Many new herbicides have become available to Wisconsin farmers recently. Although growers have been applying them for only a few years, weeds are already developing resistance to these newer herbicides, according to R. Gordon Harvey, a weed scientist at the University of Wisconsin-Madison”s College of Agricultural and Life Sciences.
? In Minnesota, some giant foxtail has become resistant to Accent.
? Illinois and Iowa soybean growers are having problems with waterhemp, a pigweed relative that has become resistant to Pursuit and Classic.
? Australian wheat farmers have encountered a Roundup-resistant weed.
Some weeds and crops naturally tolerate certain herbicides. They never were susceptible to them. But a rare genetic change in a weed that is widely susceptible to a herbicide can produce a plant that is unaffected by that herbicide. If that plant escapes other weed-control measures, it can produce hundreds of herbicide-resistant seeds. These seeds can survive in the soil for many years.
Both herbicide tolerance and herbicide resistance lead to what Harvey calls “weed-control failure.” Both can negate the effectiveness of individual herbicides or similar herbicides that kill weeds using the same mode of action.
Herbicide-resistant weeds take away tools farmers need for weed control, increase the cost of weed control and may limit farmers” planting options.
“Resistance is a real problem,” Harvey says. “We need to get farmers to take a long-term approach that will discourage the development of herbicide-resistant weeds. The bottom line is that some of these weeds are getting away. We have to be smart if we”re going to avoid problems.”
After 27 years in the CALS Department of Agronomy, Harvey has become Wisconsin”s leading authority on weed problems and herbicides. He knows that the surest way to speed up the selection for herbicide-resistant weeds is to use the same herbicide, such as atrazine or Accent, on the same fields year after year.
“I”d like to see farmers develop five-year plans that integrate different weed management practices into their farming operations. Had we rotated atrazine with other herbicides, we might have avoided the problems we have today with water pollution and atrazine-resistant weeds. If we don”t pay attention to that lesson, we”re going to see some of the same problems with the new herbicides. We may well avoid the water pollution, but we”re going to see resistance develop to these compounds and our weed problems will shift to other weed species that tolerate them.”
Accent and Pursuit are members of a new generation of herbicides that farmers apply in small amounts to kill targeted weeds. All have the same mode of action; they inhibit a particular plant enzyme called ALS.
“These ALS-inhibitors have only been available to farmers for a few years and we”re already seeing herbicide-resistant weed problems,” Harvey says. “ALS-inhibitors are valuable tools in a weed-management program and we don”t want to lose them. If farmers use them wisely now, we can make them last.”
Keeping the ALS-inhibitors, such as Accent, effective against weeds in Wisconsin will be a challenge, according to Harvey. Unlike farmers in surrounding states, many Wisconsin farmers can not use atrazine for controlling weeds because the herbicide has contaminated ground water. Where farmers can”t apply atrazine, many have turned to Accent.
Herbicide-resistant weeds are an especially serious problem for the farmers who planted corn on nearly 4 million acres in Wisconsin this year. Compared with a few insects and diseases that attack corn, there are currently about 20 species of weeds that can seriously reduce Wisconsin corn yields. Several of these weeds are dominant, but many of the others could become major problems if left unchecked. Farmers who can”t keep weeds in check face serious economic losses. Harvey tells farmers: “Don”t spend more on weeds than they”ll cost you.” But in most cases farmers can”t afford not to treat.
Harvey is a member of a national group of weed scientists that sat down several years ago to identify strategies that control weeds yet slow or prevent the development and spread of herbicide-resistant weeds. The scientists came up with a series of ideas that they are studying and trying to get farmers to adopt.
The basic idea is to keep the bank of weed seeds in farm fields under control and minimize the chance that the rare herbicide-resistant weed will survive and become a problem. The major strategies include rotating herbicides, rotating crops, cultivating, and applying herbicide combinations with compounds that kill weeds in different ways.
The idea of rotating among insecticides has helped control the development of insecticide-resistant insect populations. The same approach with herbicides has been less effective, Harvey says. Insect pests don”t remain inactive in the soil for years. But if a herbicide-resistant weed sets seed, those seeds enter the seed bank and germinate over many years. If those seeds germinate in years when a farmer rotates back to the herbicide to which they are immune, they can become a major problem.
Long crop rotations, such as a three years of alfalfa followed by a year of corn, are more helpful in avoiding herbicide-resistant weeds, Harvey says. Combining row cultivation with herbicide or crop rotations is even better. “We urge growers to cultivate at least once a year. If you cultivate once, your chance of getting that one herbicide-resistant weed that escaped other practices increases by about 60 percent,” he says.
One of the surest ways to avoid herbicide-resistant weeds is to apply two herbicides with different modes of action to the same crop. The chances of a single plant simultaneously undergoing the genetic changes that would make it resistant to both types of herbicides are extremely small. But both herbicides must be effective against each weed type. When a farmer needs to control both grass and broadleaf weeds, the farmer might have to combine three or even four herbicides.
Applying multiple herbicide combinations is expensive. Harvey has spent a good deal of time looking for the most cost-effective combination for corn. His best solution for grasses is a combination of Poast Plus and Accent on SR corn. SR corn, which was developed at the University of Minnesota, is resistant to the active ingredient in Poast Plus, a soybean herbicide that controls grasses but is approved for use on SR corn.
The problem with Harvey”s solution is that SR corn is available from only a few seed companies. “I encourage farmers to try this approach with SR corn as part of their weed-management program. It”s another tool that”s available now. If they find it useful, they need to tell their seed dealers they want them to sell SR corn.”
Harvey also praises the new corn and soybean varieties that are genetically engineered to tolerate the active ingredient in either Roundup or Liberty herbicide. Roundup-Ready soybeans hit the market in 1996 and Liberty-Link corn arrived this year; Liberty-Link soybeans and Roundup-Ready corn are expected next year.
“Roundup and Liberty are powerful, non-selective herbicides. Compared with earlier herbicides, they are less toxic to applicators and less harmful in the environment,” Harvey says.
According to Harvey, each company says it is almost impossible for weeds to become resistant to its product. But he believes they are looking at weed problems too narrowly.
“Even if Roundup and Liberty kill all the weeds that have emerged at the time of application and there are no weeds with true resistance, you will likely select for weeds that germinate after the application. Weeds that tolerate the shade in corn fields could come up later in the season. Maybe yellow nutsedge or nightshade will become major weed problems in corn. These or other weeds can still cause economic losses that amount to weed-control failure,” he says.
Too often, according to Harvey, long-term approaches to weed management take a back seat to shorter term economic decisions of both farmers and chemical companies.
“Developing a long-term approach to weed control is difficult for farmers because weed-control options are complex and just one of their many management decisions,” he says. This means farmers will have to invest more time in management, which, Harvey realizes, is a challenge.
“The whole pesticide issue is enmeshed in regulatory rules and certification programs,” he says. “For many farmers, managing weeds has become such a complex issue that they just want to call the co-op and ask them to come out and spray the same chemicals they used last year.” That”s exactly what Harvey doesn”t want to see happen.
Farmers need to plan and carry out over several years a weed-management program that incorporates different strategies and tools in order to keep weeds off balance, according to Harvey.
“It”s a little like the Green Bay Packers” offensive plan. The Packers can”t be successful if they run the same play down after down. Farmers can learn from that approach when it come to weed management,” he says. “If farmers ”mix up their plays,” they can keep their weed opponents off balance.”