How to Control Large Weeds in Fields Going to Soybeans

Tall weeds in fields either planted or not planted add extra challenge.

Tom Bechman

Published: May 28, 2010

The plan was good. Get the soybeans in the ground with a part-time helper while the farmer concentrated on planting corn. Since he does his own spraying, he didn't have everything sprayed yet. Sometimes plans go awry. This farmer didn't plan on two pushing three weeks of wet weather. He hoped to get back into fields to spray sooner than he did.

What if you find yourself in this situation? Purdue University weed control specialists Bill Johnson and Glenn Nice have noticed several fields that are either already planted to soybeans or will be planted to soybeans, covered with various weeds, from dandelions to three-foot tall winter annuals. They have a few suggestions on how whoever is farming those field can prevent this season from becoming a weed nightmare in those situations.

"If the field isn't planted yet, up the glyphosate rate to 1.5 pounds of active ingredient per acre," they suggest. "Then wait a day before planting or drill into it. That's to let the herbicide have time to get down to the roots."

If marestail is present, add First Rate, Classic or Sharpen. Some marestail is resistant to glyphosate. "If you know the marestail is ALS resistant, then go with Sharpen," Johnson says.

If you haven't planted and were going to till the field, which is the natural inclination when tall weeds are present, the weed control specialists advise no-tilling instead. Here is their logic.

"It's going to be difficult to completely control large, down weeds. Then if they recover, they become difficult to control with herbicides."

If you're going to conventionally till regardless, then consider applying glyphosate first if the weeds are over a foot tall, the weed control specialists recommend. Wait a day before tilling, and wait two to three days if there are several perennials in the field. Remember that glyphosate must be translocated to the roots to work.

The good news for anyone spraying glyphosate right now is that with warmer temperatures, glyphosate performs better. It works faster in killing weeds than when it is applied early in the season, or whenever temperatures are cool.

What you can do with soybeans that are up where the field isn't sprayed depends on which beans you planted. As long as they are Roundup Ready and glyphosate resistant, or Liberty Link seed and Ignite resistant, your choices are much wider than if they are non-GMO beans.