Recent lack of rain, and windy conditions that hampered herbicide applications, have resulted in weedy fields that are effectively reducing yields in corn and soybean. A particular concern is fields with heavy populations of common lambsquarters that have not been controlled effectively by glyphosate. In discussions with growers and applicators, the failure of glyphosate, even when applied at 48 oz per acre (1.1 lb ae/acre) to provide control, does not appear to be attributable to evolved resistance to glyphosate, but rather a combination of factors: common lambsquarters that is too large and has developed during dry conditions, and applications that were too late to provide consistent control and protection of potential crop yields.
Iowa State University
Owen, Micheal D., "Large common lambsquarters is a problem for glyphosate" (2006). Integrated Crop Management News. 1280.
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