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Corn+Soybean Digest

Defending Against Glyphosate Resistance … Basic Plays Still Key To Victory Over Weeds

If your 2004 game plan includes Roundup Ready (RR) cropping, you may want to borrow a page from a coach’s playbook. University weed specialists say the best defense against glyphosate resistance will come from a fundamental offense.

“We need to get reacquainted with some of the basic weed control practices that we have used in the past,” says Jim Martin, extension weed specialist, University of Kentucky.

According to Martin and others, some of the best offensive moves include pre- or at-planting burndown, a return to pre-emergence herbicides, crop scouting, and timely post-emergence spraying.

These are practices that many farmers have strayed from since the introduction of RR crops, notes Patrick Tranel, associate professor, University of Illinois. “Roundup Ready opened up the herbicide window and farmers didn’t have to worry as much about timing,” he explains. “They could make their applications later in the season without as much emphasis on crop scouting and weed size.”

For a while, it worked like a charm. But with confirmed glyphosate resistance in marestail and reports of increasing control problems from weeds like waterhemp, lambsquarters, and giant ragweed, researchers are calling for some back-to-basics weed control, with an emphasis on herbicide timing to eliminate weeds before they can compete with the crop and reduce yield.

Burndown Green Weeds

When deciding whether or not to use a burndown herbicide, a standard rule-of-thumb for no-tillers has been “never plant into green weeds.”

“Even small weeds can get a jump on the crop and compete for early-season moisture, sunlight and nutrients,” explains Dallas Peterson, weed specialist, Kansas State University.

With the advent of RR soybeans, some Midwest growers started combining their burndown and post-emergence glyphosate application into one and calling it a “delayed burndown.” The idea is to save money by cutting out one glyphosate treatment.

“The general impression in soybeans seems to be that you can let the weeds go longer into the season,” Peterson says. “But on a delayed burndown program, some farmers wait too long. By the time they spray, they’ve already lost yield.”

This wait-and-see approach also means that some growers are spraying glyphosate when target weeds are beyond the stage for best control.

“Years ago, marestail control wasn’t an issue because farmers used 2,4-D anywhere from seven to 30 days before planting,” says Martin. “Because of the potential for crop injury, 2,4-D forced us to be timely – we had no choice but to apply the herbicide when the marestail was smaller and easier to control.”

The recommended quart rate of glyphosate is for less than 4- to 6-inch marestail, notes Jeff Stachler, extension program specialist for weed science, Ohio State University. “But in some cases, growers are applying this rate to 12- to 24-inch marestail and wondering why they’re not getting control,” he says. “Unfortunately, we’re going to have a lot more glyphosate resistance in marestail if this practice continues.”

Be More Timely Over-The-Top

According to Bob Hartzler, weed specialist with Iowa State University, glyphosate timing is also an issue for post-emergence control of giant ragweed.

“This weed germinates early and grows fast, so it’s 10 inches tall by the time weeds like velvetleaf are two inches tall,” he says. “By the time velvetleaf gets enough height to spray, ragweed is already too big for consistent control.”

Larger ragweed plants are also reported to harbor stalk borers and other insects that can interfere with spray coverage and reduce control. “Ideally, giant ragweed should be treated when it’s 4 to 5 inches tall,” says Hartzler. “Any taller and you’re just killing off the top shoots -- the plant will still send up lateral shoots.”

In the eastern Corn Belt, growers battling giant ragweed must also contend with an extended germination pattern. Waterhemp is another troublesome weed that germinates throughout the season.

Like ragweed, “the optimum spraying time for waterhemp does not coincide with other standard weeds,” says Hartzler. “Waterhemp comes on slow and keeps on coming. That’s making it tougher and tougher to do everything in one or two trips.”

Use Residuals When Needed

In these cases, a return to pre-emergence herbicides can be a big help, says Hartzler. In fact, he says more Iowa growers used pre-emergence herbicides in RR cropping last year, and there was a notable improvement in waterhemp control.

“The weather also helped us out, but we also need to give some credit to the pre-emergence herbicides,” he says.

Whether it’s a split-post treatment or a split pre-emergence /postemergence treatment, a two-pass approach will provide more consistent, season-long weed control, plus it will control a broader spectrum of weeds, says Leon Wrage, extension weed specialist, South Dakota State University. “We especially like the pre-plus-post program because it gives you more application flexibility than a total-post program,” Wrage says. “That can be a real advantage for large-acreage growers because a split-post program has a much tighter critical timing period -- especially for corn.”

According to Tom Hoverstad, weed specialist, University of Minnesota, reliance on total-post herbicide programs in corn has aggravated recent waterhemp problems.

“Growth regulator-type herbicides used in typical total-post programs are not that strong on waterhemp,” he explains. “In corn, you’re better off with a soil applied herbicide followed by a post-emergence treatment.” In soybeans, Hoverstad recommends a two-pass post glyphosate program, or a soil-applied herbicide followed by one post-emergence application of glyphosate.

Mix Modes of Action

Ohio State’s Stachler points out that reliance on total-post programs eliminates nearly one third of the currently labeled herbicide modes of action options. “Over-reliance on one mode of action is what helped to increase ALS resistance and, now, the idea of glyphosate resistance,” he says.

To slow the spread of glyphosate resistance, Stachler advises growers to use as many different herbicide modes of action (MOA) as possible. “The only way to do that is to go back to pre-emergence herbicides,” he says. “Because when you knock those out, you lose at least 30 percent of the total herbicide MOA’s.”

Use Conventional Weed Control In Corn

To avoid continuous glyphosate, a lot of researchers favor conventional weed control in corn. They say RR technology is more suitable to soybeans because soybeans can tolerate more early-season weed pressure without losing yield. And there are plenty of effective and economical options for pre-emergence and post-emergence weed control in corn.

“Weed competition in corn, especially from grasses, is so significant because it doesn’t take much before you start hurting yield in corn,” explains Stachler. “Soybeans can compete longer against weeds and have more time to make up for any competition. You just don’t have that in corn.”

There’s a lot of value and benefit in using pre-emergence herbicides in corn, agrees KSU’s Peterson. “With some of the more recent chemistries, we have a pretty good shot at season-long control from a pre-emergence herbicide, even with waterhemp,” he says. “When you use an effective pre-emergence herbicide, you don’t have to worry about early-season weed control and the timing of your post-emergence spray.”

Scout Fields

SDSU’s Wrage says crop scouting should also be in every weed control game plan, whether growing RR or conventional crops.

“With growers working larger acreages and doing less cultivation, they aren’t going through their fields as often as in the past,” says Wrage. “Nowadays, harvest is the first time they see weed escapes. But they really need to look at their fields before then. If they let it go until harvest, a seemingly harmless weed could quickly become a major yield competitor.”

Like any good game plan, a balanced attack is better than any single strategy. Not unlike an opposing football team, weeds adapt to a challenge, and by working in some basic early-season weed control practices, farmers can win the battle against weeds today without weakening their chances for continued success in the future.

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