Weed diversity and soybean yield with glyphosate management along a north–south transect in the United States

Abstract

There are many concerns about the effects of repeated use of glyphosate in glyphosate-resistant (GR) crops, including two that are seemingly contradictory. These are (1) weed escapes and (2) loss of weed diversity. Weeds that escape glyphosate treatment represent species that likely will become troublesome and difficult to control in the future, and identifying these future problems may allow more effective management. In contrast, complete weed control directly reduces the weed component of agroecosystem biodiversity and may lower other components indirectly (e.g., weed-dependent granivores). During 2001 and 2002 effects of glyphosate and conventional weed control treatments on weed community composition and GR soybean yields were studied. Field studies were conducted along a north–south transect of sites spanning a distance of 1600 km from Minnesota to Louisiana. Low-intensity use (single application yr−1) of glyphosate allowed more escapes and maintained higher weed diversity than high-intensity use (two applications yr−1) of glyphosate, and it was equivalent to or even higher than diversity in non-GR systems. Although the same weeds escaped from low- and high-intensity glyphosate treatments, frequency of escapes was higher with less intensive use. These results suggest that limited use of glyphosate would not have profound effects on weed diversity. In addition, crop yield did not differ between GR and non-GR treatments at high latitudes, but below 40° N latitude, with a longer cropping season, yields with low-intensity glyphosate use decreased by about 2% per degree latitude because of competition from escaped weeds.

Spider mites associated to soybean in Rio Grande do Sul, Brazil

“…Such occurrence has been considered abnormal and could be associated with alterations of the soybean production system. Among the factors probably favoring the occurrence of mites are the use of new soybean cultivars, with favorable nutritional and morphological composition (Elden, 1997;Ali, 1999;Abraham, 2000;Rita & Lajos, 2001), increased use of glyphosate, which may reduce the biological diversity of the system and affect the arthropod population (Buckelew et al, 2000;Bitzer et al, 2002;Scursoni et al, 2006) and can be detrimental to entomopathogenic fungi (Morjan et al, 2002), pesticide use for pest management, which can influence, directly and indirectly, the spider mite population (Gerson & Cohen, 1989;Trichilo & Wilson, 1993;Grout et al, 1997;Childers & About-Setta, 1999;Sato et al, 2001Sato et al, , 2002James, 2003;Beers et al, 2005;Prischmann et al, 2005), and the occurrence of droughts (Flechtmann, 1972).…”

The effect of weed management systems and location on arable weed species communities in glyphosate‐resistant cropping systems

“…By comparison, a 2‐yr study along a 1600 km north–south gradient in the US, showed differences in weed diversity between single application glyphosate treatments and weedy controls in different locations between years reflecting temporal variance (Scursoni et al. ). In the current study, there was a clear distinctiveness of comparatively homogeneous weed communities associated with each hardiness zone consistent through 5 yr of observation.…”

“…), but are likely to have affected weed communities secondarily to location (Scursoni et al. ). In only 2 of 5 yr did crop rotation affect floristic similarity of the weed communities among sites, and GR trait rotations showed no effect in any year.…”

“…This limited amount of glyphosate does not have profound effects on weed diversity (Scursoni et al. ).…”

Variations in weed population densities, rate of change and community diversity in RR-soybeans and RR-maize strip crops under two herbicide strategies

“…Results are in line with the findings by Smith & Gross (2007), who found that the effects of annual crop diversity on weed communities were dependent on the crop rotation phase, and across all phases of rotation, weed community structure was affected more by crop identity than crop diversity per se. After releves made in experiments planted along a transect in the US corn belt, Scursoni et al, (2006) also suggest that higher weed diversity can be maintained with the use of a single application …”