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If you ever doubt how quickly herbicide-resistant weeds multiply, ask Sid Fogg. The story the Heth, Arkansas, farmer tells is typical of many mid-South farmers.
“A couple years ago in 2008, we had a pretty good soybean crop,” he says. After harvest, though, Fogg noticed scattered Palmer amaranth plants across some of his fields. Initially, he figured a sprayer missed some of these pigweed-family members during a postemergence glyphosate herbicide application.
Fogg came back in 2009 with wheat on 400 acres followed by a double crop of soybeans. A timely rain spurred a good early stand of soybeans. Unfortunately, it also triggered a soybean-choking early flush of Palmer amaranth. Fogg applied his normal postemergence chemical application of a full glyphosate rate laced with 6 ounces per acre of Flexstar. The Flexstar addition aimed at controlling weeds like morning glory, waterhemp, and black nightshade that are known to tolerate glyphosate.
However, prolific rainfall pushed a glyphosate-resistant Palmer amaranth explosion that overwhelmed this mix. “By the time we came back again to spray, the pigweeds were too big,” Fogg says. “We had pigweeds that were a foot tall, two feet tall.”
At that height, subsequent postemergence herbicide applications had no effect. Fogg spent about $50 per acre over 400 acres with little weed control triggered by glyphosate-resistant Palmer amaranth. In areas choked by Palmeramaranth, soybean yields dipped from 40 bushels to 20 bushels per acre.
“At that time the price was $9 per bushel, so that is $180 per acre in lost yields on top of the $50 per acre for chemical that didn’t work. In those areas, we had $230 per acre in lost revenue.”
Break Out The Hoes
The mid-South rapidly adopted Roundup Ready technology for corn and cotton when it first appeared in the 1990s. It worked well at first. “Ten years ago, there were no weeds in soybean fields in Arkansas,” says Ford Baldwin, co-owner of Practical Weed Consultants LLC and former University of Arkansas (U of A) Extension weed scientist.
The first chink in glyphosate’s armor appeared in 2003, when glyphosateresistant marestail surfaced. Glyphosateresistant common ragweed popped up in 2004, followed by glyphosate-resistant Palmer amaranth in 2005. Now, glyphosate-resistant Palmer amaranth reigns in the mid-South.
“If you see a Palmer amaranth in your (Arkansas) field, you need to assume it is glyphosate-resistant,” says Jason Norsworthy, U of A weed specialist.
This is compounded by multiplemode resistance, often to glyphosate and ALS herbicides. “There are 16 different herbicide-resistant biotypes of weeds in Arkansas,” says Norsworthy.
How Resistance Starts
Regardless of location, all herbicide resistant weeds share a similar origin.
“There are resistant genes in every weed population,” says Wes Everman, Michigan State University Extension weed specialist. “It’s just a matter of selection pressure. We look at South vs. North. There are different cropping systems. In the North, as we look at corn and soybeans, there are normally two weed-control applications (annually) and sometimes one.”
This contrasts with a more frequent weed-control treatment system in the South. Prior to Roundup Ready, fields often received seven or more weed-control treatments via preseason tillage, preplantincorporated or preemergence chemicals, cultivation, and several postemergence chemical treatments, says Everman. Many mid-South growers followed this multiple-treatment strategy in glyphosate- tolerant systems in order to curb multiple weed flushes.
“When you have multiple applications in a season, you have multiple selection throughout the season,” says Everman.
“It may be just that one-in-a-million plant with the resistant gene. But it can multiply fast, particularly if you continuously apply glyphosate in cotton.”
Low application rates and delayed applications also piqued glyphosate-resistant weeds in Arkansas.
“We used to cut rates of glyphosate when it was expensive,” says Norsworthy.
“Not only did farmers cut rates, but also they delayed applications to save on herbicide costs. So you get tall weeds controlled by fewer applications and lower-than-recommended applications. Eventually, we got resistance.”
After 2009, Fogg called in U of A researchers to establish plots on his farm to study various weed treatments. On his own farm, he revamped his weedcontrol strategy for Palmer amaranthinfested acres, applying a preemergence herbicide prior to coming back with a postemergence application of Ignite on LibertyLink soybeans.
“We didn’t get rain for seven weeks, so the preemergence didn’t get enough incorporated,” says Fogg. “But we applied Ignite when the pigweed was 3 inches tall, and the beans looked pretty good.”
The drought continued and sliced his average soybean yields to around 20 bushels per acre. This was due to drought, though, and not losses from early weed competition.
Following is a strategy similar to the one Ken Smith, U of A Extension weed specialist, recommends for Arkansas growers:
- Start clean with a burndown.
- Apply a residual preemergence herbicide.
- Include Dual or other postresidual herbicides over the top with first postemergence application of chemicals other than glyphosate like Flexstar or Ignite (on LibertyLink soybeans).
“Dual over the top with Ignite gives it the residual it needs,” Smith says. “Ignite has no residual.”
The upside? Timely applications with herbicides containing multiple modes of action other than glyphosate keep weeds off balance, thereby forestalling resistance to any one mode of action.
The downside? Such a program can cost $35 to $40 per acre. That’s a pocketbook shocker in light of declining glyphosate prices.
“We have such a severe Palmer amaranth infestation that we have no other option,” says Smith.
Lessons For The Midwest
So far, Midwestern glyphosate-tolerant systems continue to work well in most cases. Unless farmers change strategies, though, (such as mixing modes of action and using preemergence herbicides), glyphosate-resistant weed biotypes will continue to increase.
“Right now, we’re on the edge of a precipice we could step off in the next two years,” says Mike Owen, Iowa State University Extension weed specialist.
As in the mid-South, diversity of herbicides and crops rules in forestalling
herbicide resistance. Start clean with an early preemergence application of a residual herbicide followed by timely postemergence herbicides, advises Owen.
“It may include glyphosate, it may not,” says Owen. “We suggest rotating (weed control) traits. Perhaps one year we use a LibertyLink crop; the next year we use a Roundup Ready technology crop.” This strategy entails economic and agronomic benefits.
Owen cites a benchmark study in which weed scientists from six land grant universities are examining weed management practices over several years to improve the sustainability of Roundup Ready cropping systems.
“We are suggesting growers plant on half of a 40- or 50-acre field using our best management programs,” says Owen.
“On the other half, they use whatever they have been doing. What we have found is better economic return and better weed control and no evolved resistance where growers are using the diversified weed-management program.”
Such a strategy could also halt a repeat of the Palmer amaranth invasion mid-South growers are battling. Fogg figures he’s now on the right track with steps like a mix of herbicides with multiple action modes applied at different times.
“Without them, you (Midwestern farmers) will be in the same boat we are in,” he says.