While volunteer corn has been a “weed” in soybean fields ever since corn fields have been rotated to soybeans, we have not thought much about volunteer corn with Roundup Ready soybeans. Prior to significant adoption of glyphosate-resistant (e.g. Roundup Ready) corn, any volunteer corn would easily be controlled in soybeans when glyphosate was applied. However, glyphosate-resistant corn produces resistant volunteers the next year if ears are dropped in the field (Fig. 1 and 2). 

 

Figures 1 and 2. Volunteer glyphosate-resistant corn can form clumps from ears or individual plants in soybean fields.

 

We can review two studies for answers. In Minnesota, clumps of volunteer corn (7 to 10 plants/clump) were tested at three locations over 2 years in row soybeans (Andersen et al. 1982). Averaged across the six experiments, they found soybean yield was reduced 1% for every 75 clumps of volunteer corn per acre. In a similar study in Illinois, weed scientists determined a slightly greater density of volunteer corn was required to cause a similar yield loss (Beckett and Stoller 1988). Soybean yield loss was reduced 1% for every 91 clumps/acre of volunteer corn in one year and 1% for every 116 clumps/acre in a second year. The clumps had 10 corn plants in the Illinois study. These results only illustrate the effect of volunteer corn on soybean yield. Volunteer corn can also cause dockage if present at higher levels in the harvested soybeans.   

 

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

What is the best management?

I think the key to volunteer corn management is to know if the field had glyphosate-resistant corn the previous year. If so, scout to determine if volunteer corn is present. It will be much more cost effective to include a postemergence grass herbicide in the initial glyphosate application than to make a second application just to control the volunteer corn. A second application may require a higher herbicide rate plus the additional application cost. In total, this may cost $10/acre or more than if the postemergence grass herbicide was tank mixed with the first application.

 

Andersen, R. N., J. H. Ford, and W. E. Lueschen. 1982. Controlling volunteer corn (Zea mays) in soybean (Glycine max) with diclofop and glyphosate. Weed Sci. 30:132-136.

 

Beckett, T. H. and E. W. Stoller. 1988. Volunteer corn (Zea mays) interference in soybeans (Glycine max). Weed Sci. 36:159-166.