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2-foot-tall marestail in soybean field
BIG MARESTAIL: Once marestail reach this size after soybeans are up, weed control options are limited, says Purdue Extension’s Bill Johnson.

How would you control these weed situations?

Here are examples of tough weed scenarios with recommendations from an expert.

Weed issues came fast and furious in 2019. What if you could have a do-over?

Here are two real-world examples from 2019. Bill Johnson, Purdue University Extension weed control specialist, provides insight into the best options for these tough scenarios.

“The best advice is to do what you can to stay out of these situations,” he says. “The weather doesn’t always cooperate, but plan ahead as much as possible and get a jump on weed control.

1. Big marestail in young soybeans. Refer to the picture above. Marestail are 2 feet tall, with split-row, no-till soybeans in an early vegetative stage. “Should you plant into a situation like this?” Johnson asks. “I don’t like planting into a field with marestail this tall because options are limited.”

The better option would have been a fall herbicide application, he notes. However, during the past two falls, that’s been difficult. A spring burndown before planting with residual herbicides also in the tank would have been an option. There can be waiting intervals before planting, and there was no time to wait in 2019.

The next question is what technology is in the soybeans for herbicide tolerance, he says. “If they are straight Roundup Ready soybeans, there isn’t much of an option to clean up this mess,” he notes. “We assume today that marestail is resistant to both glyphosate and ALS herbicides.”

If that’s your situation, you could apply 2 quarts per acre of glyphosate, plus a half rate of both FirstRate and Classic, but lower your expectations of control, he says. If the soybeans are resistant to dicamba, 2,4-D and/or Liberty, you could use those products. Even then, with marestail this tall, don’t expect miracles, he says.

2. Mixed weeds in young soybeans. Refer to the picture below. The same considerations about taking care of weeds before you plant apply, Johnson says.

variety of weeds in soybean field
WEED VARIETY: There are several different species of weeds to consider when determining what might be the best solution in this field of young soybeans.

Johnson notes common ragweed, goosegrass and even some small morningglory in this field, plus volunteer corn. Knowing what’s in the field is essential.

If the soybeans are only tolerant to glyphosate, Johnson would suggest the highest rate of glyphosate plus either a PPO herbicide, such as Flexstar or another herbicide in that class, or an ALS herbicide such as First Rate or Classic. Those would help catch morningglory. Add Select or Assure to control volunteer corn.

If the soybeans are Xtend-, Liberty- or Enlist-tolerant, you could use the respective product at the proper rate for this weed size. In either case, Johnson recommends adding a grass herbicide, such as Select or Assure, to pick up volunteer corn. Assume that volunteer corn also has Roundup, Liberty or both these herbicide-tolerant traits.   

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