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Weed resistance gains

IF YOU'VE turned a deaf ear to industry concerns about glyphosate-resistant weeds in corn and soybeans, now might be a good time to listen. Most experts say the problem is here to stay and, if anything, is on the increase.

“We're not going to stop glyphosate-resistant weeds now. There's too much selection pressure,” says Chuck Foresman, weed resistance strategies manager, Syngenta Crop Protection.

The reason is the sheer number of U.S. corn, soybean and cotton acres on which glyphosate is applied annually — now approaching 180 million acres. In addition, many of those acres are treated with glyphosate two times or more per season, increasing the risk of resistance. To date, experts have identified seven common weed species that exhibit some level of resistance to glyphosate: common lambsquarters, common ragweed, Italian ryegrass, marestail (horseweed), Palmer pigweed, rigid ryegrass and common waterhemp. (See “Confirmed resistance,” page 42, for details on where resistant weeds have been confirmed.)

Slow to act

Recently, manufacturers such as Syngenta and Monsanto have stepped up efforts to encourage growers to implement stewardship programs to preserve glyphosate as a weed-control tool. Both companies recently launched Web sites to provide weed-control strategies that can help minimize the risk of a buildup of resistant weeds in your fields. The Monsanto site is at, and the Syngenta site is at

Despite increased efforts by industry and university experts to address glyphosate resistance, many growers still seem unconcerned, reports Mike Owen, Iowa State University (ISU) extension weed management specialist.

Bill Johnson, extension weed specialist at Purdue University, agrees. He says glyphosate-resistant marestail has shown up in at least 28 Indiana counties. However, only 48% of growers statewide even scout their fields for weed and pest problems, he says. That's according to a 2005 survey of more than 600 Indiana corn and soybean growers.

Owen contends that one reason growers are slow to act is because, historically, manufacturers have developed new chemistries that have addressed resistance problems. But because development of new herbicides today offers little financial incentive, primarily because of the cost-effective Roundup Ready system, manufacturers say there will be no glyphosate replacement chemistry in the near future.

“We need to rely on products already available,” Foresman says. “There is no silver bullet for this.”

Many Delaware farmers already understand this fact. Growers there began fighting glyphosate-resistant marestail in 2000. A recent University of Delaware survey found that 43% of Delaware growers with resistant marestail in their fields now spend $2 to $7/acre more on herbicide applications to control it; 20% spend an additional $8 to $15/acre to control it.

But many U.S. corn and soybean growers have not encountered weed-resistance problems of any magnitude. That's another reason growers are slow to take proactive measures to counter weed-resistance issues, according to Bob Hartzler, ISU Extension weed management specialist. (See “Glyphosate Resistance in the Cornbelt” at for details.)

Pay now or pay later

Another reason growers are slow to take a proactive approach to counter potential weed-resistance issues is because using preemergence herbicides, which add another mode of action to that of glyphosate, requires extra time and money.

However, Foresman says growers are forgetting that some weeds cause significant economic losses early in the growing season, even before postemergence applications are made, especially in corn. This problem is addressed in an online article written earlier this winter by Bill Johnson of Purdue and Mark Loux and Jeff Stachler of Ohio State University.

They say that weeds emerging with corn can quickly reduce yield potential. “In the absence of preemergence herbicides, weeds should be controlled with post herbicides when they are 2 to 4 inches tall to avoid the risk of yield loss from early-season weed interference. This corresponds to no later than approximately 23 days after corn planting, or before corn exceeds the V4 stage,” they say.

The weed-control specialists further explain the timing. They say that weeds can grow with a corn crop for approximately three weeks after planting with little yield detriment. However, for the following four weeks, emerged weeds must be controlled to prevent yield loss, regardless of whether they are controlled with post applications at a later date.

“When allowed to grow to a size of 6 to 9 inches (grasses) or 16 inches (ragweed), these weeds cause up to 20% corn yield loss, even though they can be effectively controlled by glyphosate at this large size,” the specialists point out.

They add: “The critical weed-free period is the reason why it is almost impossible to maximize weed control and corn yield when the herbicide program consists of a single post glyphosate application without the use of residual herbicides.”

Read more about the extension specialists' explanations and recommendations for effectively controlling weeds with glyphosate while minimizing resistance issues in the January 24-February 7, 2006, issue of the OSU C.O.R.N. newsletter, located online at

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