Maximum soybean growth stage for glyphosate applications clarified

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.      

Recently, Monsanto has clarified the maximum stage for which glyphosate can be applied to Roundup Ready (glyphosate-resistant) soybeans. Currently stated on glyphosate labels, glyphosate can be applied from soybean cracking to “throughout flowering”. Throughout flowering is defined as the R2 soybean growth stage. The R2 soybean growth stage is where one of the two uppermost nodes on the main stem has an open flower. The R3 growth stage starts when one of the four uppermost nodes with a fully developed trifoliate leaf on the main stem has a pod that is 3/16 inches long. According to this definition, glyphosate can be applied through the R2 growth stage, but applications after the R3 stage begins will be off-label.

The clarification of this definition is important when making glyphosate applications to control late flushes of weeds or weed escapes in soybeans. These applications must be made prior to R3 soybeans. The clarification of this definition is also important with the trend for tank-mixing other pesticides, fungicides or insecticides, with glyphosate in soybeans. Many times the push to tank-mix these products is to clean up weed escapes in these fields and for the cost savings of a single application.

While a cost savings can be a fairly good reason to tank-mix these products it is important to remember that glyphosate can not be applied after the R2 soybean stage and that at this stage, insects such as soybean aphids, or diseases may not have reached threshold levels in soybeans to make the insecticide or fungicide application useful or economical. Additionally, if glyphosate applications are postponed until insects or diseases are at threshold levels chances are you will be off-label (past R2 soybean) or have already reduced your soybean yield due to weed competition.

Application methods also differ between glyphosate and insecticides or fungicides. Generally, glyphosate applications are made at lower pressures and spray volumes to reduce herbicide drift. However, for optimal control of insects or diseases, insecticides and fungicides are usually applied at higher volumes and pressures to increase coverage of the target species. With these considerations in mind, there probably will be very few times when it would be appropriate to tank-mix glyphosate with an insecticide or fungicide.

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