Glyphosate susceptibility, weed community response, and competitive load following 12 years of selection pressure in a glyphosate resistant-cropping system

A long-term field study was conducted near Scottsbluff, NE from 1998 to 2009 to identify weed-shifts in response to glyphosate use in glyphosate-resistant cropping systems. The study was designed as a split-split plot randomized complete block design (RCBD) where the whole plot factor was crop rotation, the split-plot factor consisted of glyphosate use histories, and the split-split plot factor was presence or absence of a preemergence (PRE) herbicide. Glyphosate use histories consisted of four different herbicide treatments: a non-glyphosate treatment designed to achieve at least 95% control of all weed species; an annual alternating non-glyphosate and glyphosate treatment at 840 g acid equivalent (ae) ha-1; a low glyphosate treatment of 420 g ae ha -1; and a high rate glyphosate treatment of 840 g ae ha-1. All glyphosate applications were made twice per season. In 2010 (after the conclusion of this study), a series of studies were initiated to investigate the effect of glyphosate use on glyphosate susceptibility of the weed community, weed community composition, and competitive load. Crop rotation had no significant influence on glyphosate susceptibility, weed community composition, or competitive load. Decreased susceptibility of the weed community to glyphosate was observed in treatments historically exposed to glyphosate at 840 g ae ha-1. The weed community following 12 years of the study was dominated by common lambsquarters, redroot pigweed, and kochia; which together composed more than 99% of the weed community. Common lambsquarters made up a majority of the weed community where glyphosate was applied continuously at either 840 or 420 g ae ha-1. Treatments historically receiving glyphosate at 840 g ae ha-1 had the lowest overall weed density, while plots historically receiving glyphosate application at 420 g ae ha -1 had the highest weed density. The use of a PRE herbicide decreased weed abundance. Competitive load was calculated for the weed community in competition with corn, dry bean, and sugarbeet. Regardless of crop, competitive load was lowest in treatments historically receiving glyphosate application at 840 g ae ha-1, and greatest in plots historically receiving glyphosate application at 420 g ae ha-1. The use of a PRE herbicide reduced competitive load for dry bean and sugarbeet. The influence of a PRE treatment on competitive load against corn was mixed, with reductions in weed competition only seen in plots historically receiving alternating glyphosate application and those receiving continuous glyphosate applications at 420 g ae ha-1. Results of these studies indicate that glyphosate, applied at recommended rates, provides efficacious control of weeds even after long term selection pressure shifted the weed spectrum to an adapted species. Use of residual PRE herbicides is an effective way to reduce the competitiveness of weed communities in glyphosate-resistant cropping systems.