When a spring that started off cold and dry turns warm and wet it becomes challenging to stay ahead of weed infestations. The appearance of glyphosate resistance in our area has necessitated more aggressive weed management. Using preplant and post residuals, getting 2,4-D back into our no-till burndowns, tank-mixing products for multiple modes of action, targeting sprays on smaller weeds and using higher rates of glyphosate are all examples of “more aggressive weed management.”

The presence of resistant weeds means we can’t just “clean up a mess” like we could a few years ago with a spray of glyphosate. We got spoiled being able to get good control of gigantic weeds and this led to some less than desirable weed management practices such as over dependence on one herbicide and often a single herbicide pass. Some of today’s fields require a good residual program up front, followed by additional residual applied in-season before the PRE residual breaks down. We call this practice “overlapping residuals.” In addition, you may need more than one mode of action post applied to get complete control of emerged weeds.

For many reasons we really need to target weeds that are less than 6” tall when making our POST applications. For many years soybean growers have been fearful of making POST applications too early because they worry about late flushes of weeds and escapes. These days we have FAR MORE issues with weeds due to spraying too late than too early. Large weeds are much harder to kill and coverage becomes much more of an issue because of small weeds growing under the big ones and under the soybean plant.

In a year like this with extreme weather delays, waiting to spray can be even more costly. You are much better off to spray a few days too early than 2 – 3 weeks too late. I have seen many no-till fields with weeds like marestail that were not controlled completely with the burndown. These weeds likely will not be controlled effectively POST and will be there season-long—and adding more seed back to the soil bank.

In many tilled fields scattered lambsquarters and giant ragweeds survived the tillage pass and are now VERY large. Fields without a residual product with good activity on waterhemp or fields that could not be sprayed timely due to rain delays are going to pose a problem if any of those waterhemp are resistant to glyphosate. The other products we have to control waterhemp POST are most effective on weeds less than 6” tall.

With the threat of tall weed escapes, maybe you need to get out the old corn knife.

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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.