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Current Crop Weather Good for Weeds, Too

Jeff Caldwell Updated: 07/14/2014 @ 4:36pm Agricultural content creator and marketer.

The weather has been great for the corn and soybean crops in much of the Midwest so far this year. So much so that some states' crops are in as good condition at this point in the growing season as they've been in decades.

However, what's good for the crops is also good for the weeds that invade them. That means some of your fields might be harboring weed stands just as healthy as the corn and soybeans among them. Experts say that means you'll need to make a couple of key changes to your weed control plans this year.

First, your preemergence herbicides may have been knocked out of commission by heavy rain. If so, weeds like waterhemp could be coming back with a vengeance, says Iowa State University (ISU) Extension agronomist Bob Hartzler.

"The heavy rains across the state have made timely postemergence weed control difficult, even in fields treated with preemergence herbicides. This leads to questions about what can be done to ensure control of weeds larger than specified on herbicide labels," he says. "Keep in mind that if there were a way to improve the consistency of a herbicide on large weeds, the manufacturer probably would have included that practice on the label."

All hope is not lost if your weeds are already shooting to the sky: Though it's been a common practices for a long time, herbicide cocktails can be tough to mix the right way so you don't do more harm than good. If you want to do this, though, just do your homework first and read the labels carefully.

"Unfortunately, products registered for use in soybeans that can be added to a Group 14 herbicide have minimal activity on waterhemp, thus little or no benefit exists in using these mixes in terms of improving waterhemp control," Hartzler says, using the example of waterhemp in soybeans. "The addition of these products may, however, help control other weeds on which they have good activity, such as velvetleaf. Remember, a combination of multiple herbicides can increase the likelihood of significant crop injury compared to the products individually."

You can also just put on more than you normally would, something that can be done in a number of ways. Hartzler says you can simply up the spray volume, slow down while you're spraying, adjust nozzles, or lower your boom height to get more chemical where it needs to be. Considering the relative lateness for applications, plus the potential for higher weed height overall, you might need to keep the sprayer wheels turning as fast as possible. If that's the case, you still may be able to up the punch your herbicide packs by some of the other steps Hartzler suggests.

"Applicators using ground sprayers for herbicide may be using higher speeds to catch up with weeds and cover more acres in a narrow time window. If so, they should consider the travel speed they’ve used to calculate appropriate nozzle size," according to a report from ISU Extension agronomist Clarke McGrath and ISU Extension ag engineer Mark Hanna. "If field speed increases by 25%, spray pressure for a given nozzle must increase by 56% to maintain output. A better choice to reduce smaller droplets and minimize drift would be to increase nozzle size."

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