Weed Warriors: Herbicide Handoff

March 8, 2013 08:18 PM
Weed Warriors: Herbicide Handoff

Overlapping products puts a protective barrier in fields to shut out weeds

The philosophy of start early and don’t let up drives Jason Luckey’s approach to weed control on his west Tennessee farm. The grain and cotton producer starts each spring with burndown products so fields are clean at planting. He then uses a combination of pre-emergence and post-emergence herbicides with residual to continue his control efforts.

"I put products down early and then layer them in-season over the top," Luckey says.

Weed scientists refer to this practice as overlapping residual herbicides to keep a constant protective barrier in fields to prevent weed establishment. Southern farmers have successfully
adopted this concept in Roundup Ready cotton and soybeans for several years now to battle glyphosate-resistant Palmer amaranth (pigweed). Now, the concept is catching on with farmers further north who are plagued by waterhemp, in particular.

Weed Warrior

"Overlapping herbicides works well on waterhemp because it typically flushes multiple times, like Palmer amaranth, during a single growing season," says Jeff Springsteen, BayerCrop Science marketing manager for selective corn and soybean herbicides "Essentially, you’re matching your weed control strategy to the waterhemp’s natural biology," adds Kevin Bradley, University of Missouri Extension weed scientist.

Waterhemp can grow more than 1" per day. As a result, it can get beyond the control reach of most herbicides quickly. Left unchecked, it can wreak havoc in crops.

Protect yield. Season-long competition by waterhemp (more than 20 plants per square foot) reduced soybean yields 44% in 30" rows and 37% in 7.5" rows, according to research by Larry Steckel, University of Tennessee weed scientist.

Early season competition in corn from a heavy waterhemp infestation (more than 30 plants per square foot) reduced yields 15% by the time waterhemp was 6". With lower waterhemp densities (less than 10 plants per square foot), corn yields were reduced by only 1% when waterhemp was controlled by the time it reached 6".

By overlapping herbicides, weeds never get the chance to emerge and impact yield potential. The trick to making the practice work is figuring out how close together to make the herbicide applications, Bradley says.

For soybeans, he recommends applying a pre-emergence herbicide as close to planting as possible. Then, make a second herbicide application roughly two to three weeks postemergence to get grasses and anything that broke through the first herbicide application.

"At the time of that second postemergence application, growers can also apply an overlapping residual herbicide to prevent subsequent flushes of waterhemp later in the season, which almost always occur," Bradley says.

Choose carefully. In the process of overlapping herbicides, make sure you pick modes of action that are effective against your specific weed species, says Gordon Vail, technical product lead for Syngenta. He highlights a common misstep farmers make: "If one mode of action is an ALS inhibitor and you have ALS-resistant weeds, that means a two-mode-of-action product only has one mode of action that works against that weed," Vail explains.

For corn, Springsteen recommends applying a residual herbicide such as Corvus to fields prior to crop emergence. Make the second herbicide application between the V4 and V6 corn growth stages, depending on weed growth and weather conditions.

Corvus and Balance Flexx herbicide offers reactivation properties. "A little rain reactivates the herbicide, which can enlarge your window of treatment and control," Springsteen says.

In the process of overlapping herbicides, Bradley cautions farmers to use full labeled rates of herbicides for best results. "Skimping on rates can set up a scenario for resistance," he says.
Sometimes tough weeds break through even the best control efforts. In that case, consider Luckey’s strategy to prevent stray weeds from going to seed: chop them out by hand.

You can e-mail Rhonda Brooks at rbrooks@farmjournal.com.


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