Weed Control Strategies - 5

OMAHA (DTN) -- Dicamba-tolerant corn and soybeans stacked with the Roundup Ready trait are getting closer to market. However, growers need to realize the new trait technology will require closer attention to detail than glyphosate-only programs.

One example is the use of additives. Growers are accustomed to loading up spray volume with ammonium sulfate (AMS) to enhance glyphosate activity. DTN has discovered that the same AMS addition could possibly increase dicamba volatility in both the older and newer formulations. Granted, companies are still working out the bugs in the system and these new dicamba products haven't come to market yet. However, it's important to start discussing the best management practices for these new technologies.

Richard Zollinger, a North Dakota State University weed scientist, agreed that in the past AMS has been a must with glyphosate. "AMS enhances Roundup absorption and translocation and deactivates antagonistic hard water salts (Na, Ca, Mg, and Fe) which bind to glyphosate salts," Zollinger told DTN in an interview. "As water evaporates, sulfate from AMS binds with antagonistic salts and prevents binding with glyphosate. In addition, ammonium from AMS binds with glyphosate, resulting in greater absorption."

The current label for glyphosate suggests adding AMS at 8.5 to 17 pounds per 100 gallons of water. However, Zollinger said that analysis of water across the U.S. shows that 4 to 6 pounds per 100 gallons of solution is generally adequate to overcome most hard water in the U.S. Zollinger maintained that most growers are overusing AMS.

With proper spray techniques, drift retardants and avoiding windy days, glyphosate drift can be minimized because the glyphosate doesn't volatilize and drift as vapor.

However, herbicide products like 2,4-D and dicamba can volatilize and drift "off-site" as vapor, even with good spray technique practices and especially during warm summer days. That's why manufacturers are coming out with new formulations of 2,4-D and dicamba that are less volatile. What those new formulations don't address (yet) is the fact that it appears the addition of AMS can exacerbate dicamba volatility. It also appears AMS does not increase 2,4-D volatility.

Dicamba has been marketed as several different salts including dimethylamine (Banvel), diglycolamine (DGA)(Clarity), potassium salt (Marksman) and sodium salt (Distinct, Celebrity Plus, NorthStar). In a greenhouse dicamba volatility study conducted by Bob Hartzler at Iowa State University in 2001, the author reported that dimethylamine salt of dicamba caused 64% injury to soybean, whereas the potassium and sodium salts did not cause a visible response. In a corresponding field study, they found that the sodium salt reduced but did not eliminate volatilization injury to soybeans compared to the dimethylamine salt.

John Combest, Monsanto chemistry communication manager, told DTN that Monsanto has recently evaluated AMS and determined that it can contribute to increased dicamba volatility potential. "Therefore, Monsanto will restrict the use of AMS with Roundup Xtend and XtendiMax herbicides that come premixed with dicamba," Combest said. Monsanto has not yet officially released the formulation, since it is still in the development phase and has not yet been registered by EPA.

"Monsanto is developing and evaluating multiple low-volatility formulations of dicamba for use with the Roundup Ready Xtend Crop System. We will be communicating more details about the specific formulations as we advance through the registration process with EPA," he added.

BASF also plans to market a dicamba-based herbicide called Engenia to complement the upcoming trait technology. Luke Bozeman, technical marketing manager with BASF, said Engenia greatly reduces the risk of secondary dicamba movement, including volatility that was a risk with some earlier formulations. "Innovative formulations help manage off-target risk; however, it is also important for the applicator to manage spray drift through proper application," he added. "Managing spray drift is the most important means of preventing off-target movement. Some other dicamba formulations, such as in Banvel, are potentially more volatile and may increase risk of secondary drift."

BASF recognizes that adding AMS to the spray tank may further increase the risk of secondary drift. While Engenia has lower volatility risk, BASF anticipates being aligned with Monsanto and restricting the use of AMS with Engenia. "Look for water conditioning additives other than AMS to add to your spray solution. We also anticipate less need for AMS for broadleaf weed control in an Engenia plus glyphosate tank-mix," Bozeman said.

Managing volatilization and vapor drift also requires a higher level of spray management. "Volatility is only a minor component of off-site movement," Combest said. "Particle movement (drift) is the primary means and risk for potential off-site movement of dicamba and application requirements must be followed to minimize this potential."

BASF and TeeJet have been sponsoring the On Target Application Academy to educate grower and applicators about spray stewardship. The academy provides grower and applicators with hands-on training on the latest advancements in application technology that include: Selecting the right formulation; nozzle selection; practicing good spray stewardship to prevent spray drift and utilizing an agronomically sound approach to weed control that includes the use of effective soil residual herbicides. "By paying attention to all of these factors, growers can control their most difficult to control broadleaf weeds while keeping spray drift under control," Bozeman said.

At the recent Commodity Classic, Monsanto announced it will have 19 "Learning Xperience" sites across the country this summer to begin educating stakeholders about the dicamba-tolerant crop system it calls the Roundup Ready Xtend Crop System.

Dan Davidson can be reached at Daniel.davidson@telventdtn.com

(PS/ES/AG)

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