Volunteer Corn in Soybean Fields

BY Dairyland Seed Agronomy Team

The photo above shows a volunteer corn plant in a soybean field. Another word to describe this volunteer corn plant is, “it’s a weed”, and a weed, is a plant that is growing where it is not supposed to be. A weed competes with the intended crop for space, water, light and nutrients, and the value or cost of water and nutrients has not gone down in recent history. Over the years I have been told that “a volunteer corn plant here and there in your soybeans is not worth the cost to kill it”. In one memorable instance “my elevator doesn’t dock for it, and really, when is the last time you were able to get $8-9 for your corn” which, considering recent soybean prices being $11 to $13, you could insert those numbers instead.

Some of the more apparent concerns with volunteer corn is the fact that it reduces your soybean yield 10 to 20 percent. Various studies from land grant institutions have shown this. So, how much corn equates to a 10 percent yield loss? Only about six volunteer corn plants per 100 foot of row will achieve that amount of loss. Another rule of thumb is 2.4 volunteer corn plants per 100 square foot can reduce yield by 10 percent. (For those of us that used to walk beans with a hoe and kill weeds, that hoe is about 5 foot in length. So if you took two hoe lengths long and two hoe lengths wide that would give you 100 sq. ft.) These yield losses are on top of the water and nutrients that were not used by the intended crop.

10% Yield Loss

6 Plants per 100 foot of row

2.4 Plants per 100 sq. ft.

Some of the unintended consequences of not controlling volunteer corn is that this weed can attract corn rootworm beetles to those plants, and then potentially lay eggs. That reduces the benefits or advantage of a crop rotation, creating a bigger threat to next year’s corn crop. There has been some discussion that if the previous year’s corn crop was a Bt corn rootworm event, by not controlling volunteer corn in soybeans, there is the potential of reduced levels of expression of Bt in those plants. We could select for resistance to these Bt events. If this were to happen, it is not a good scenario for anyone.

Another concern that I have is by having these volunteer corn plants we are also allowing diseases such as northern corn leaf blight, gray leaf spot, Goss’s Wilt and tar spot, as well as numerous others an opportunity to infect plants in the rotation year, which potentially sets up the next or future crops for more disease pressure. Which again defeats one of the reasons of rotating from one crop to the other.

Lastly, if you have already controlled your volunteer corn, good job. If you are still in the process of doing that, and using a volunteer corn product to accomplish this, remember you will need to increase your volunteer corn herbicide rate 30 to 50 percent when tank mixing with a broadleaf herbicide. The addition of surfactants and oils will increase the effectiveness of your volunteer corn control, however check the herbicide labels of the products you are using to see if this allowed and or rates recommend. If you have questions about this or other Agronomic topics, please contact your Dairyland Seed Agronomist.