Most of us have memories of walking soybeans. With the advent of glyphosate-resistant soybeans, you thought those days were only a fleeting memory and something you could describe to your grandkids.

If the trend continues, your grandkids may be walking beans, too, and maybe even the current farming generation. More and more weeds are appearing in Roundup-Ready bean fields throughout the Cornbelt, and that is upsetting to farmers who’ve gotten used to seeing clean soybean fields.

This edition of the farm gate will focus on the increasing numbers of marestail, giant ragweed, and lambsquarters that are being found in bean fields, and then offer some strategies for addressing your problems. If you find this helpful, thank Mark Loux and Jeff Stachler at Ohio State University.

The OSU weed whackers say, “We know that some instances of poor control can be attributed to generally poor management of glyphosate, due to the tendency for some growers to over-simplify their approach to weed management in order to reduce the number of applications and costs. Lambsquarters and giant ragweed can both be generally tough to control with a single postemergence treatment. Growers who omit preplant burndown treatments, apply when weeds are large and old, and use rates too low for the weed size and age, place themselves at risk for control failures.” Whether or not that applies to your situation, let’s shift away from the blame, and talk about how serious the glyphosate resistance might be. “While we typically observe about an 8X level of resistance to glyphosate in marestail, our greenhouse research indicates that the level of resistance in lambsquarters and giant ragweed is more on the order of 1X to 4X. So, plants can survive glyphosate rates up to 3 lbs ae (acid equivalent) of glyphosate per acre, but they usually suffer substantial injury even at lower rates.”

Don’t quit farming just yet! Loux and Stachler say, “The expression of the low level of resistance is variable, so that the resistant populations may be controlled under appropriate management of glyphosate (small plants, high enough rate, multiple applications) and when environmental conditions are favorable.” Develop a system for weed management that does not rely solely on glyphosate in both corn and soybeans. The OSU report recommends, “reduce the reliance on glyphosate alone for weed control), such as rotation of Roundup Ready systems with non-Roundup Ready systems, use of tillage or a multiple-herbicide burndown program to start the crop weed-free, use of preemergence herbicides, and combination of glyphosate with other postmergence herbicides.”

Now, that is the overall system; but what about specific treatments for each of those three weeds?

1) MARESTAIL (horseweed) Apply burndown herbicides when plants are small – less than 4 inches tall. Refer to the cited newsletter for recommendations on pre-plant burndown, as well as strength of glyphosate, and what to do about a post emergent treatment.

2) LAMBSQUARTERS – Most effective preplant burndown results from a combination of glyphosate plus 2,4-D ester. Refer to the cited newsletter for recommendations on pre-plant burndown, as well as strength of glyphosate, and what to do about a post emergent treatment.

3) GIANT RAGWEED – Most effective preplant burndown results from a combination of glyphosate plus 2,4-D ester. Refer to the cited newsletter for recommendations on pre-plant burndown, as well as strength of glyphosate, and what to do about a post emergent treatment.

Summary:

If you have marestail, giant ragweed, and lambsquarters in your soybeans, your neighbor probably has them as well, because of their growing resistance to glyphosate. Your plan of action should be to blend weed management systems, as well as create a burndown and postemergent treatment plan that includes herbicides with other modes of action along with your glyphosate application. Without a strategy for success, you will spend a lot of time trying to find a hardware or farm supply store that still carries weed hooks.

Stu Ellis

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