Options for controlling large weeds in soybeans …

Heavy and frequent rainfall throughout the month of June has made timely postemergence weed control in soybeans very difficult, if not impossible. Some producers have even resorted to aerial herbicide applications.

To improve weed control and help manage resistant weeds the use of residual products and tank mixtures has become more common and needs to become routine. This year a good PRE put down ahead of planting was a life saver. Not only did a good PRE greatly reduce your weed pressure, it bought you more time to make your in-crop POST application. In my area it is easy to spot the fields that did not get a residual down early or no-till fields that did not start with a good burndown. This year will be a good but hard learning opportunity for some.

If you are faced with the daunting task of trying to take 18 – 24”+ tall weeds out of soybeans, what do you do? Glyphosate is one of very few herbicides with good activity on large weeds. If these large weeds happen to be resistant to glyphosate, now what? I’m afraid there are not any good or reliable options—beyond the corn knife. Most other broadleaf herbicides that can be used POST in soybeans are designed to control small weeds (< 6” tall). If a full label rate of glyphosate (44 – 64 oz, depending on formulation) will not control a large weed, then most likely no other product or tank mix will either.

Its relative easy to defoliate large weeds with contact “burner type” herbicides, but defoliating them will not kill them. You will feel good about the practice for about a week until you see the new growth emerge—and then they seem to get even tougher. There are usually weeds of many different sizes in a field, so your tank mixture might get better overall control because it will get more of the small weeds. But it will not control the big, ugly ones you are most concerned about. In some cases it is questionable if there will be a benefit to adding another herbicide with glyphosate. It is a good IPM (Integrated Pest Management) approach to use multiple modes of action, but to be of value they need to have an “effective” mode of action. Contact herbicides are not effective on very large weeds.

In many cases this year we are forced to make the best of a bad situation. In the future we need a good PRE at or before planting followed by earlier POST application timings with tank mixtures and a second residual product to get the kind of weed control many growers and landlords expect today.

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About the Author: Lance Tarochione

Lance Tarochione is a technical agronomist with Asgrow/DEKALB in west central Illinois. His work has focused on crop production, research and product development, and through his role at Monsanto® he currently supports the Asgrow® and DEKALB® brands in seven counties in western Illinois.