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Diversification Prevents Weed Resistance to Glyphosate

THE ADVENT of Roundup Ready technology made weed-control programs much simpler — perhaps too simple. Industry and university extension professionals are sounding the alarm that producers need to rethink how they plan and implement their herbicide programs.

“Producers have had more than a decade with Roundup Ready technology, which provided a simple and effective weed-control program,” says Dale Shaner, plant physiologist with the USDA's Agricultural Research Service. “But we are starting to see clean fields that are now having problems. Weeds are starting to come back, and we need to control them.”

Continual use of glyphosate over the years has created resistant biotypes of certain weeds, which glyphosate won't kill. Scientists have identified nine glyphosate-resistant biotypes in the United States, but the list includes some significant weeds: horseweed (marestail), giant ragweed, common ragweed, Palmer amaranth and waterhemp.

Although most producers have yet to see glyphosate resistance on their farms, it may be only a matter of time. Current surveys indicate that almost 20% of U.S. producers have found glyphosate-resistant weeds on their farms. Glyphosate resistance isn't something that is going away, and odds are good that the number of acres where resistance is confirmed will continue to increase.

“The sheer simplicity of the glyphosate system drove its adoption,” says Dan Westberg, technical marketing manager for BASF. “But that also has led to problems, and we're starting to run the wheels off glyphosate.”

Glyphosate is still a very effective herbicide and provides control to nearly 170 different weeds. However, experts caution that any system in which one chemical is favored, and used, continuously will speed development of a weed specie's resistance.

What is probably the most worrisome weed right now is glyphosate-resistant waterhemp. “Waterhemp has already shown resistance to triazines and ALS [acetolactate synthase] inhibitors, and now glyphosate,” Shaner says. “And waterhemp will probably continue to spread. That's the weed that will really cause headaches.”

Mix it up

Experts agree that a key component in preventing glyphosate-resistant weed development — and effectively controlling weeds — is to mix it up. That means rotating crops, herbicides and modes of action to ensure that weeds aren't getting the same dose of the same herbicide.

“There are areas of the country where we are starting to see the population of resistant weed biotypes explode,” says Jim Bloomberg, product development manager for Bayer CropScience. “We need to go back to the table to look at some older chemistries that worked previously and integrate them with the new selective and nonselective herbicides available in the marketplace today. Together, these products could form a solid integrated weed management program.”

Companies have been aggressive in developing new products based on old chemistries that fell out of favor when Roundup Ready systems took hold. Putting down a preemergence herbicide with residual control is making economic sense. Studies show that doing so not only nets a cleaner field from the start, but also nets additional bushels at the end of the season.

“If you continually apply glyphosate, you are more likely to develop resistant weeds, says Mark Loux, professor of weed science at The Ohio State University. “But the real story is that if you are trying to control weeds with just glyphosate, you may be losing yield for other reasons.”

Missing an early application window of glyphosate can mean larger weeds, which are already reducing yield. After reaching 4 in. in height, some weeds can grow at a rate of 1 in./day. That same field may be clean at the end of the year, but the damage has already been done. Waiting to apply glyphosate when weeds are 9 in. — rather than 6 in. — can cost 2 to 5 bu./acre in soybean yield.

“And an $8 to $12 application of a residual herbicide can control those early weeds, reduce competition for nutrients and water, and ultimately mean more yield,” Loux says.

“Pay close attention to preemptive management of weeds,” says Chuck Foresman, manager of weed-resistance strategies for Syngenta. “Glyphosate is a great product, but it may need help in some weed-control programs.”

To get the most effective weed control, that means alternating modes of action, using a preemergence herbicide. “We have lots of ammo to battle some of the weeds that are giving us problems,” Foresman says. “It is naive to think glyphosate will take care of weeds every year. Mixing up your weed-control program is the best way to control weeds and to reduce the chances of developing glyphosate-resistant weeds.”

Even if glyphosate-resistant weeds aren't present in your fields, it's an important issue to be aware of and to manage so it doesn't become a problem. “With lower glyphosate prices, it may entice some producers to simply put one more application of glyphosate down, and that's only perpetuating the problem if it's not a problem already,” says Mark Woodruff, product manager for corn herbicides at Dow AgroSciences. “Weed pressure can easily rob 7% out of corn yield. If you are producing 200-bu. corn, the money lost in potential yield more than makes up for the $10 spent on residuals.”

Basic steps

The message of reducing the chances of glyphosate resistance as well as keeping fields clean of early season weed competition is important for everyone. “Many growers may not realize they have an issue with glyphosate-resistant weeds,” says Susan Macy, product manager for DuPont. “Glyphosate is a key tool that growers don't want to lose. It still has a lot of utility, but will lose its effectiveness if additional residual chemistries aren't used, incorporating preemergence followed by postemergence treatments.”

And the message is simple:

Know the weeds in your field, and the weed pressure;

Rotate your crops and even cultural practices;

Start with a clean field;

Rotate modes of action, and consider rotating herbicide-tolerant systems;

Include residual herbicides in preplant herbicide applications;

Apply herbicides correctly, at the full rate and at the optimum timing;

Control escapes;

Clean equipment.

“We have had nearly a generation of growers who have relied on glyphosate as a very large tool to control weeds,” Foresman says. “Today's weed control will require folks to become familiar again with some older herbicides that remain effective and may provide lifesavers for controlling problem weeds.”

However, economic pressures come into play, and producers already stretched thin will continue to look at ways to save money. “That's one problem with the message of proactive management,” Loux says. “It's difficult to look out 10 to 15 years to prevent something that might occur when a producer is so focused on next year.” That can be especially true when it comes to rented cropland.

Glyphosate continues to work well, and it remains effective. But for how long, and for which producers, will remain to be seen.

“The bottom line is that investing a little now on a comprehensive weed-control program will pay later,” Loux says. “Losing glyphosate will mean additional costs down the road, whether it is additional tankmixes or newer herbicide-tolerant traits.”

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