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CO2 Effects on Weed-Challenged Soybeans
Ziska, LH. And Goins, E.W. 2006. Elevated atmospheric carbon dioxide and weed populations in glyphosate treated soybean. Crop Science 46: 1354-1359.

What was done
Genetically modified (Round-up Ready) soybean (Glycine max (L.) Merr.) plants were grown in the field within aluminum chambers maintained at ambient and ambient + 250 ppm atmospheric CO2 concentrations for two full growing seasons under conditions - both environmental and managerial, including herbicide (glyphosate) application - that led to a variety of different weed densities developing among the soybean plants.

What was learned
Based on linear relationships developed by the authors for soybean seed yield vs. weed biomass in the ambient and CO2-enriched treatments, we calculate that for weed biomasses of 0, 200, 400, 600 and 800 gm-2, the seed yield enhancements provided by the extra 250 ppm of CO2 were on the order of 25, 33, 50, 90 and 250%, respectively, ultimately becoming infinite at the point where seed yield dropped to zero, i.e., at a weed density of approximately 920 gm-2. Also of note was the fact that soybean seed yield in the CO2-enriched treatment was calculated to not drop to zero until weed biomass reached a value of approximately 1250 gm-2.

What it means
As has been shown to be the case with certain other environmental stressors, such as plant diseases, lack of water and high temperatures, the atmospheric CO2 enrichment of this study boosted crop yield by an ever-increasing percentage as the stress of the expanding weed population grew ever larger, helping the soybean plants most when they needed it most.

Reviewed 6 September 2006