ARS | Publication request: Changes in the weed species in the major agronomic crops of the United States: 1994/1995 to 2008/2009

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 4, 2011
Publication Date: April 1, 2012
Citation: Webster, T.M., Nichols, R. 2012. Changes in the weed species in the major agronomic crops of the United States: 1994/1995 to 2008/2009. Weed Science. 60:145-157.

Interpretive Summary: The Southern Weed Science Society has conducted an annual survey of the most common and troublesome weeds for over 40 years. Glyphosate use increased dramatically in soybean, cotton, and corn. As a consequence, the exposure of weed populations to the EPSPS mechanism of action was greater than that previously observed for any other herbicide mechanism of action. The introduction of transgenic, glyphosate resistant cultivars increased use of glyphosate, decreased the use of other herbicides, and decreased the use of tillage. The net effect was a major change in the field conditions affecting weed emergence, survival, and propagation. The changes in weed flora in the southern US can be explained, in part, by the revolutionary changes in weed management practices and dominance of glyphosate in cotton and soybean. In corn where adoption of glyphosate-resistance technology had lagged behind cotton and soybean, or is non-existent, as in the case of wheat, changes in the weed species composition were not as dramatic. In corn and wheat, 12 of the top 15 most troublesome weeds in 1994 were also ranked in the top 15 in 2008. In contrast, only eight of the top 15 most troublesome weeds of cotton and soybean were common in 1994 and 2009. The Southern Weed Science Society’s surveys provide a benchmark of weed prevalence each year. This analysis of changes in these surveys over time illustrates the effects of changing management on the weed flora of southern crops.

Technical Abstract: This analysis documents changes in the weed flora of the Southern US since advent of herbicide-resistant crops. In corn, the top five weeds were morningglories, Texas millet, broadleaf signalgrass, johnsongrass, and sicklepod. These same weeds were the top five in 1994. In cotton, morningglories and nutsedges were among the top five most troublesome weeds in 1995/2009. Palmer amaranth, pigweeds, and Florida pusley were also among the five most troublesome species in 2009. In soybean, morningglories, nutsedges, and sicklepod were among the top five weed species in 1995/2009. Palmer amaranth and horseweed were the 2nd and 4th most troublesome weeds of soybean in 2009. In wheat, the top four weeds in 1994/2008 were the same, including Italian ryegrass, wild garlic, wild radish, and henbit. The number five weed and species that increased most in importance was annual bluegrass. The changes in weed flora in the southern US can be explained, in part, by the revolutionary changes in weed management practices and dominance of glyphosate in cotton and soybean. In corn where adoption of glyphosate-resistance technology had lagged behind cotton and soybean, or is non-existent, as in the case of wheat, changes in the weed species composition were not as dramatic. In corn and wheat, 12 of the top 15 most troublesome weeds in 1994 were also ranked in the top 15 in 2008. In contrast, only eight of the top 15 most troublesome weeds of cotton and soybean were common in 1994 and 2009.