While visiting some fields in Henry County this past week, I found several with volunteer corn that will need to be controlled in the post-emergence pass. Although tillage or pre-emergence herbicides were used to prevent or take out early germinators, the volunteer corn has come up and is competing with the current soybean crop.  

Consequences 

Dropped ears and lost kernels from the combine can overwinter and grow the next soybean growing season, becoming a weed needing control due to competition for nutrients and water that young soybeans require to thrive and maximize yield potential. Volunteer corn populations among soybeans also provide a haven for corn rootworm, competing with the next year’s corn crop as well.   

A two-year South Dakota State study (Alms et al. 2008) conducted in 2007-08 indicated that densities of 5,000 corn plants per acre reduced soybean yield by 20% – a 12-bushel yield loss for 60 bu/acre soybeans. This density of volunteer corn equates to one plant in every 3.5 feet of row.   

We often see clumps of volunteer corn growing in a soybean field due to ears dropped the previous fall. These clumps of volunteer corn can affect yield as well. A university study in the 1980s by the University of Minnesota (Andersen et al. 1982) and the University of Illinois (Becket and Stoller 1988) indicated that soybean yield can be reduced 1% for every 75-115 clumps per acre.   

It’s common to see weed-free soybean fields where volunteer corn either pops up later or was too small to be controlled during the first post-herbicide application was applied. This often happens because other weeds were controlled with glyphosate, glufosinate and 2,4-D or dicamba – chemistries that most corn is either resistant to, due to trait packages, or tolerant of because of their growth regulator classifications.  

When volunteer corn is missed early and an ACCase inhibitor, such as a FOP and DIM (growing point disintegrators), is not included in the tank mix, a second pass with that chemistry will likely be needed later. By then, the volunteer corn has usually grown to a stage where higher labeled rates must be applied.  

Recommendations 

Scout all soybean fields scheduled for their first post-emergence application to check for emerging volunteer corn. If so, most applications can be done with the lower rates on the herbicide labels. Most of the FOPs (Assure II, Fusilade DX, Fusion) and the DIMs (Select, Arrow and any others) can be applied at the 4 oz rate for 0-12 inch volunteer corn and bumped up to 6 oz for 12-24 inches. Select Max is slightly different: the rates are 6 oz for 0-12 inch, 9 oz for 12-24 inch, and 12 oz for 24-36 inch volunteer corn control.   

Tank Mixing Watch Outs 

When applying 2,4-D in the tank mix, there can be antagonism or tie-up issues when mixing with ACCase inhibitors.  My main recommendation is to increase the rate to avoid the antagonism issue.   

A big thank you to the UNL Crop Watch team for a great article I found from 2018 worth revisiting!