Source: Stu Ellis, University of Illinois

If you had weeds in your soybeans, you applied Roundup. If you had weeds in your corn, you applied atrazine. If you had other chores to do, you did them and did not worry about weedy fields. But now there are weeds that don't want to be controlled, and with ragweed, waterhemp and marestail resisting your herbicide application in glyphosate resistant corn and soybeans, your fields are not as clean as Dad tried to keep them. What is the problem here?

Some weed specialists might say it is too much reliance on glyphosate in soybeans. But Ohio State University weed specialist Mark Loux looks at your challenge a different way, and says it may be rooted in your weed management program when the problem fields are planted to corn. In Loux's assessment it may be easier to control glyphosate-resistant weeds as part of your corn herbicide program than trying to do it with your soybean program. He says weather and crop development are potential contributors to your challenge, but so is a potential failure to use enough residual herbicide in your pre-emerge and post emerge programs. Loux says other contributors could be the inability of your pre-emerge program to control giant ragweed, as well as a timing issue in your post-emerge program for soybeans.

Loux wants you to focus on two issues in your pre-emerge program and post emerge program for corn. 1) Does your residual pre-emerge herbicide perform adequately until the post emerge herbicide can be applied? 2) Does the residual herbicide provide enough control of the more difficult to control weeds, or a long enough period of control for such weeds as foxtail, panicum, giant ragweed, waterhemp and pigweed so that the combination of pre and post emergent herbicides work through the end of the growing season?

He says corn needs help from herbicides until it can develop a canopy at 20 inches tall. The canopy will help with weed suppression, but herbicides need to help with suppression until the canopy is formed. He says the pre-emergent herbicide should do most of the work, and followed by the post emergent herbicide, which is a "finishing tool." And if the post emergent herbicide has to do the bulk of the work, your yield has been compromised. He says a 75 percent atrazine rate should be the minimum in a pre-mix, assuming there is a post herbicide application. Loux adds, "For any of these weeds, it's important to have a higher rate and/or more comprehensive PRE treatment that improves and extends control, and results in smaller weeds at the time of the POST (giant ragweed), or provides almost complete early-season control to allow for a later POST application."

Loux says giant ragweed tolerates many pre-emergent herbicides and requires higher rates to control, but also it has an ability to emerge over the course of many weeks, and not all at once with other weeds, plus it has a low level resistance to glyphosate. As a result, he says the most effective program occurs when the pre-emergent herbicide is applied close to a full rate, and includes two different herbicides that exhibit good control on giant ragweed. He suggests: Lexar/Lumax, mixtures of atrazine or an atrazine premix with any of the following: Corvus, Balance Flex; SureStart/TripleFlex, Hornet or Verdict. But he says you will also need a grass herbicide. Those will provide good residual control and keep weeds under control until the post emergent application is made. But he says if your ragweed has developed a strong ability to resist glyphosate, and then combine it with Status, dicamba or Callisto.

For cost considerations, Loux suggests a generic atrazine premix product that is used at a 100 percent rate, which is still less than some primary herbicides, such as Parallel Plus versus Bicep II Magnum. He also says that using less than a 100 percent rate will open the door for problems that can be caused by the weather. Loux says corn is a valuable crop and trying to save $5 or $10 on pre-emergent herbicides may result in a $25 loss in yield.

Summary:
Some weeds are becoming difficult to control with just a glyphosate spray in soybeans, but if the field is rotated to corn, the use of corn herbicides with better residual weed control may be a way of reducing the population of glyphosate tolerant weeds. However, your goal is for weed control, so full rates of herbicides are your likely choice. And relying on your pre-emergent herbicide program is a better way to protect your corn yield than relying upon a post-emergent application.