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Corn+Soybean Digest

Weeds To Watch In Roundup Ready Beans

Nothing's perfect. And, in spite of its popularity, that includes Roundup Ultra herbicide. Under ideal conditions, it can keep fields weed-free without help from a residual soil-applied herbicide - sometimes with a single application.

But when the inevitable happens, some weeds are tough enough to survive Roundup's non-selective nature. Even if those weeds don't compete with the crop, they can still cause problems at harvest.

In Nebraska, extension weed scientist Alex Martin lists morningglory, giant ragweed, velvetleaf and waterhemp as likely candidates for weed escapes in Roundup Ready soybean fields.

"Weed species that germinate late or tend to have multiple flushes have an advantage," says Martin. "Particularly if a weed is under stress when Roundup is applied, it will respond to the herbicide, show symptoms, but not die. You won't get the same effect as you would with the same rate of herbicide applied when the plant has lush growth."

Farther south, Ford Baldwin, University of Arkansas weed scientist, also sees possible problem weeds in Roundup Ready soybeans.

"Several things will happen when a person gets on a Roundup Ready program and stays on it," he says. "Anytime you take the residual herbicide out of a program, you'll see a shift to late-germinating, more shade-tolerant type weeds. We'll see a shift to more morningglory and hemp sesbania, or "coffeebean."

"Roundup is capable of controlling those weeds with two applications, but poor environmental conditions and bobbled applications allow for escapes."

Reduced tillage and drilled beans also leave a slot for an increase in perennial weeds in Roundup Ready beans, according to Baldwin.

"The solution is to use herbicide rotation, or add other herbicides to your Roundup Ready program," he says. "Farmers who have gotten away from soil-applied herbicides are discovering how good they really are. Roundup is a great technology, but I think it will have to be used in a program with other herbicides."

Iowa farmers have seen their share of problems with waterhemp, velvetleaf, morningglory and yellow nutsedge, according to Bob Hartzler, Iowa State University weed specialist.

"Waterhemp can be difficult to control if the weather's cool when Roundup is applied, or if weeds get more than 8" tall," says Hartzler. "Farmers don't plan for that to happen, but it does. Weed size and the environment interact and make it difficult to predict exactly how plants will react."

On velvetleaf you can start to see problems if the weather is either too cool or hot and dry, and if the weeds get too big.

"Yellow nutsedge remains a concern," Hartzler says. "Some dealers are starting to add Classic to Roundup for more control. I think that's a trend we'll continue to see.

"We don't have any weeds that can't be controlled with Roundup by itself. But a lot of times growers are trying to go with a single application. They may find it more efficient to add something else to the tank rather than increase the rate of Roundup they use."

Last year was a banner year for weed control in Roundup Ready soybeans in northwestern Missouri, reports University of Missouri extension agronomist Don Null.

"Everything was growing fast and succulent," he says. "I haven't talked to a single producer who is unhappy with Roundup Ready weed control."

In dry years, like 1997, however, Null also lists velvetleaf, morningglory and waterhemp as possible problems if they get too big. In some cases, crabgrass and barnyardgrass also go on the list.

"If you want to use a single Roundup application, I'd wait until the weeds are 4-8" tall," Null advises. "Unless we are in drought conditions, that doesn't scare me a bit. If you spray when the weeds are 1-3" tall, you're likely to see a second flush, and that will mean you have to make a second Roundup application."

In Indiana, growers have seen problems with giant ragweed, velvetleaf, morningglory and smartweed when weather prevented timely applications, says extension weed specialist Tom Jordan.

"We've seen a few problems with black nightshade," Jordan reports. "It comes in late and hides in the canopy. Honeyvine milkweed, or clinging milkweed, won't be adequately controlled if there aren't enough leaves on it. Large, established pokeweed, lambsquarters and pigweed can be a problem, too."

Jordan is quick to add, however, that farmers using a single Roundup application in drilled beans, which most Indiana beans are, have been satisfied with the results.

"When and how many applications you need to make is really weather-driven," he says. "While most farmers have been happy with a single application, I think generally it's going to take more than that due to weather conditions. That may be tankmixing a second herbicide that's species-specific and real strong on something real big."

All the weed scientists caution that it's unlikely Roundup will always be as effective as it is today.

"You don't have to be a genius to know there's going to be change," says Nebraska's Martin. "Five years from now we won't think that Roundup gives as good of control as it does today. There's too much genetic diversity in the weed population. Does Pursuit give as good of control today as when it first came out? Does atrazine?"

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