West Lafayette, Indiana
April 30, 2003

Farmers applying popular herbicides to their fields one day might receive an unwelcome chemical reaction: weeds ignoring the products altogether.

Scores of crop-damaging weeds are developing immunity to even the strongest herbicides in farmers' arsenals, said Bill Johnson, Purdue University Cooperative Extension Service weed specialist. What's more, fewer chemical methods for controlling the undesired vegetation are being introduced to replace them.

It all adds up to trouble for producers, but all is not lost if farmers change some current practices, Johnson said.

"We are developing glyphosate-resistant weeds at a rate of about one new species per year over the last four years," he said.

"There are about 250 species of herbicide-resistant weeds in the world. The highest number is in areas where production row-crop agriculture is most intense and relies almost exclusively on herbicides for weed control. That would be North America, Australia and Europe."

Glyphosate is the active ingredient in Roundup, considered the king of herbicides. So dominant is Roundup that 83 percent of United States soybean acres are expected to be planted to Roundup-tolerant varieties this year, according to the U.S. Department of Agriculture. Indiana farmers are projected to plant 91 percent of their soybean acres to Roundup Ready varieties.

Statewide, several weeds are demonstrating resistance to herbicide ingredients and brand-name products, Johnson said.

"In Indiana we have glyphosate-resistant marestail; jimsonweed resistant to atrazine; giant and common ragweed resistant to First Rate, pigweed resistant to Scepter, Classic and Pursuit; and lambsquarter resistant to atrazine."

Herbicide resistance in weeds started slowly in the 1970s and picked up steam in the early 1980s. Between 1990 and 2000 the number of confirmed herbicide-tolerant weeds worldwide jumped from about 125 to more than 240.

The rapid increase in weeds unaffected by herbicides was caused, in part, by the use of herbicides with identical control methods - known as modes-of-action - on both soybeans and corn, Johnson said. The same is now happening to herbicides with aceto-lactase synthase (ALS) inhibitors. ALS inhibitors kill weeds by preventing them from producing essential amino acids necessary for growth.

Weed-control chemistry isn't keeping up with weed physiology, Johnson said.

"For the most part we haven't lost active ingredients in corn or soybean production, but we're not getting new active ingredients introduced, either," he said.
 
"In the 1980s and through the early part of the 1990s, we probably were getting one or two new herbicides with a relatively new mode-of-action every couple of years. We haven't gotten a new mode-of-action introduced into research programs in probably four or five years."

Developing effective new herbicides is a time-consuming and expensive task, Johnson said.

"There's probably a misconception out there that companies can turn on the spigot and turn out a new active ingredient, when in fact it takes probably $100 million and 10 years of research to get a new product to the marketplace," he said.

Although weeds are gradually winning the control war, farmers still have a fighting chance. Johnson recommends producers avoid using similar mode-of-action herbicides on two or more crops. Also, planting soybeans in narrow rows helps minimize weed emergence later in the crop season, and can be effective with corn, as well.

"One of the best things we can do is rotate crops," Johnson said. "We rotate crops for insect and disease problems, and we need to rotate crops for weed problems. Most of our corn and soybean crops in Indiana are rotated. However, there are certain areas where soybeans are grown continuously.

"The other thing we can do - if soil erosion is not a big problem - is introduce tillage back into our systems. That could be some primary tillage in the fall, some secondary tillage in the spring or simply
using a rotary hoe and cultivator in the crop. There aren't too many weeds that have developed resistance to being torn out of the ground by a piece of steel."

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu
Source: Bill Johnson, (765) 494-4656, wgjohnso@purdue.edu

Related Web sites:

Purdue University Weed Science Page: http://www.btny.purdue.edu/weedscience/
International Survey of Herbicide Resistant Weeds: http://www.weedscience.org/