Managing Glyphosate-Tolerant Corn in Soybeans (Field Facts)


Use of glyphosate herbicides with glyphosate-tolerant corn is a simple, inexpensive, broad-spectrum weed control program. This has led to the rapid adoption of this technology. However, this production system comes with a unique challenge - glyphosate-tolerant volunteer corn plants that survive as grain and germinate in the soybean crop the next season. Growers can use two approaches to managing this problem -- preventing ear and kernel losses in the fall, and controlling volunteer plants in the soybean crop with cultivation or herbicides.

Volunteer corn in soybean field.

Volunteer corn in soybean field.


Preventing Volunteer Corn

Volunteer corn results from ear and kernel losses in the field the previous season. Ears may drop for a number of reasons, including genetic weakness of the ear attachment, drought stress, and corn borer feeding in the ear shank. Ears may also be lost on plants that are severely lodged or may randomly bounce off the corn head. Kernel losses result from shelling at the corn head, incomplete threshing of cobs, or combine adjustments that allow kernels to be lost out the back of the combine. Growers are encouraged to follow the suggestions below to prevent glyphosate-tolerant volunteer corn in their soybean fields.

  • Use Pioneer® brand hybrids with one of the European corn borer insect protection traits to eliminate ear drop problems caused by corn borer feeding in ear shanks. On refuge acres, select hybrids with the best available natural tolerance to second brood corn borer and have good ear retention characteristics. Pioneer provides an ear retention rating to aid in selecting hybrids for this purpose.
  • Harvest fields in a timely manner to avoid harvest losses. Scout and monitor fields in the fall and be aware of any corn borer feeding, drought effects, stalk rots, or root lodging problems. Those fields may need to be harvested first, even if other fields are dryer. Lodged corn resulting in whole ears being lost is often the primary contributor to volunteer corn problems the next year. Schedule harvest based on crop condition – not just grain moisture.
  • Make sure combines are properly adjusted to reduce harvest losses, and check behind the combine frequently to monitor combine performance.
    • Gathering losses occur due to missed ears or shelling at the corn head.
    • Cylinder losses are kernels attached to pieces of cob that were not shelled by the combine cylinder.
    • Separating losses are loose kernels that were not shaken out of the cobs and husks and were lost out the back of the combine.

Check the link below or your local extension website for information on measuring and pinpointing harvest losses, and adjusting combines to minimize losses.

If harvest losses are extremely severe, growers may attempt to germinate lost corn in the fall. A light tillage operation that puts much of the remaining corn kernels and ears into the top one to three inches of soil can stimulate grain to germinate and then die out in the winter. This approach should be reserved to areas where soil erosion is not a problem and the soil is still warm enough for corn to germinate. Otherwise, it is best to leave the field alone and no-till the soybean crop the next spring. No-tilling allows wildlife to forage the seed during the winter and eliminates spring soil disturbance. Soil disturbance in the spring can cause lost grain to be planted and germinate and grow with the soybeans.


Controlling Volunteer Corn in Soybean Fields

Non-harvested corn that germinates and grows in soybean fields can be controlled with cultivation or herbicides. Cultivation is an option on less-sloping fields where tillage and reduction of surface residue will not result in soil erosion. However, because it is time-consuming and fuel-intensive, cultivation will not compare favorably with herbicide application in most cases.

If the volunteer corn is tolerant to glyphosate, it will not be controlled with glyphosate herbicides. This would be the case if glyphosate-tolerant corn was planted in the field the previous year. It is also possible that conventional corn could have obtained the glyphosate trait due to adventitious presence of the trait (such as offsite pollen movement last year.) This could have occurred in commercial hybrid corn fields planted near glyphosate-tolerant corn. Offsite pollen drift can also produce low levels of this trait in hybrid seed corn. For this reason, growers should obtain their seed corn from companies with a good track record of providing pure, high quality seed by using high production standards including ideal isolation requirements.

Volunteer corn in soybeans can be controlled best with burn-down and post-emergence herbicides. Although some soil-applied herbicides can suppress volunteer corn, the post-emergence grass control products are much more effective.

If the overall pressure is not high, it is possible to skip the early burndown and wait to apply a grass control product tank-mixed with glyphosate after Roundup Ready® soybeans emerge. However, the timing of the post application is then more critical to ensure that weed and volunteer corn growth are not competing with the soybeans and hurting final yields. Several post-emergence grass herbicides are effective in killing volunteer corn in soybeans (Table 1).

Table 1. Commonly used post-emergence herbicides for control of glyphosate-tolerant volunteer corn in soybeans.

Commonly used post-emergence herbicides for control of glyphosate-tolerant volunteer corn in soybeans.

*See product labels for restrictions and other details of use.

Tank Mixes
If growers are aware early on of the need to control volunteer glyphosate-tolerant corn in their Roundup Ready soybean fields, they can conveniently do so by using tank-mixtures of a labeled grass herbicide with their glyphosate herbicide (glyphosate herbicides will kill or severely injure soybeans not containing the Roundup Ready trait.). In these tank mixes, it may be necessary to include a spray adjuvant to ensure effectiveness of the grass herbicide. However, no additional adjuvant is needed with some glyphosate formulations, so check product labels carefully. Use of crop oil concentrates or methylated seed oils in tank mixes with glyphosate herbicides is NOT recommended, because it can result in crop damage and/or reduced herbicide performance. Liquid nitrogen fertilizer or sprayable grade ammonium sulfate (AMS) may also be recommended in some cases. Read and follow label directions for both products carefully when tank mixing herbicides.

Dense clumps of volunteer corn in soybean field.

Good spray coverage is important to control dense clumps of volunteer corn.


As with other annual weeds, it is best to spray the volunteer corn early to get the best results – for example, before corn is taller than 12 inches. When used with their suggested adjuvants, the grass herbicides listed in Table 1 are labeled to control taller volunteer corn. However, in some tank mixes, the grass herbicide is relying on the preloaded surfactant in the glyphosate product as its adjuvant. This may not optimize the performance of the grass herbicide compared to its recommended adjuvant. For that reason, apply early when volunteer corn is easier to control.

Spraying Considerations
Volunteer corn often grows in dense clumps due to germina-tion of many kernels on a dropped ear. In these dense clumps, some plants may be shielded by others during post-emergence spraying operations. Growers can reduce this problem by using higher spray volumes with correct nozzle types and alignment to provide good spray coverage.

Contact your local Pioneer sales professional if you have questions regarding management of volunteer corn plants in your soybean fields.

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