Glyphosate Management in Soybeans

June 17, 2009 at 7:30 am

The results of weed-crop interference studies show that weeds in soybeans should be treated with herbicide before they exceed a size of 6 to 8 inches in order to ensure that weed interference is not a limiting factor in soybean yield. Weeds may not have reached this size in later-planted soybeans, or where pre-emergence herbicide activity reduced weed populations and growth. In soybeans planted late May through June, POST herbicides can be applied sooner after planting when weeds are small, which maximizes POST herbicide activity and reduces the need for higher glyphosate rates.

Glyphosate rates should generally be based on weed size and age, environmental conditions, and the previous history of glyphosate effectiveness in the target weed population. You can use the lowest labeled rate of glyphosate, 32 oz/A Roundup Original and similar products, and achieve 100% control when the following are met: 1) weeds are less than 6 inches tall; 2) weeds have not survived tillage or a previous herbicide application; 3) glyphosate is applied following use of a pre-emergence herbicide; and 4) environmental conditions are extremely favorable for herbicide. Increasing the glyphosate rate to a range of 48 oz/A to 64 oz/A of Roundup Original and similar products can greatly increase effectiveness when weeds are more than 6 inches tall and/or other conditions are not optimum for herbicide activity.

Glyphosate-resistant corn increases the potential for volunteer corn problems in soybean when glyphosate is used for weed control. Fortunately, good volunteer corn control options exist in glyphosate-resistant soybean. Most postemergence grass herbicides are very effective in controlling volunteer corn. The exception is that Poast Plus can be less effective. Postemergence grass herbicides can be tank mixed with glyphosate, but the adjuvant requirements may be greater than the typical ammonium sulfate added with glyphosate and may depend if a glyphosate formulation requires no additional surfactant or if surfactant is required. The question of whether volunteer corn needs to be controlled depends both on the competition from the corn and the potential for dockage because of corn in the harvested soybeans. Based on previous studies from University of Wisconsin, soybean yield loss from volunteer corn is in the neighborhood of 1% yield loss for every 75 to 100 “clumps” of volunteer corn per acre.

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