Weed density and diversity under glyphosate-resistant crop sequences

The effect of regular and exclusive application of glyphosate in crop sequences on associated weed composition, richness and diversity was studied over five years. Three sequences (wheat-soybean, soybean monoculture and soybean-maize) including soybean and maize glyphosate-resistant cultivars were investigated under two tillage systems (conventional or no-tillage). Regardless of sequence and tillage system, regular glyphosate application appeared to reduce richness and density of the most competitive weeds such as early-emergence annual broad-leaved and grassy annuals and increase that of the less competitive late-emerging annual broad-leaved weeds in no-tillage systems. Some late-emerging annual broad-leaved species, e.g. Bowlesia incana Ruiz et Pav. were present from the beginning of the experiment and others species, e.g. Parietaria debilis G. Foster were not present before initiating the study. This species was the only tolerant to glyphosate at the recommended dose of the herbicide. Species richness in pre-glyphosate application counts showed no response in the wheat-soybean rotation but decreased in the soybean-maize rotation and soybean monoculture in both tillage systems. In post-glyphosate application counts, richness increased for all sequences. Diversity showed an inconsistent response with time. It can be concluded that the effect of glyphosate application was a more important factor than crop sequence to explain weed community changes in summer crops. It may be predicted that continual glyphosate application for longer periods of time might lead to the development or higher increases in abundance of weeds tolerant to the herbicide.