Act Fast to Kill Weeds

Weeds taking the fields as crop season winds down. Recommendations given for corn, soybeans and wheat. Compiled by staff

Published on: Nov 5, 2004

Winter annual weeds are up to no good again, and farmers will need to act fast to stop them dead in their tracks.

A dry October that was ideal for crop harvest has given way to a wet November. The soggy conditions have rejuvenated weed growth, says Bill Johnson, Purdue University Extension weed specialist. Farmers planning to apply herbicides to fields this fall should do so at the earliest opportunity, he says.

"Farmers need to get into their fields and see if they have winter weeds coming up," Johnson says. "Until the recent rains most fields were relatively clean because, quite frankly, we didn't have the moisture to stimulate germination of the weeds. Now, with the rain and relatively mild weather, I've noticed a lot of fields greening up. I'm seeing dandelion, henbit, chickweed -- many winter annual weed species that are a bear to manage in the spring, which could be easily controlled now in the fall."

Most herbicides labeled for use on winter annual weeds should be applied before freezing temperatures set in and slow weed growth, Johnson says. Products that attack weed leaves and stems -- including glyphosate-based herbicides and 2,4-D -- need active plant tissue to work best.

"The best time to apply these herbicides is when you get a couple of days with air temperatures in the 50s," Johnson says. "Realistically, we're probably getting close to the end of the time when we're going to have multiple days in a row with air temperatures in the 50s. Most of these products are labeled to go on until the ground freezes. So if you think about the window of opportunity from that standpoint, we probably have at least a month or so in which we can apply these products."

Producers have greater latitude when using residual herbicides, Johnson says. "We have a relatively wide window when using residual products," he says. "I've become much less concerned about whether weeds are actively growing, because those products will provide activity through the fall and into the winter."

Fall herbicide formulas vary for fields that will be planted to corn and soybeans next spring. Johnson outlined application options for each:

Corn

  • Most winter annuals -- Simazine, used in combination with 2,4-D.
  • Grassy winter annual species and dandelion -- Basis, plus 2,4-D.
  • Dandelion only -- A glyphosate-based herbicide, plus 2,4-D.

Soybeans

  • Most winter annuals and dandelion -- Canopy XL, plus Express and 2,4-D.
  • Grassy winter annuals -- Canopy XL, plus Express and a glyphosate herbicide.

Corn or soybeans

  • Most winter annuals, biennials and dandelion -- A glyphosate product, plus 2,4-D.
  • Winter annuals only -- Sencor, plus 2,4-D.

Crop rotation also factors into fall herbicide decisions, Johnson says. "There are a couple of herbicide programs that don't tie you into a specific crop rotation," he says. "Fall-applied glyphosate plus 2,4-D, fall-applied Sencor plus 2,4-D, glyphosate alone, Sencor alone or Gramoxone plus 2,4-D does not tie you into growing either corn or soybean the next year. If, for example, you use Simazine or Basis in the fall, that pretty much ties you into growing corn the next year. If you use Canopy XL and Express in the fall, that ties you into growing soybean the next year."

Farmers growing winter wheat should apply herbicides to fields before planting, Johnson says. He recommended Harmony GT, Harmony Express or Peak. Growers should avoid 2,4-D, because the herbicide can damage wheat heads and, thus, reduce yield, he says.

"Unfortunately, we don't have a lot of good fall-applied herbicides for wheat after the crop has emerged," Johnson says.

For more information on fall herbicides, read "Fall-Applied Herbicides in Soybean, Corn and Wheat" by Johnson and Glenn Nice, Purdue weed scientist.