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Nip those weeds in the bud

By Staff | Sep 3, 2009

During a recent field day north of Wall Lake, Iowa State University Extension agronomist Mark Licht, right, assisted ISU Extension Weed Specialist Bob Hartzler explain why early-season weed control is critical.

WALL LAKE – If a small foxtail is allowed to reach 4 to 6 inches tall, the weed can grow half an inch per day, quickly turning into a significant yield robber. Even corn plants “know” how detrimental these early-season weeds can be.

Plants have a photo-receptor that can detect small changes in the wavelength of light, said Bob Hartzler, an Iowa State University Extension weed scientist who spoke at a recent ISU field day north of Wall Lake. “When weeds are present, light bouncing off their plant tissue alters the wavelength of the light that hits the corn, which allows the corn to sense that weeds are growing adjacent to it.

In response, the corn produces leaves and stalks, rather than putting energy into the roots, so it can grow taller than the weeds.”

While scientists don’t fully understand this complex system, it shows the importance of early-season weed control. This also highlights the difference between weed control and weed management, Hartzler said. “A lot of Iowa farmers are controlling the weeds, making sure their fields are clean by the end of the season, but they aren’t managing the weeds as effectively as they could be to protect the crops’ yield potential.”

Foundation herbicides pay off

Erin Hodgson, left, an Iowa State University Extension entomologist, discussed soybean aphid treatments during a recent field day north of Wall Lake.

In many cases, Hartzler said, broadleaf weeds are much more competitive over a full growing season than grasses. Just look at how much larger a velvetleaf can grow compared to a foxtail.

However, grasses can create much more of a problem early on, especially in corn. “Grasses are very good scavengers of nitrogen early in the season,” said Hartzler, who cited Wisconsin research that shows that a corn crop would need 20 to 40 pounds less of nitrogen, if foxtails were controlled with a foundation, or pre-emergence herbicide.

Since it’s impossible to predict early in the growing season how damaging weeds will be that year, a foundation herbicide is the best insurance a farmer has, said Hartzler, who noted that yield losses occur when weed control is delayed after weeds reach 2 inches tall. These yield losses can accumulate quickly. Research has shown that 8-inch tall weeds can trim a crop’s yield potential by 12 percent.

“A foundation herbicide will thin out the weed population, and buys you a lot of time for when you must get in the field for post-emergence applications,” said Hartzler, who said a product that offers full-season protection is not necessary in this system. “Choose products that offer good effectiveness against the dominant weeds in your field, and look for products with a good margin of crop safety.”

Unless a corn or soybean field has been managed so well that it’s weed populations are extremely low, it’s wise to use a pre-emerge/post-emerge weed control system, rather than just a sole post-emerge program, Hartzler said.

He also noted that mixing an insecticide with the herbicide “to save a trip over the field is never a good management practice.

“You need to know what bugs are in your field, and insecticides must be applied at the right time to control these pests. Instead of throwing in an insecticide with your herbicide, you may be better off with separate applications.”

Managing soybean aphids

As soybean fields entered the seed set period throughout Iowa this summer, ISU continued to receive questions about how long to scout for aphids. This year, populations have been extremely variable in Iowa, with a limited number of fields exceeding the economic threshold of 250 aphids per plant with increasing populations on 80 percent of the plants. In August, however, there were more reports of increased aphid activity throughout the state, said Erin Hodgson, an ISU Extension entomologist.

“You should keep scouting for soybean aphids through seed set,” said Hodgson, who urged farmers to scout every seven to 10 days before making an assessment. “Use speed scouting, or count the number of aphids per plant.”

Soybean aphids can fly between fields and are highly mobile. They are also highly prolific, since 15 to 17 generations can exist per year. This summer’s cool weather has been perfect for aphids, said Hodgson, who noted that a cool July favored the aphids’ trajectory of growth in August.

When temperatures range from 70 to 80 degrees, aphids can produce a tremendous amount of nymphs. In fact, aphids can double their population every three to four days in the right conditions, said Hodgson, who noted that some fields in the Decorah area had 2,000 aphids per plant this summer.

Concerning late-season aphids, Hodgson said there is much evidence to show that treating aphids when they exceed the economic threshold up to R5.5 will protect yields. However, if aphid populations are still increasing past seed set, it’s a gamble whether late-season insecticides will help.

“We don’t have much replicated data on this,” Hodgson said. “Sometimes it may be worth it, but it becomes a field-by-field judgment call.”

Contact Darcy Dougherty Maulsby by e-mail at yettergirl@yahoo.com.

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