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Issue No. 12, Article 8/June 15, 2007

Postemergence Herbicide Applications in Soybean: Pull the Trigger or Wait for Rain?

Recently several folks have inquired whether they should go ahead and make the initial postemergence application of glyphosate in soybean or wait until more favorable weather (i.e., rain) arrives that would relieve the "stress" on the weeds and encourage another weed emergence event. Dry soil conditions and a desire to reduce expenditures by making only one postemergence herbicide application are frequently mentioned during these inquiries. The following thoughts and suggestions related to soybean weed control under the current (dry) environmental conditions are provided for your consideration.

Dry soil conditions can, in fact, reduce weed control efficacy of some postemergence herbicides. Certain weed species, such as common lambsquarters, can become increasingly difficult to control under dry soil conditions, even though the weeds continue to increase in size (albeit more slowly). Dust generated by the ground application equipment may also contribute to reduced performance of certain herbicides, such as glyphosate. Dr. Chris Boerboom, Extension weed scientist at the University of Wisconsin, recently wrote an excellent article summarizing current research on the effects of dust and postemergence herbicides. You can read the article ("Dust Interferes with Glyphosate Activity") in the May 31, 2007, issue of the Wisconsin Crop Manager.

Keep in mind, however, that existing weeds continue to compete with soybean plants for needed resources, including soil moisture. Results from field experiments conducted over many years, locations, and environments have demonstrated that early-season weed competition can significantly impact soybean yield. The deleterious effects of early-season weed interference on soybean growth and yield can be even more acute under adverse growing conditions, such as the very dry conditions in many areas of Illinois. Additional weed emergence may be reduced or delayed until soil moisture conditions improve, but weed interference caused by existing weed vegetation will continue to reduce soybean yield potential. At the current price of soybean, it doesn't take many bushels lost to interference to equal the cost of a second postemergence application of glyphosate. Therefore, delaying an initial glyphosate application until additional weeds emerge may in fact cost more than making two timely applications.

Pay close attention to glyphosate rate and additive selection, especially in areas with very limited soil moisture. Application rates should be increased above 0.75 lb ae/acre if drought conditions are prevalent, weeds are over 6 to 8 inches tall, and/or "tough" weed species, such as annual morningglory, common lambsquarters, or perennial broadleaves, are prevalent in the field. Ammonium sulfate should be included with all glyphosate applications; this is especially important where hard water is used as the spray carrier, velvetleaf is present, or other products are tank-mixed with glyphosate. The rate of ammonium sulfate should be at least 8.5 pounds, and preferably 17 pounds, per 100 gallons of water. Many additive blends include some amount of ammonium sulfate but may not provide these suggested amounts at their recommended use rates. Also, research has indicated that poor spray distribution/deposition can result from certain combinations of drift retardants, drift-reducing spray nozzles, and low application volumes.

Remain vigilant for instances of poor weed control following a glyphosate application that may not be completely attributable to adverse environmental conditions. We have received several reports of waterhemp surviving after a glyphosate application in fields where all other weed species were effectively controlled.--Aaron Hager

Aaron Hager

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