Large populations of volunteer corn are being reported in some fields in Minnesota this year. What impact the volunteer corn will have on this year’s crop yield and the viable management options available will depend upon in which crop the volunteer corn is present. Making the assumptions that the majority of the volunteer corn present is glyphosate resistant and that glyphosate-resistant crops were planted in the field this year, your only management option in corn at this time is cultivation. In soybean you have the herbicide options of the ACCase inhibiting herbicides such as: Select Max (clethodim), Fusilade DX (fluazifop-P), Fusion (fluazifop-P & fenoxaprop) and Assure II (quizalofop); note Poast Plus (sethoxydim) is not as active as the other herbicides on volunteer corn. The ACCase inhibiting herbicides are generally targeted on 12-24-in.-tall volunteer corn. The ALS herbicide Raptor can also effectively control smaller (2-8 in.) volunteer corn.

In 2007, researchers at South Dakota State University indicated that volunteer corn is much less competitive in corn than soybean. The South Dakota study (Alms et al. 2007) evaluated the full season effect of a range of volunteer corn densities (800-14,000 plants/acre) on both corn and soybean and reported yield losses that ranged from 0% to 13% in corn and 0% to 54% in soybean. A 2007 University of Minnesota (U of M) study reported yield loss potential in corn that was very similar to the South Dakota study. Iowa State reported one volunteer corn plant per 10 ft. of row reduced corn yield 1.3%. This lower impact on corn is likely due to a volunteer corn’s reduced demand for resources and the competitive vigor of the planted F1 hybrid. Volunteer corn has a lower yield potential than the planted F1 hybrid resulting from delayed emergence. In the U of M study, volunteer corn plants lagged from one to six leaf stages behind the crop and few plants produced an ear by harvest.

If volunteer corn populations are high and conditions remain dry, inter-row cultivation is the most cost-effective option in corn unless the herbicide-resistance traits of volunteer corn and the planted corn differ; then herbicide control is possible. Volunteer conventional corn can be controlled with glyphosate or Ignite; volunteer Liberty Link (LL) corn can be controlled with glyphosate; volunteer Roundup Ready (RR) corn can be controlled with Ignite. Unfortunately, stacked LL/RR corn must be cultivated. Ignite can be less effective than glyphosate due to its contact activity, but it will still significantly reduce the competitiveness of the volunteer corn.

Volunteer corn can significantly reduce soybean yield. A study conducted in Minnesota in 1979 and 1980 (by Andersen et al. 1982) evaluated the effect of clumps of volunteer corn per row of soybean (approximately 8 plants/clump) on soybean yield. When averaged over six experiments, soybean yield was reduced 1% for every 75 clumps/acre. The authors also indicated that at a density of 75 clumps/acre or greater, delays in herbicide application of approximately three weeks (from mid-June into July) resulted in a reduction in soybean yield.

Like any weed, early emerging volunteer corn that competes longer into the growing season with soybean will have a greater impact on yield, especially if drought conditions persist. However, volunteer corn emergence is often delayed and extends throughout the growing season making the timing and economics of the time of weed removal more difficult to determine. To save a trip across the field the ACCase inhibiting herbicides mentioned previously can be tankmixed with glyphosate, but adjuvant requirements may need to be adjusted depending upon the glyphosate and ACCase herbicide formulations used (see specific labels for details). Also, different ACCase rates are associated with different volunteer growth stages. Often growers would like to wait later into the growing season before treating for volunteer corn, to allow for the extended emergence period. This response is understandable but must be tempered by the fact that the early emerging volunteer corn plants will cause the greatest yield loss and extending the herbicide application too late into the growing season can diminish herbicide effectiveness, potentially resulting in reduction in soybean yield.

Entomologists Bruce Potter and Ken Ostlie are proposing additional reasons to remove volunteer corn in soybean earlier rather than later. First, allowing volunteer corn plants to grow into July allows time for any hatching corn rootworm larvae to feed and possibly survive on volunteer corn within soybean fields. Survival rates depend on whether or not volunteer corn expresses the Bt-RW gene. An additional concern is an increase of corn rootworm exposure to sub-lethal levels of the Bt proteins insecticidal properties and potentially accelerating development of resistance. Second, volunteer corn plants can serve as sites for corn rootworm egg laying, potentially increasing rootworm populations in the next corn crop. Therefore, the presence of volunteer corn reduces the crop rotation effect. While much is still unknown in this area of research, removing volunteer corn plants in mid-June rather than July would help to mitigate these potential problems.