Organic crop production is often limited by the inability to control weeds. An 18-yr data set of weed cover in organic crop rotations at the long-term Farming Systems Project at Beltsville, MD, was analyzed to identify meteorological and management factors influencing weed abundance. A path analysis using structural equation models was employed to distinguish between the direct effect of factors on weed cover and the indirect effect on weed cover through effects on crop competitiveness. Grain yield of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] served as a surrogate for crop competitiveness and was found to be the most important factor influencing weed cover. Precipitation during late vegetative and early reproductive crop growth had a strong positive effect on crop yield, and thereby a negative indirect effect on weed cover, but this effect was partially offset by a positive direct effect on weed cover. Delayed crop planting date and crop rotational diversification including crops other than summer row crops had a moderate negative effect on weed cover, while having minimal effect on crop performance. Rotary hoeing also had a direct negative effect on weed cover, but a corresponding negative effect on crop performance resulted in a diminished total effect on weeds. Results demonstrate the complex interactions that define the relative abundance of weeds faced by organic growers, but, generally, factors that enhanced crop competitiveness provided the most effective weed management.
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Vol. 66 • No. 4