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Tips to control giant ragweed

Giant ragweed is still high atop the public enemy list. Crop consultants believe it’s a battle you can win, even if some giant ragweed is resistant. Here are suggestions to wrestle ragweed down and pin it to the mat.

Providing tips are current panel members of the Indiana Certified Crops Advisers group. They include Danny Greene, Greene Consulting Inc., Franklin; Ryan McAllister, Beck’s Hybrids, Parker City; and Jeff Nagel, Ceres Solutions, Lafayette.

We had real problems with giant ragweed in corn and soybeans last year. We applied a light rate of residual and then glyphosate. Do we have resistant weeds?

Greene: Before you conclude that you have resistant weeds, rule out factors that may have reduced weed control. A University of Minnesota Extension bulletin lists several, including misapplication, unfavorable weather conditions, improper timing and weed flushes after application of a non-residual herbicide.

If these factors weren’t your problem, consider these questions:

Were other weeds listed on the label controlled effectively?

Did the same herbicide or a herbicide with the same mode of action fail in the same area of the field previously?

Do field histories indicate extensive use of the same herbicide or herbicides with the same mode of action?

If you answer one or more “yes,” consider weed control program changes.

McAllister: You don’t necessarily have glyphosate-resistant weeds. Ask yourself how big they were when you sprayed post, and at what rate. If giant ragweed is your problem weed, apply a heavier rate of residual up front. First Rate can be tank mixed with glyphosate for soybeans, unless they happen to be ALS resistant. Status could be tank mixed with Roundup in corn to knock down larger ragweed that glyphosate alone won’t control.

I’ve also seen stalk borers tunneling into giant ragweed plants and disrupting the flow of glyphosate to roots. You will notice top death down to the point of tunneling. Then you may see excessive branching of new growth from the entry point up.

Most weeds can be controlled if sprayed timely. If you get multiple flushes, bite the bullet with an earlier application and use a second application later for optimum control.

Nagel: The key for the future is to keep giant ragweed from setting seed. Try these strategies:

For corn, start with a soil-applied herbicide. Utilize a reduced rate of a grass/broadleaf premix and add additional atrazine to help on large-seeded broadleaf control. Corvus, Lexar/Lumax and SureStart are some of several good choices for a soil-applied residual herbicide. Post-apply glyphosate at 1.5 times the normal rate on 4- to 6-inch giant ragweed. We’ve also had good results tank mixing Status or other broadleaf herbicides with a 1x rate of glyphosate. We don’t recommend multiple passes of glyphosate alone.

For soybeans, Scepter, Canopy DF, Envive, Valor XLT, Sonic and others are options for soil-applied residuals. Post-apply glyphosate at 1.5 times the normal rate and spray 4- to 6-inch weeds. There are few economical tankmix partners that control giant ragweed in soybeans.

You must spray timely so giant ragweed isn’t allowed to get too tall for control. Most problems we see in soybeans are where growers only use glyphosate, use the wrong rate and/or try to spray weeds pushing label height or beyond.

This article published in the January, 2011 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.