The Resistance Movement
It’s time to steward agriculture technology
There’s a resistance movement going on in America’s crop fields. Weeds, insects and diseases that were once no match for modern chemistry are fighting back, and winning.
Farm Journal Field Agronomist Ken Ferrie hopes that farmers are paying attention to the resistance scoreboard. "This isn’t the 1970s and 1980s when we had new products coming through the pipelines on a yearly basis," Ferrie says.
The good news is that there are paths to stave off resistance through management and stewardship.
During the next few months, a Farm Journal series of articles will explore how resistance is evolving and what you can do about it.
Resistance is not new, nor unique to the U.S. The first report of insect resistance came in 1908. Plant pathogens started fighting back against fungicides in 1940, and the first reports of weed resistance were confirmed in the late 1950s. The difference today is that the products at risk, such as glyphosate, Bt and strobilurin, are widely used across the agricultural landscape.
USDA’s National Agricultural Statistics Service says insect-tolerant crops containing Bt traits were planted on 73% of cotton acres and 65% of corn acres the past year. While multiple Bt proteins and a non-Bt protein are being employed in insect-tolerant trait strategies, the most recent Bt resistance case involves Cry3Bb1, a critical component of some current pyramided products.
Less than a decade ago, fungicides were mostly a way to protect fruits and vegetables. In 2011, more than 20 million acres of corn and soybean received at least one application of fungicide, with strobilurin-based products among the most popular yield protectors.
Of all these challenges, nothing has galvanized the industry quite like weeds that no longer succumb to glyphosate. In 2011, U.S. farmers planted 94% of their soybean acres, 75% of their cotton acres and 72% of their corn acres with herbicide-resistant varieties. The majority of these acres are glyphosate-resistant and many received multiple applications. Since 2000, there have been 21 weed species confirmed resistant to glyphosate.
Ferrie says reversing the trend will require changing a mindset. "Growers look at weed control as a cost," he says. "As a result, they attempt to kill weeds in the cheapest, easiest way. If an herbicide program is working, there hasn’t been a lot of incentive for them to change the system. Unfortunately, it’s usually the guy in trouble that’s typically looking for options," he says.
By comparison, insect and disease control tend to be viewed more as a way to improve profits and tack on extra yield. "Our pest thresholds haven’t changed, but crop prices have, and that triggers treatment at lower infestation levels. Growth of insecticide and fungicide use is a result of growers willing to throw more money at the crop to protect or boost yield," Ferrie says.
The result is the same: Constantly using more of the same product with more frequency is a recipe that leads to resistance. In general, pests develop a resistance to a chemical through natural selection. The most resistant weeds, pests and diseases survive and pass on their genetic ability to survive to their offspring.
Pat Tranel, a University of Illinois molecular weed scientist working to unravel the secrets of waterhemp, believes agriculture might still be in a dangerous state of denial.
"Convenience, memories of prior effectiveness, cost and a chronic unwillingness to change complicate behavior. By the time we do act, the problem has typically been with us three to four years," Tranel says.
"We’re seeing 11.5 new cases of herbicide resistance each year, and it shows no sign of slowing down. Herbicide resistance has reached a point where management can no longer be an afterthought," he adds.
One Worrisome Weed
If there is a poster child for worrisome weeds, it’s waterhemp. Recently, a population in southeast Nebraska has been confirmed to be resistant to 2,4-D herbicide. That discovery makes waterhemp the first broadleaf weed in the U.S. to become resistant to six different herbicide modes of action.
The discovery is also generating attention because new trait technologies are currently being designed to resist 2,4-D (a synthetic auxin) in soybeans, corn and cotton.
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| Waterhemp has become the first broadleaf weed to exhibit resistance to six classes of herbicides in the U.S. |
In 2009, University of Nebraska Extension weed scientists received a report that a waterhemp population in a warm-season grass field was no longer being controlled by 2,4-D. Seed collected in 2009 and 2010 was tested in greenhouse studies and found to be at least 10 times more resistant to 2,4-D than other waterhemp populations.
Waterhemp is the predominant pigweed (Amaranthus) species in eastern and south central Nebraska fields. It has become a widespread problem throughout much of the Corn Belt in recent years.
Mark Bernards, a weed scientist at Western Illinois University, helped identify the resistance while on staff at the University of Nebraska–Lincoln. "Waterhemp is well adapted to reduced tillage cropping systems that rely on postemergence herbicides for weed control," he says.
"It has an emergence pattern that lasts throughout the summer, is cross-pollinated and produces a large number of seeds," he adds.
Bernards says the herbicide use pattern in the Nebraska field where the 2,4-D–resistant population was found included an annual burndown of atrazine, metolachlor and 2,4-D followed by a postemergence application of 2,4-D. Sequential auxin treatments were made during a 15-year period.
While the auxin-resistant waterhemp is thought to be isolated, it is considered a wakeup call to keep rotating modes of action. Repeated use of any herbicide has the potential to select for herbicide-resistant weeds.
Multiple resistance. Bernards says waterhemp is teaching farmers that it is a mistake to depend on any one herbicide tool for control. "My biggest concern is that there is still a heavy reliance on glyphosate in these new technologies. If you put this technology on a field that already has glyphosate-resistant waterhemp and use only glyphosate plus 2,4-D, you are exerting tremendous selection for 2,4-D–resistant plants."
A 2010 survey by University of Illinois weed scientists found that a third of glyphosate-resistant waterhemp populations in Illinois also demonstrated resistance to PPO inhibitors, and virtually all were resistant to ALS inhibitors. Common waterhemp has also exhibited resistance to triazines and HPPD inhibitors. Waterhemp is the first weed to evolve resistance to HPPD-inhibiting herbicides.
Since the 1950s, only 29 synthetic auxin-resistant weed species have been discovered worldwide. The nine species resistant in the U.S. are barnyard grass (Louisiana and Arkansas); field bindweed (Kansas); kochia (Montana, North Dakota and Idaho); prickly lettuce and yellow starthistle (Washington); smooth crabgrass (California); spreading dayflower (Hawaii); waterhemp (Nebraska); and wild carrot (Michigan and Ohio).
Dean Riechers, a University of Illinois weed physiologist, says that auxinic herbicides have escaped the resistance problems experienced by more modern herbicides because they have a complicated mode of action and are normally applied with other herbicides in tank mixes.
"Auxin-resistant weeds may incur a fitness penalty that causes them to grow more slowly or produce less seed. We think that may reduce their frequency in natural weed populations," Riechers says.
The new Enlist Weed Control System from Dow AgroSciences combines herbicide tolerance traits with 2,4-D choline and glyphosate. Additionally, it provides tolerance to glufosinate in soybeans and cotton and the fop chemistries in corn. Mark Peterson, global biology leader for Enlist, says the technology is being developed with resistance management principles in mind.
"We know plants adapt, and weeds are no different," Peterson says. "Years of research in weed science have demonstrated that continuous use of a single mode of action will ultimately result in the development of a resistant population. If a single weed management practice is used continuously, resistant weed biotypes are likely to arise."
