Volunteer corn is something to watch as more glyphosate-resistant crops hit the market.

Weeds are nothing more than plants that are out of place. In the late 70s and early 80s postemergent weed control in soybeans was tough, especially if fields suffered grass infestation. In those cases, weed control often came down to a corn knife or hoe. Back then, you based how bad the weed problem was by how many rows you could walk. Then came herbicides with a new mode of action called lipid inhibitors. This group of herbicides, referred to by many as the “fops” and “dims,” brought us Assure II, Fusion, Poast Plus, Select and a few others. These products, when sprayed post-emergent over soybeans, allowed growers a new way to control grasses.

But since the late 90s, the soybean market has been dominated by Roundup Ready soybeans. Many of the soybean fields today receive only applications of glyphosate to control both grasses and broadleaf weeds. So let’s take a look at one grass that may or may not become a major player again in soybean fields.

Back when you walked soybean fields with a hoe or corn knife, this weed was known simply as volunteer corn. Then we had volunteer IMI corn. But now we have to consider the reality of volunteer Roundup Ready corn.

With the soybean market dominated by glyphosate-resistant soybeans, and with an increasing percentage of Roundup Ready corn acres, we have to recognize that the stage is set for glyphosate-resistant corn to begin appearing in the soybean canopy.

How do you control Roundup Ready corn? One obvious answer: not with glyphosate. In fact, this past summer I think many growers didn’t realize that they had a potential problem until after the first in-crop application of glyphosate on soybeans. I admit that this is not a new problem to some growers. However, if the Roundup Ready corn market share continues to grow, glyphosate-resistant corn becomes a potential problem for more and more growers.

According to the Missouri Agricultural Statistics Service, there were approximately 2.9 million acres of corn planted in Missouri in 2003. Of this, the statistics office estimated 2.8 million acres of corn to be harvested. If Roundup Ready corn made up 14 percent of the market share in Missouri (as is the average for the Midwest), then approximately 406,000 acres were planted to Roundup Ready corn.

I know some of this corn was taken for a silage crop, thus volunteer corn probably will not be an issue in all fields. But the bottom line is that there is a potential for problems now and in the future if Roundup Ready corn acres continue to increase.

Unlike the common field weed, there is some deductive reasoning to be used when addressing glyphosate-resistant corn.

Where did the Roundup Ready corn come from? Obviously, it’s possible that the field could have had Roundup Ready corn in the rotation. But also consider the possibility that Roundup Ready corn could have made it to the field in a planter that hadn’t been cleaned thoroughly. Or did the neighbor plant glyphosate-resistant corn across the fence, causing some pollen drift and cross pollination?

It has been suggested that to minimize corn pollen drift growers would need approximately 660 feet between fields. But reality tells us that fields are going to be planted right next to each other and that productive ground is not going to be separated by a large buffer strip. Pollen drift can also be minimized when pollination occurs at different times between fields. But coordinating hybrid maturity dates with neighbors isn’t a cultural practice many growers have adopted.

Simply put, however glyphosate-resistant volunteer corn gets in the field, it needs to be considered a potential weed problem.

The corn harvest in 2003 was variable across the state of Missouri. There were some really good yielding fields. There were some bad yielding fields. And there were some disasters. So how much corn escaped combines this harvest season? If a bushel of corn contains approximately 90,000 kernels on the average, just leaving one bushel per acre spread evenly across the ground can result in a generous flush of volunteer corn plants. We know that a lot of corn spread by combines germinated this fall because the fields were green with corn. However, not all of this harvest-loss corn will germinate in the fall. Some will hold out until the soil warms in spring. Another thing to consider is that oftentimes an entire ear falls to the ground, which can allow for germination to spread from fall to spring.

Think of it this way: One plant per 100 square feet is equivalent to just 435 plants per acre. However, if these volunteer corn plants are left alone and allowed to compete with the soybean crop, they may cause a 0.4 to 2 bushel-per-acre loss. Can you afford this type of loss? Take a look at Table 1 and Table 2, which use the WeedSoft model to predict the potential yield reduction volunteer corn can have on soybean yield. This model assumes that if the weed is not removed and allowed to compete all season, the calculated yield reduction would occur. This model was executed using an expected weed-free yield for soybeans of 50 bushels per acre. Table 1 looks at the weeds appearing when the soybeans are in the first trifoliate stage. Table 2 looks at the weeds if they are not a problem until the soybeans are in the pre-bloom stage. Remember, weed competition can have a minor effect on the crop if the problem is taken care of early. But, if left alone to compete all season, weeds can have a major impact on yield.

How soon should volunteer corn be sprayed? In general, when you can control this weed before it is 6 inches tall, its competition for light, water and nutrients will have had little effect on the soybean crop. In some cases, especially with low weed pressure, you can get away with controlling it when it’s 6 to 12 inches tall—and still have minimal effect on the soybeans. The standard caveat to remember is that the larger the corn plant gets, the greater opportunity for it to rob yield from soybeans.

My recommendation for soybeans is to always start with a clean field. Whether you use tillage or a burndown program for no-till, start clean. Then, as the soybeans begin to grow, scout the field for weed problems. If you find volunteer corn showing up in the field ask the question: Could this volunteer corn be Roundup Ready corn? If you know for sure or highly suspect that it is Roundup Ready corn, then immediately plan on tank mixing another herbicide in with the glyphosate to kill the volunteer corn. Do not wait until after you have applied a first in-season glyphosate application to realize the potential problem.

Table 3 lists several products that will control volunteer Roundup Ready corn. Adding additional herbicides to control glyphosate-resistant volunteers definitely adds to your cost of treatment. However, not applying the correct herbicide in a timely fashion may cost you even more.

Table 1. Predicting the potential yield reduction due to volunteer corn growing in soybeans at the first trifoliate stage using WeedSoft.

Table 2. Predicting the potential yield reduction due to volunteer corn growing in soybeans at the pre-bloom stage using WeedSoft.

Table 3. Herbicides that can be used for control of Roundup Ready volunteer corn in soybeans.