Double check labels, scout weed size for maximum weed control

June 13, 2016 5:31 PM
Blog Post

Corn sprayingburned soybeans

Corn spraying is about wrapped up, for fields yet to go just take a minute to double check corn stage/size and compare to herbicide labels. We don’t want anyone to run into herbicide injury, as this corn is really growing quickly and might be pushing the upper limits of some product labels.

Soybean spraying

Soybean spraying is getting started. Check weed sizes vs. herbicide rates so you can maximize herbicide effectiveness. I am getting calls on some pretty sizeable weeds in fields, especially ragweed and waterhemp. Some of the ragweed (and marestail in some no-till and min-till fields) is already beyond being able to consistently knock it down, so I’m pretty reluctant to advise any exotic tank mixtures in an attempt to control them now. We’re more likely to burn the tops out of them than actually control them; odds are they’ll just branch out from other buds on the stems and be back in a few weeks. Aside from the frustration of not killing the weeds, sometimes our exotic tank mixes (aka tailgate cocktails) can be pretty hard on the beans. We are usually better off to target the primary weeds in the field that we can still control, which in most of the fields I am seeing and talking with growers about is waterhemp.


Scout a little and double check weed size, and use enough of your chosen burner (HG 14’s like Cobra, Flexstar, Blazer, etc.) to do the job right the first time. Double check what additives you can use; while we want to be aggressive and use the most effective additives we can, some labels restrict what we can do with some mixtures. I ran into a situation where a grower sprayed a tank mix of a residual Group 15, a Group 14 burner, and glyphosate. It is a great plan to add a residual herbicide to your early post applications in soybeans, but it also might mean shifting what additives you use and what rates are allowed. In this grower's case, a few fields where additional surfactants were used, in addition to loaded glyphosate… the beans are showing signs of recovery, but they took a pretty good hit from the excess of surfactants in the mixture. In the fields sprayed after we got the additive package back within label recommendations, the beans are at least a growth stage ahead of the injured beans. Just a reminder to talk with your chemical dealer and double check labels, hopefully keep crop response to a minimum while we try to maximize weed control.

Related content:
2016 Herbicide Guide for Iowa Corn and Soybean Production
Controlling Large Weeds
Pesticide Labels: There are no Substitutes
Is That a Palmer Amaranth or Waterhemp?
The Hidden Cost of Late-Season Waterhemp

Photo: Burned soybeans taken by Clarke McGrath



Clarke McGrath On-Farm Research and Extension Coordinator - Iowa Soybean Research Center

Clarke McGrath currently serves as the On-Farm Research and Extension Coordinator.

Prior to joining Iowa State University, McGrath spent nearly a decade as a retail agronomist and manager in the GROWMARK system, earni...