N2-fixation and weed competition: breaking the connection between crops and weeds

View the project final report

• 2004 annual report
• 2005 annual report
• 2006 annual report
• 2007 annual report

Commodities

• Agronomic: barley, corn, peanuts, sorghum (milo), soybeans
• Vegetables: sweet potatoes

Practices

• Crop Production: conservation tillage
• Education and Training: demonstration, extension, on-farm/ranch research
• Farm Business Management: budgets/cost and returns
• Pest Management: competition, integrated pest management, mulches - killed, mulches - living, physical control, prevention, weed ecology
• Soil Management: green manures, soil analysis
• Sustainable Communities: sustainability measures

Weed control remains as one of the most challenging problems for profitable organic and sustainable agricultural systems. In this project, we propose a new strategy for weed management in the highly weathered, low fertility soils of the southeastern U.S. Crop rotations currently favored for the region place great importance on N2-fixing crops. In preliminary research with soybean and peanut, it has been established that much of the nitrogen driving weed growth (35 to 80%) actually comes from the N2-fixing crop plants themselves. The nitrogen is transferred to the weeds through hyphae of mycorrhizal fungi that connect plant root systems underground. Also, research results indicate that different amounts of N are transferred from different soybean varieties, which implies that varietal selection could become an important part of weed management strategies. This project will have two major objectives. One is to critically examine the impact of N transfer on weed vigor and competitiveness using high and low transfer soybean varieties and a group of methodologies that includes competition studies and quantification of N transfer by 15N natural abundance. The other objective is to evaluate weed control in a rotation that includes a low N-transfer soybean and a sweetpotato variety that has a low N requirement, and compare its effectiveness in reducing weed competition to that of traditional rotations. The project will be located at the Center for Environmental Farming Systems, N.C. Agricultural Experiment Stations, and fields of two cooperating farmers.

1. Critically evaluate the role of N transfer in the development of weed patches.
a. Determine N transfer distances and penetration into weed patches, and the impact of transfer on weed population dynamics.
b. Determine the influence of N transfer on weed competitiveness and reproduction.
c. Determine the impact of N transfer on the N nutritional status of weed seeds and the competitiveness of offspring.
2. Compare weed growth, competitiveness, and seed production in traditional organic production systems with the new, modified system that uses a “low N transfer” soybean variety in rotation with a sweetpotato variety that has a lower N requirement.