Changes in the Prevalence of Weed Species in the Major Agronomic Crops of the Southern United States: 1994/1995 to 2008/2009

Abstract

Changes in the weed flora of cropping systems reflect the impacts of factors that create safe sites for weed establishment and facilitate the influx and losses to and from the soil seedbank. This analysis of the annual surveys of the Southern Weed Science Society documents changes in the weed flora of the 14 contiguous southern states since the advent of transgenic, herbicide-resistant crops. In 1994 and 2009, the top five weeds in corn were morningglories, Texas millet, broadleaf signalgrass, johnsongrass, and sicklepod; in this same period Palmer amaranth, smartweeds, and goosegrass had the greatest increases in importance in corn. In cotton, morningglories and nutsedges were among the top five most troublesome weeds in 1995 and 2009. Palmer amaranth, pigweeds, and Florida pusley were also among the five most troublesome species in 2009; the weeds with the largest increases in importance in cotton were common ragweed and two species with tolerance to glyphosate, Benghal dayflower and Florida pusley. In soybean, morningglories, nutsedges, and sicklepod were among the top five weed species in 1995 and 2009. Two species with glyphosate resistance, Palmer amaranth and horseweed, were the second and fourth most troublesome weeds of soybean in 2009. In wheat, the top four weeds in 2008 were the same as those in 1994 and included Italian ryegrass, wild garlic, wild radish, and henbit. Crop production in the southern region is a mosaic of various crop rotations, soil types, and types of tillage. During the interval between the surveys, the predominant change in weed management practices in the region and the nation was the onset and rapid dominance of the use of glyphosate in herbicide-resistant cultivars of corn, cotton, and soybean. Because of the correspondence between the effects of glyphosate on the respective weed species and the observed changes in the weed flora of the crops, it is likely the very broad use of glyphosate was a key component shaping the changes in weed flora. Only eight of the top 15 most troublesome weeds of cotton and soybean, the crops with the greatest use of glyphosate, were the same in 1995 and 2009. In contrast, in corn and wheat where adoption of glyphosate-resistant cultivars lags or is absent, 12 of the 15 most troublesome weeds were the same in 1994 and 2008. These findings show on a regional scale that weeds adapt to recurrent selection from herbicides, currently the predominant weed management tool. Future research should seek methods to hinder the rapid spread of herbicide-tolerant and evolution of herbicide-resistant weed species. As new tools are developed, research should focus on ways to preserve the efficacy of those tools through improved stewardship.