for planting is to process seed through a seed cleaning and sizing machine, and then save the larger seed as planting stock. The purpose of this study was to test the efficacy of this approach. We took several bushels of seed of three soybean varieties and sized the seed into five classes. These seed size classes were then planted and compared for canopy grow-off and ability to compete with weeds. Over three locations, the larger seed showed an increased ability to produce a large canopy size and suppress weeds, as compared to the small seed size classes. These results suggest that this seed sizing tactic might be a useful approach in organic farming to improve soybean’s competitive ability with weeds.

Technical Abstract: Organic soybean 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 mm to 6.75 mm diameter. 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.