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The lone danger

When Wayne Heckathorn was a teenager in the late 1950s, an uncle would try to entice him to walk a soybean field oozing with waterhemp.

“I'd say to him, ‘No way!’” recalls Heckathorn, who today farms with his son Chad near Elk Point, SD. “But if we didn't do it, the waterhemp would be so tall that it would dwarf a 105 John Deere combine at harvest.”

Fortunately, herbicides such as Amiben and Sencor/Lexone pummeled this pigweed family member in the early 1970s. “From then on, it wasn't a problem,” Heckathorn says.

Just another herbicide success story? Read on.

In the late 1980s, many Midwestern soybean farmers started to rely solely upon herbicides, including Pursuit and Scepter, that kill weeds by disrupting the acetolactate synthase (ALS) enzymes. “In some areas, farmers used ALS inhibitors on 80% of soybean fields,” says Reid Smeda, a University of Missouri weed scientist.

Despite giving phenomenal weed control at the time, these herbicides wilted against waterhemp. Although waterhemp had not infested fields for years, its seeds had lingered in the soil and waited for an emergence opportunity. The sole use of ALS herbicides triggered waterhemp's resurgence.

Although not economically damaging in all cases, waterhemp now infests 90% of Iowa fields, says Bob Hartzler, Iowa State University extension agronomist. Waterhemp's spread is hastened by its prolific seed production that often averages 550,000 seeds/plant. This weed, which also infests corn, may clip yields 3% if just six plants per 100 ft. of soybean row are present.

Due to overuse of Pursuit and Scepter, ALS-resistant waterhemp also exists. “It's a numbers game,” explains Zach Fore, a University of Minnesota cropping systems specialist. “As you increase selection pressure, you'll select out the resistant types of weeds.”

What about Roundup?

Waterhemp's resurrection holds a powerful lesson as you plan your 2002 herbicide buys. Reliance upon only one control measure can quickly render that tool useless.

“The reason that you see so much waterhemp is that biological organisms adapt and adjust,” Fore says. “We have become predictable in the crops that we plant and the herbicides that we use.”

This even applies to biotechnology's golden boy, Roundup Ready soybeans.

“I keep thinking of that scene in the movie Jurassic Park, where the actors are hiding in a car as the dinosaur plows through a restrictive fence,” says Kevin Branick, a Pioneer Hi-Bred International agronomist based in Sioux Falls, SD. “One character says, ‘You can't contain life.’

“When we talk about weed species and variations in a population, we try to contain and squelch it, and Roundup is about as close as we can get to doing it,” Branick continues. “Yet in some way, shape or form, it may flip around and become resistant.”

Thus far, glyphosate (Roundup's generic name) resistance is rare. Resistant ryegrass surfaced in Australia in 1996, followed by resistant goosegrass in Malaysia in 1997. In 2000, researchers detected resistant horseweed in Delaware.

“We've been using Roundup for 28 years, and we are only dealing with three confirmed cases of resistance globally,” points out David Heering, Roundup technical manager for Monsanto.

But agronomists and weed scientists are equally concerned about “tolerant” weeds that farmers may initially label as resistant.

Resistance in a weed population is selected by repeated control measures, Hartzler says. He says in several fields where waterhemp infested Roundup Ready soybeans, Roundup did not control waterhemp the first time it was used.

“To me, that doesn't fit that definition of resistance,” he says.

Yet, Hartzler adds that there are some biotypes within a weed species that may tolerate higher herbicide rates than others. That's especially true with waterhemp. Because this weed has male and female plants, each plant contains unique genetic traits. This diversity has resulted in some plants with the ability to survive the typical Roundup use rate of 1 qt./acre.

In 1999, University of Missouri researchers detected tolerant waterhemp that withstood recommended Roundup rates even after the field had been planted to corn the preceding three years.

“We're still seeing good control of weeds with the 1-qt./acre rate,” Smeda says. “But we do see cracks in the armor.”

Shifty weeds

Farmers also face weed shifts from Roundup Ready technology similar to shifts keyed by ALS herbicides more than a decade ago.

“A weed shift will be a greater issue with Roundup than resistance,” says Jay Zielske, a Pioneer agronomist based in Mankato, MN. “You'll likely see more late-emerging weeds. Roundup works great on early-emerging grasses. But it has no residual for later-emerging weeds like waterhemp.”

This sets the stage for infestations of waterhemp, which may germinate from April into August. Other late-emerging weeds include buckwheat, smartweed, velvetleaf and black nightshade. If farmers follow the same pattern of use with Roundup as they did with ALS herbicides, this shift will occur rapidly, Zielske says.

