soybean plant
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ISU: Don’t use dicamba postemergence on soybeans

Weed scientists are only recommending dicamba be used preemergence on dicamba-tolerant beans.

At last month’s Integrated Crop Management conference at Iowa State University, ISU Extension weed management specialists Mike Owen and Bob Hartzler gave presentations on the use of dicamba herbicide for weed control in the new dicamba-tolerant soybean systems.

The new formulations of dicamba herbicide came on the market for use in Iowa for the first time in 2017. After widespread problems with drift of the new dicamba herbicides onto non-tolerant soybeans and causing damage, the ISU weed scientists are recommending Iowa farmers avoid using dicamba postemergence on dicamba-tolerant beans in 2018. ISU recommends dicamba only be applied preemergence or as a burndown.

“We reviewed the situation in Iowa and other states, and have decided to recommend the preemergence and burndown type of applications, but we are not recommending postemergence applications of dicamba,” says Owen. “We are fully aware this limits the utility of this weed control technology.”

New label changes ignore volatilization issue
Owen and Hartzler also looked at the label changes for dicamba, mandated by the Environmental Protection Agency after the widespread drift problems occurred in 2017. Drift problems were even worse in states south of Iowa. “None of the label changes do anything to address volatilization drift,” says Owen. “These changes in the label requirements will help reduce particle drift and help avoid spray tank contamination. But they ignore the volatilization issue that we believe exists with dicamba herbicide products.”

Dicamba isn’t a new herbicide. But the new formulations which the manufacturers say have less volatility are new. “Iowa farmers have had a love-hate relationship with dicamba for a long time,” notes Hartzler. “Years ago, dicamba was a corn herbicide applied on up to 80% of the corn acres in northwest Iowa due to that area’s high pH soils. The use of atrazine was limited on high pH soils, so dicamba was a good, effective weed control alternative.”

New formulations provide effective weed control
Owen and Hartzler say a bright spot in 2017 was that the new dicamba herbicides did provide good weed control when applied on dicamba-tolerant bean varieties. “Dicamba was never strong on pigweed, so we wondered how effective it would be on waterhemp,” says Hartzler. “We were impressed.”

Three new dicamba herbicide formulations were marketed for the first time in Iowa in 2017. They are marketed as low volatility products to be used on dicamba tolerant bean varieties. The three herbicides are available again for 2018: Engenia from BASF, FeXapan from DuPont, and Monsanto’s Xtendimax with Vapor Grip Technology.

Record number of drift incidents reported in Iowa
The Iowa Department of Agriculture had 270 off-target herbicide cases reported in 2017 in Iowa, and 175 of them involved growth regulator herbicides, such as dicamba. It was the first time the department has ever had over 200 cases in a year, notes Hartzler. Manufacturers of the new dicamba formulations say the drift damage was due to applicator error and farmers not following label recommendations: incorrect nozzle selection or boom height, excessive wind speed or spraying when wind was blowing toward a sensitive crop, having an insufficient buffer, and sprayer contamination.

Owen and Hartzler agree that in many cases, off-target movement is due to such factors. “But we know the volatility of dicamba also plays a role,” says Hartzler.

A historical perspective on dicamba herbicide
At the 2017 North Central Weed Science Society annual meeting Hartzler was asked to provide the opening presentation on “A historical perspective on dicamba.” It was a symposium focusing on issues with the use of dicamba, including volatilization and particle drift. The slides and a summary of Hartzler’s presentation are at Powerpoint presentation pdf. A summary of his presentation, posted as a blog on the ISU Integrated Crop Management website, follows here:

The auxin-like activity of the phenoxyacetic and benzoic acids was discovered in the early-1940s. The herbicide dicamba was first described in 1958; Velsicol acquired the patent for the molecule, and dicamba was first approved for use in the U.S. in 1962. In subsequent years, the label was expanded for use on a wide range of grass crops (such as corn) and for non-crop areas. Dicamba has been described as either a benzoic acid or carboxylic acid compound, and mimics the activity of indole-3-acetic acid (Group 4 herbicide).

Dicamba effective on herbicide resistant weeds
According to USDA data, dicamba was used on less than 10% of U.S. corn acres in 1979. Use increased to 15% of corn hectares by 1990, then as herbicide-resistant weeds spread, dicamba use on corn increased to 28% of hectares in 1995. Prior to the introduction of herbicide-resistant crops and Group 27 herbicides (HPPD inhibitors), dicamba primarily competed with atrazine and 2,4-D for broadleaf weed control in corn.

Atrazine was preferred over dicamba and 2,4-D by most farmers due to its preemergence use, greater margin of crop safety, and lower risk of off-target injury. Dicamba use was much higher in northern states with high pH soils due to the carryover risk associated with atrazine. Dicamba was used on more than 70% of the 1985 corn hectares in north-central and northwest Iowa, compared to 12% of U.S. corn hectares. High pH soils in this region prevented use of atrazine rates greater than 1 pound per acre when rotating corn to soybeans or other sensitive crops.

Soybean sensitivity to dicamba has always been an issue
The high sensitivity of soybeans to dicamba has been an issue since its introduction. In a 1971 University of Illinois Extension bulletin, Dr. Ellery Knake discouraged the use of dicamba on corn due to the risk it posed to adjacent soybeans. Two other weed scientists (Behrens and Leuschen) published a paper in 1979 reporting on factors that influence volatility of dicamba, including temperature, rainfall following application, application surface (soil vs. foliar interception), and formulation. A wide range in volatility was found among the salts of dicamba evaluated.

The first dicamba product (Banvel) contained the dimethylamine salt of the parent acid. Over the years, several different salts of dicamba have been introduced, often with the intent of reducing dicamba volatility. Low volatility formulations include Banvel II (sodium) in 1981, Clarity (diglycolamine) in 1990, and most recently Xtendimax/Fexapan with Vaporgrip Technology (diglycolamine) and Engenia (BAPMA).

Continued research will hopefully provide answers
Current research will determine the reductions in volatility and drift problems achieved with these new dicamba formulations. Increasing problems with herbicide-resistant weeds have led to an increase in dicamba use, and the introduction of dicamba-tolerant crops will continue this trend of more dicamba use. The International Survey of Herbicide Resistant Weeds lists 36 weed species with evolved resistance to Group 4 herbicides, seven of these species are reported to be resistant to dicamba.

For more information, download ISU’s 2018 Herbicide Guide for Iowa Corn and Soybean Production available at store.extension.iastate/Productwc94-pdf.




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