Farm Progress

How to keep resistant corn out of LibertyLink soybeans

Glyphosate-resistant volunteer corn is a competitive weed that can cause significant soybean yield reduction if not controlled.

June 26, 2017

3 Min Read
CORN PEST: A soybean field infested with volunteer corn in south-central Nebraska. If not controlled, volunteer corn can contaminate the harvested soybean, resulting in dockage.

By Parminder Chahal and Amit Jhala

Glyphosate-resistant volunteer corn is a major weed in corn-soybean cropping systems. It results from the harvest loss of glyphosate-resistant hybrid corn planted in the previous season.

Volunteer corn is a competitive weed that can cause significant soybean yield reduction depending on density. If not controlled, it will contaminate the harvested soybeans, resulting in dockage. In addition, volunteer corn encourages the dispersal and survival of western corn rootworm and gray leaf spot disease, limiting the benefits of a corn-soybean rotation.

The preemergence herbicides registered in soybeans are not very effective for volunteer corn control. Therefore, the only effective option for controlling glyphosate-resistant volunteer corn in glyphosate-resistant soybeans is the postemergence application of acetyl-coenzyme A carboxylase (ACCase) inhibiting-herbicides, also known as graminicides, such as Assure II, Fusilade, Select Max, or Poast Plus.

Overreliance on glyphosate for weed control in corn and soybeans has resulted in the evolution of glyphosate-resistant weeds. Six weeds have evolved resistance to glyphosate in Nebraska. Therefore, there is a need to diversify weed control programs by using herbicides with distinct modes of action or alternate herbicide-resistant crop technology. For example, LibertyLink soybeans have provided growers an opportunity to apply Liberty to control some troublesome weeds, including glyphosate-resistant weeds.

Liberty is a nonselective, contact, postemergence herbicide for broad-spectrum weed control in LibertyLink soybeans. Liberty can be applied sequentially in LibertyLink soybeans, with a maximum cumulative rate of 65 fluid ounces per acre per growing season. According to the label, a single Liberty application up to 29 to 36 fluid ounces per acre can be made in LibertyLink soybeans, and if needed, another application of 29 fluid ounces can be made postemergence before soybeans start flowering. A recent update to the Liberty label allows a maximum cumulative rate of 87 fluid ounces per acre per growing season. Two applications, each of 32 to 43 fluid ounces per acre, could be made.

A field study was conducted at the South Central Agricultural Laboratory (SCAL) near Clay Center in 2013-14 to compare the efficacy of Liberty applied at different rates in single or sequential applications or in tank-mixed combination with graminicides for control of glyphosate-resistant volunteer corn. Liberty applied in single or sequential applications provided greater than 80% control of glyphosate-resistant volunteer corn at two, four and 12 weeks after early postemergence application.

Graminicides (Assure II, Select Max, Poast Plus, Fusilade) can be tank-mixed with Liberty; however, this combination slightly reduces glyphosate-resistant volunteer corn control at two weeks after postemergence. Therefore, depending on control achieved by Liberty plus Assure II or Select Max and volunteer corn infestation and recovery, additional Liberty can be applied if flowering has not begun in Liberty Link soybeans.

Keep in mind that Liberty can only be applied in LibertyLink soybeans, and not in Roundup Ready or Roundup Ready 2Xtend soybeans. Application of graminicides (Assure II, Select Max, Poast, Fusilade) will be the only option for volunteer corn control in Roundup Ready or Roundup Ready 2Xtend soybeans.

Chahal is a UNL weed science graduate student and Jhala is a Nebraska Extension weed management specialist. This report comes from UNL CropWatch.

 

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