Weed Community and Competitive Load Following 12 Years 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-resistant cropping systems. The study was designed as a split-split plot RCBD where the whole plot factor was crop rotation, the split plot factor consisted of glyphosate use patterns, and the split-split plot factor was presence or absence of a PRE herbicide. Glyphosate use patterns ranged from treatments receiving no glyphosate to continuous, exclusive use of glyphosate. In 2010 weeds were allowed to establish without herbicide treatment or crop competition and then counted. To interpret the weed density counts, a competitive index of each weed species as if it was in competition with corn, dry bean, and sugarbeet was used to calculate the competitive load for the weed spectrum resulting from historical treatments. The previous crop rotation had no effect on competitive load regardless of the crop index used. For the corn competitive index, treatments utilizing a PRE herbicide, or continuous use of glyphosate at 840 g ae ha^-1 resulted in the lowest competitive load. For both the dry bean and sugarbeet competitive indices, the use of a PRE herbicide significantly decreased the competitive load, regardless of glyphosate use history. Continuous use of glyphosate at 840 g ae ha^-1 resulted in the lowest competitive load of any glyphosate use history for both the sugarbeet and dry bean indices; however there was no statistical difference between this treatment and alternating glyphosate applications with conventional herbicides when using the sugarbeet index.