By Debalin Sarangi and Amit Jhala

Six weeds have been confirmed resistant to glyphosate in Nebraska due to repeated application of glyphosate in Roundup Ready corn-soybean production systems.

The increasing prevalence of herbicide-resistant weeds is compelling growers to diversify weed management practices. LibertyLink technology in corn and soybean provides an opportunity to apply Liberty (active ingredient: glufosinate; site-of-action group 10) as a postemergence on top of the crop canopy.

Liberty is a water-soluble nonselective herbicide that can control a broad spectrum of emerged broadleaf and grass weeds. It is a contact herbicide, which means it only kills the plant parts contacted by the herbicide. Small weed size and uniform spray coverage are necessary for optimum control of emerged weeds.

Considerations for Liberty use
Following are spraying guidelines to ensure maximum weed control with Liberty:

• Target weeds at early growth stage. Liberty should be applied when weeds are less than 4 inches tall. The bigger the weed at the time of control, the smaller the chance for effective weed control. Moreover, shoot regrowth can be seen when Liberty is sprayed on a weed more than 4 inches tall.

• Follow instructions on herbicide label. The Liberty label states the herbicide can be applied at 32 to 43 fluid ounces per acre as a burndown (before planting), and at 29 to 43 fluid ounces per acre in LibertyLink corn and soybeans as a postemergence (with a cumulative maximum of 87 fluid ounces per acre per year). Liberty should be applied at a full rate to avoid the selection pressure on weeds. All Liberty applications must be made more than 60 days before harvesting corn forage and 70 days before harvesting corn grain and soybean.

It's recommended that ammonium sulfate (AMS) be mixed with Liberty at 1.5 to 3 pounds per acre, depending on the tank-mix partners, to improve weed control efficacy, especially for difficult-to-control weeds, such as velvetleaf, common lambsquarters, giant ragweed and pigweed. A minimum of 15 gallons of carrier volume per acre is needed to ensure good spray coverage. The volume can be increased up to 20 gallons per acre in a dense weed canopy.

Medium to coarse spray droplets are necessary to reduce spray drift to sensitive crops; however, flood jet nozzles should not be used for Liberty applications. Examples of recommended nozzles are TeeJet VisiFlo Flat, XR TeeJet Extended Range Flat, Turbo TwinJet Flat or TurboDrop XL. Liberty should be applied when wind speed is less than 10 mph to reduce drift.

• Spray on a warm, sunny day. Liberty’s efficacy is maximized by warm temperatures, high humidity and bright sunlight. Liberty should be applied from two hours after sunrise to two hours before sunset. Avoid spraying when a heavy dew or fog is present and weeds are under stress due to drought, cold or extended cloudiness. Rainfall within four hours of Liberty application may wash out the herbicide and reduce its efficacy.

Italian ryegrass has been confirmed resistant to Liberty in California and Oregon. It's important to diversify weed management practices to delay the evolution of resistance to Liberty. Rotating crops, selecting for herbicide-resistant traits and using herbicides with multiple "effective" sites-of-action is recommended to reduce selection pressure.

Sarangi is a former weed science postdoctoral research associate, and Jhala is an Extension weed management specialist at the University of Nebraska-Lincoln. This report comes from UNL CropWatch.