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Corn+Soybean Digest

Weeding Through Weed Control

Ted Bay has had a rough couple of years battling weeds in soybeans. Delayed planting has given weeds a leg-up early on, and he's also battled weather-induced delays during his second application of glyphosate before canopy.

In normal growing years, a two-pass glyphosate program has worked well - the first application when beans are 3-4 in. tall, then just before canopy. “But timing can be an issue, especially if we have wet weather delays,” Bay says. “Because I hire the application, there can be a time lag, and that has shifted me to call earlier than I normally would.”

Bay also serves as Extension crops and farm management agent in Grant County, WI, so he not only experiences weed-control issues on his own farm, he sees issues faced by other county farmers. “And we've noticed that any delays in that second application can make a dramatic difference in weed control,” he says.

Chris Boerboom, Extension weed scientist at the University of Wisconsin, says mistimed applications may be costing producers more than they think. “Spraying too late is giving weeds the ability to compete with soybeans,” he says. “And even if the weeds are dead at harvest, that competition early in the growing season is reducing yields.”

Surveys on soybean fields in Wisconsin in 2008 showed that nearly half the fields may have suffered a $20/acre yield loss because they were sprayed too late. “Producers might not notice a 3-bu. yield decrease because of weed competition, but it's there if the glyphosate is sprayed late,” Boerboom says.

He adds that at current input prices, producers may want to rethink their weed-control strategy. “What we do know is that an increasing number of soybean growers are already spraying more than once with glyphosate,” he says. “With an increase in glyphosate price, producers may want to replace that second application with a pre-emergence herbicide. With a pre-emergence herbicide down, one shot of glyphosate often does the trick.”

That strategy protects the crop from early season weed competition, especially if the first glyphosate application is delayed. Second, controlling that first flush of weeds gives the first application of glyphosate a better opportunity to work.

“Economically, trading a pre-emergence application for a second glyphosate application is close to a wash,” Boerboom says. “But the advantage is better weed control and a reduction in weed competition, which will pay off by not suffering a potential 2- or 3-bu./acre yield hit.”

Boerboom calls that yield hit the invisible yield loss. “If a producer gets 50 bu. to the acre rather than 53, and the field is perfectly clean at harvest, they may not notice a difference on the yield monitor,” he says. “But that early season competition is just enough to knock down yields.”

One of the biggest changes in weed-control options is the introduction of LibertyLink soybeans for the 2009 growing season. Producers can use Ignite (a new form of Liberty herbicide) over the top of LibertyLink soybeans as an alternative to the Roundup Ready system.

And while the system has several similar traits to the Roundup system, Boerboom cautions that it is not a direct replacement. “Ignite is a contact herbicide, as compared to the translocation of glyphosate. Timing for Ignite needs to be monitored more closely to be effective and have a high level of performance,” he says.

The new system does offer a new mode of action for weed-resistance management.

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