Start Scouting for Volunteer Corn

Chris Boerboom, Extension Weed Scientist

The increasing adoption of glyphosate-resistant corn (e.g. RR, GT) increases the potential for volunteer corn problems in soybean when glyphosate is used for weed control. After all, the resistance trait is inherited and is passed from one generation to the next. If you remember your biology or genetics course, a heterozygous corn hybrid (F1) is made when resistance is inherited from one inbred parent and the other parent is a conventional inbred. When this F1 hybrid is grown in a production field, all the plants are resistant, but the resistant trait is only carried in half of the corn plant’s pollen and in half of the ovules (eggs). The F2 seed that is harvested will be a mix of resistant seed and conventional seed. The ratio should be three resistant seeds for each conventional seed. If any of this corn or ears fall to the ground and becomes volunteer corn, most will be resistant. The volunteer corn that was sprayed with glyphosate in Figure 1 shows this segregation. Six plants are resistant and 2 plants were killed by the glyphosate, a perfect 3:1 ratio. Clearly, volunteer corn where 75% of plants have resistance is too much to ignore. 

Figure 1. Volunteer corn in soybeans that has segregated for the glyphosate resistant trait.

Fortunately, good volunteer corn control options exist in glyphosate-resistant soybean. Most postemergence grass herbicides (ACCase inhibitors) are very effective in controlling volunteer corn. The notable exception is that Poast Plus can be less effective. These herbicides can be tank mixed with glyphosate, but the adjuvant requirements may be greater than the typical ammonium sulfate (AMS) added with glyphosate and may depend if a glyphosate formulation is “loaded” (requires no additional surfactant) or “non-loaded” (surfactant is required). 

Examples of herbicides for volunteer glyphosate-resistant corn control in soybean.
 

Herbicide	Volunteer corn height	Rate/acre	Adjuvants
Arrow	0-12 inches	4 oz	Add 1 pt/a crop oil plus 2.5 lb/a AMS
	12-24 inches	6 oz	Add 2.5 lb/a AMS if loaded glyphosate; 0.25% surfactant or 1 pt/a crop oil if non-loaded glyphosate
Assure II or Targa	0-12 inches	4 oz	Add 0.125% surfactant if loaded glyphosate; 0.25% surfactant if non-loaded glyphosate
	12-18 inches	5 oz
	18-30 inches	8 oz
Fusion	0-12 inches	4 oz	Add 0.25% crop oil even with loaded glyphosate
	12-24 inches	6 oz	Add 0.25-0.5% surfactant or 0.5-1% crop oil
Fusilade DX	0-12 inches	4 oz	Add 0.25% crop oil even with loaded glyphosate
	12-24 inches	6 oz	Add 0.25-0.5% surfactant or 0.5-1% crop oil
SelectMax	0-12 inches	6 oz	Add AMS if loaded glyphosate; 0.25% surfactant + AMS if non-loaded glyphosate
	12-24 inches	9 oz
	24-36 inches	12 oz

Of course, a grass herbicide is not needed unless the previous year’s corn hybrid was a RR or GT hybrid. Therefore, a key for success is record keeping and tracking hybrids and fields. If glyphosate-resistant volunteer corn is present in a field, a second question is when to apply the grass herbicide – with the first glyphosate application or with the second application if two are planned. I would be inclined to add the grass herbicide to the first application for the reason that the corn will be easier to kill with lower rates and to remove the corn so it competes less with the soybeans. The risk is that not all of the volunteer corn has emerged. However, the late corn is probably not going to be as damaging to soybean yields as the earlier corn and late corn is also less likely to produce mature seed. I can see arguments on both sides in terms of timing. 

Volunteer glyphosate-resistant corn in this year’s RR or GT corn cannot be controlled other than with cultivation. If this year’s corn has LibertyLink (LL) resistance or is a LL stack, Ignite can be used to control or suppress volunteer glyphosate-resistant corn. Ignite may not kill all of the volunteer corn if it is too large, but it should severely stunt it. Similarly, volunteer LL corn can be controlled with glyphosate. However, if last year’s corn was a RR/LL stack, there are no herbicide control options.