“In some respects, people aren't really thinking about weed shifts caused by Roundup,” he adds. “They're using the same reasoning they did with Pursuit by saying, ‘We'll deal with it when it's a problem. Maybe there will be another product that will control it.’ Then again, there may not be a product that can deal with the proliferation of waterhemp and other late-emerging weeds.”

Good news, bad news

Roundup's effectiveness and popularity are its greatest weaknesses, says Chuck Benbrook, president of Benbrook Consulting Services, Sandpoint, ID. Excessive reliance upon Roundup Ready technology, which U.S. farmers used to plant two-thirds of soybean acres in 2001, concerns Benbrook.

“That is a burnout pace for any technology,” he says. “It's like that line that Jim Carrey uses in a movie, ‘Somebody stop me!’ There is not a single system that you can use on that much cropland and not have problems emerge.

“Some farmers may say, ‘So what?’” Benbrook continues. “‘We have resistance to triazine, imidazolinone, sulfonylurea herbicides and now Roundup. Fine, we'll move on.’ But that is a shortsighted attitude on their part. I consider Roundup to be an extremely valuable herbicide. It doesn't leach, it's not toxic, and it doesn't injure the soybean plant or the environment. A good farmer will want to preserve its efficacy for 100 years rather than 10 years, and they can if they use it wisely.”

Tap your toolbox

To ease these weed worries, farmers have several options. Not all methods include herbicides, Fore says.

“When we get new tools, we forget about old tools,” he points out. “The interesting thing is that successful organic growers have the equivalent of a toolbox with just a Crescent wrench and Vice Grips. They take what they have and use it. The harrow and rotary hoe are tools. So is changing the planting date. Maybe they plant later and do a tillage trip later on instead of applying a certain herbicide.”

Conventional farmers may apply some of these techniques to their own farms, Fore says. To ensure an optimum stand, farmers should first buy seed free of weeds and disease and use equipment in good working condition.

“If you have a good stand, you can get things started off on the right foot,” Fore says. “If you have a good herbicide program and a poor crop, you can still have weed problems.”

Diversified crop rotations may also throw weeds off-balance. “In southern Minnesota, we have corn and soybeans,” Fore says. “In northwestern Minnesota, we plant wheat and soybeans. The tight rotations we have now really select for weeds that grow well under those conditions. We used to have perennial and fall-seeded crops. If we had more of a crop mix in our system, we would have less weeds.”

Mix it up

Farmers locked into two-crop rotations may buy herbicides with different modes of action to confuse weed populations.

Heckathorn tries to mix herbicide families on fields every two to three years. A mainstay treatment of the farm is LibertyLink corn and Roundup Ready soybeans.

“But we'll never use just two chemicals across the farm,” he says. He mixes in Dual II Magnum, acetachlor and Balance Pro on corn. Herbicides used on soybeans include Treflan, Pursuit, Flexstar, Cobra, Prowl, Raptor and Ultra Balzer. Selection hinges upon present weed species, crop safety and planting flexibility.

“It's just like a baseball game,” Branick says. “When a pitcher throws fastballs all the time, hitters get used to them. But when he throws a curveball, fastball and slider, it's tough for a batter to overcome.”

Buying other chemicals as a setup for Roundup is another way to ease weed worries, Zielske says. Soil-applied herbicides such as Axiom, Boundary and Authority all are good setup products to knock out weeds that Roundup may miss, such as waterhemp.

This strategy may help prevent the mistakes made with ALS herbicides. “Pursuit spoiled us,” Zielske says. “When we stuck with the traditional soil-applied DNA [dinitroaniline] herbicides with Pursuit, waterhemp control was good. But trouble started when we didn't include the DNA herbicides.”

Tillage is another option. “When I first came here 20 years ago, 95% of all corn and soybeans had a lay-by cultivation,” Hartzler says. “You'd eliminate 99% of waterhemp that way.”

Heckathorn cultivates when the crop is about 12 in. high on corn and conventional soybeans. “We cultivate strictly because of waterhemp,” Heckathorn says. “There's no resistance to the cultivator.”

Learn from history

The overuse of ALS herbicides years ago may have a silver lining, Heckathorn says.

“As farmers, we all did the same thing by overusing those herbicides,” he adds. “We had a real mess. Now everyone is taking a more serious approach to seed management.”

The effort is justified, Smeda says. “Nature always bats last,” he reminds. “Every pesticide selects for its own failure.”

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