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Title: Elevated atmospheric carbon dioxide and weed populations in glyphosate treated soybean

item Ziska, Lewis
item Goins, Ernest

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2006
Publication Date: 10/1/2006
Citation: Ziska, L.H., Goins, E.W. 2006. Elevated atmospheric carbon dioxide and weed populations in glyphosate treated soybean. Crop Science. 46:(3)1354-1359.

Interpretive Summary: Genetically modified or Round-up ReadyTM soybean constitutes about 80% of the current U.S. soybean crop. Although soybean growth is likely to increase with the increase in atmospheric carbon dioxide, it is also likely that weed growth will increase as well. Consequently, it is unclear if current weed management methods will be effective in maintaining weed growth. In the experiment described here, we examined a genetically modified soybean line at current and future levels of carbon dioxide over a two year period. We found that while soybean growth was stimulated by enhanced carbon dioxide levels, weed growth was stimulated to a greater extent during years with normal precipitation. During a normal precipitation year, application of glyphosate (i.e. Round-up)did not adequately control weed growth. Hence, while increasing carbon dioxide may cause soybean yields to increase, any increase in profits may be offset by the higher cost of weed control to maintain these yields. Overall, these data suggest that while precipitation may influence the types of weed species that occur, rising carbon dioxide will have a significant effect on weed species, weed biomass and herbicide effectiveness. This data will be of interest for chemical companies which manufacture herbicide, farm managers, extension agents, scientists and the general public.

Technical Abstract: Genetically modified (Round-up ReadyTM) soybean was grown over a two year period (2003-2004) at ambient and at projected levels of atmospheric carbon dioxide (CO2, 250 ppm above ambient), with and without application of the herbicide, glyphosate [N–(phosphonomethyl) glycine], to assess potential impacts of rising atmospheric carbon dioxide concentration on chemical efficacy of weed control. Precipitation was markedly different between each year of the study, with 2003 being the wettest year on record (since 1895) and 2004 being near the historic precipitation mean for this location. For both years, soybean (grown with glyphosate application), showed a significant vegetative response to elevated carbon dioxide, but no consistent effect of seed yield, with subsequent reductions in apparent harvest index (AHI). For 2003, observed weed populations for all treatments consisted entirely of C4 grasses, with no carbon dioxide induced effects noted with respect to weed biomass (unsprayed plots) or glyphosate efficacy (sprayed plots). However, in 2004, weed populations were mixed and included C3 and C4 broadleaves as well as C4 grasses. In this same year, a significant increase in both C3 broadleaf populations and total weed biomass was observed as a function of carbon dioxide concentration (unsprayed plots). In addition, a carbon dioxide by glyphosate interaction was observed with significant C3 broadleaf weed biomass remaining after glyphosate application in the elevated [CO2] treatment (sprayed plots). Overall, these data reinforce the idea that environmental factors such as precipitation may determine the relative composition of C3 and C4 weeds, but emphasize the subsequent consequences for CO2-induced changes in weed biomass, composition and subsequent glyphosate efficacy.