Weed-crop competition relationships differ between organic and conventional cropping systems

Authors: RYAN, MATTHEW - Pennsylvania State University; SMITH, R - Pennsylvania State University; MORTENSEN, DAVIS - Pennsylvania State University; Teasdale, John; CURRAN, WILLIAM - Pennsylvania State University; SEIDEL, RITA - Rodale Institute; SHUMWAY, DURLAND - Rodale Institute

Submitted to: Weed Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2009
Publication Date: 9/29/2009
Citation: Ryan, M.R., Smith, R.G., Mortensen, D.A., Teasdale, J.R., Curran, W.S., Seidel, R., Shumway, D.L. 2009. Weed-crop competition relationships differ between organic and conventional cropping systems. Weed Research. 49(6):572–580.

Interpretive Summary: Organic farming has become an increasingly attractive alternative to conventional grain producers because of price premiums and reduced costs. Organic farmers have identified weed management to be a top research priority and production constraint, as the efficacy of organic weed management is often more variable than conventional herbicide-based methods. The Rodale Institute Farming Systems Trial (FST) provides a unique 27-year history of conventional and organic corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) production, associated weed management, and resulting weed competition; the latter of which is a subject that has received little attention. In this assessment involving a collaboration of Penn State, Rodale Institute, and ARS, a long-term dataset was used to evaluate weed population dynamics (through biomass variation) and resulting corn and soybean yield. Weed biomass variation was greatest in the organic systems and was sensitive to fluctuations in rainfall timing and amount. Weed management efficacy was more stable and consistently high in the conventional system. Although corn yields were higher in the single conventional system at the onset of the trial, within four years corn yields in the two organic systems equaled or exceeded conventional corn despite weed biomass averaging six times higher in organic corn. Organically grown soybean initially yielded higher than conventionally grown soybean, but organic soybean yields decreased slightly over time. These results suggest that organically grown corn is well buffered in years of “poor” weed control and offers hope that further improvements in weed management could increase organic corn yields. The findings also demonstrate the need for careful planning when developing an integrated cropping system to limit the negative effects of “poor” years when growing weed sensitive crops such as soybean. These results will be useful to organic farmers and agricultural professionals working with organic and/or sustainable agricultural systems.

Technical Abstract: Organic farmers have identified weed management to be a top research priority and production constraint, as the efficacy of organic weed management is often more variable than conventional herbicide-based methods. The Rodale Institute Farming Systems Trial (FST) provides a unique 27-year history of conventional and organic corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) production, associated weed management, and resulting weed competition; the latter of which is a subject that has received little attention. In this assessment, a long-term dataset was used to evaluate weed population dynamics (through biomass variation) and resulting corn and soybean yield. Weed biomass variation was greatest in the organic systems and was sensitive to fluctuations in rainfall timing and amount. Weed management efficacy was more stable and consistently high in the conventional system. Although corn yields were higher in the single conventional system at the onset of the trial, within four years corn yields in the two organic systems equaled or exceeded conventional corn despite weed biomass averaging six times higher in organic corn. Organically grown soybean initially yielded higher than conventionally grown soybean, but organic soybean yields decreased slightly over time. These results suggest that organically grown corn is well buffered in years of “poor” weed control and offers hope that further improvements in weed management could increase organic corn yields. The findings also demonstrate the need for careful planning when developing an integrated cropping system to limit the negative effects of “poor” years when growing weed sensitive crops such as soybean.