Cream the Weeds, Not Your Corn

May 31, 2010 07:00 PM

Rachel Duff, AgWeb Contributing Editor
Are weeds growing as fast as the corn in your fields? If so, you're probably looking to zap those weeds with an herbicide. Remember, herbicide restrictions are important in keeping the corn plant healthy.
Each herbicide has a certain list of restrictions that need to be followed, says Bob Hartzler, extension weed specialist for Iowa State University. Some products can harm corn after a certain time, leave herbicide residues on the grain or not be effective on certain weeds. It's important to follow the label to prevent crop injury and promote weed control.
Almost all products can be used in the current crop window. The majority of corn will be ready in the next week for these herbicides to be applied. It's also important to remember that as corn gets larger, so do the weeds, Hartzler says.
Before applying a herbicide, it's important to take a trip through the fields to see what growth stage the crop is in, says Aaron Hager, University of Illinois Extension Weed Specialist, in a recent University of Illinois publication. Undesirable weather conditions can make corn plants physiologically older than they might seem according to their height, so when checking the field, the leaf/collar number should be taken into account as well as the plant height.
Being timely is the most important aspect of herbicide application. Weed shifts are occurring rapidly, Hager says. Instead of waiting until the corn is out of the field, apply quickly and decide if that herbicide was effective at the time. Applying when weeds are small is also important to gain control early.
Saving trips across the field can be important in a time crunch. "Some labels suggest changing from one type of additive to another when the corn crop is under stressful growing conditions," Hager says. "However, attempting to save a trip across the field by applying a post-emergence corn herbicide with a liquid nitrogen fertilizer solution such as UAN as the carrier is not advisable. Applying high rates of UAN by itself can cause corn injury, but adding a post-emergence herbicide can greatly increase corn injury."
Hot temperatures are good for corn growth but can cause problems for herbicide application. The high temperatures can make the herbicide volatile because the vapors are moved by the wind and could unintentionally carry the herbicide to other plant life and cause injury to it.
Many farmers are dealing with wet conditions, and some of those fields are in a no-win situation, Hartzler says. If it's been too wet to get into the field, the weeds have had a chance to grow. It can be tempting to mix herbicides if the field has been too wet the past couple of weeks, but there's really no benefit in mixing any more than two herbicides. If farmers mix more than one herbicide, the instructions for the most restrictive corn growth stage on the herbicides' labels should be followed, Hager says. Mixing more herbicides than that can be counter productive in the cases of wet fields, Hartzler says. In the case of a wet field, identify the biggest problems, choose the herbicide and hope for the best.
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