Submitted to: Agronomy for Sustainable Development
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/19/2009
Publication Date: 4/16/2010
Publication URL: hdl.handle.net/10113/42207
Citation: Teasdale, J.R., Cavigelli, M.A. 2010. Subplots facilitate assessment of corn yield losses from weed competition in a long-term systems experiment. Agronomy for Sustainable Development. 30:445-453. Interpretive Summary: Inconsistent weed control is a major limitation to organic farming. The Farming Systems Project (FSP) is a long-term experiment that was established at Beltsville, Maryland, to compare three organic farming rotations with two conventional cropping systems. The effect of weed competition on corn yield was evaluated in paired weedy and weed-free subplots within the FSP cropping systems during the first ten years of this experiment. Weed competition caused a reduction in corn yield, particularly in organic systems, but the degree of yield loss because of weed competition varied considerably among years from 4 to 76%. Corn yield was often lower in organic compared to conventional systems, but weed competition only accounted for a portion of that difference. Limited nitrogen availability in the organic systems accounted for most of the remaining difference in yield between organic and conventional systems. These results demonstrated that organic farming rotations need to be carefully designed to reduce weed populations and provide adequate nitrogen availability. This information will be useful to agricultural scientists and professionals working with organic farming systems.
Technical Abstract: Weeds can potentially limit crop yield, particularly in organic systems where herbicide technologies are unavailable. Weedy and weed-free subplots were established within full plots of a long-term cropping systems experiment, the Farming Systems Project, at Beltsville, Maryland, USA, to determine the effect of weed competition on maize (Zea mays L.) yields in six out of the first ten years of this experiment. Four systems were addressed, 1) a conventional no-tillage (NT) system, 2) a two-year organic maize-soybean (Glycine max (L.) Merr.) rotation (Org2), 3) a three-year organic maize-soybean-wheat (Triticum aestivum L.) rotation (Org3), and 4) a four-year organic maize-soybean-wheat-hay rotation (Org4). Weed abundance was determined by estimating percent of soil area covered by weeds which was shown to be linearly related weed biomass but the precise conversion coefficient was dependent on year and species composition. Weed competition in organic systems reduced maize grain yield in all years but annual variation ranging from 4 to 76% was greater than system differences. Also, the rate of yield loss per unit of weed cover varied by approximately six-fold across years. This variability was explained in part by rainfall; the highest yield loss rates occurred in years with below average rainfall and the lowest yield loss rates occurred in years with above average rainfall. Maize grain yield was higher in NT than all organic systems in 3 of 6 years and yield in the longer, more diverse Org4 rotation was higher than that in the less diverse Org2 rotation 3 of 5 years. Weed-free yields estimated from subplot data showed that most of the yield differences between systems remained after elimination of weed competition effects. Regression analysis showed that 59% of the variation in weed-free yields could be explained by nitrogen availability in years with above-average rainfall but little or none of the variation in weed-free yield could be explained by nitrogen availability in years with below-average rainfall. Results suggest that in above-average rainfall years, nitrogen availability was more limiting to organic maize yield than weed competition during the initial transition years of this experiment.