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Dicamba-tolerant soybean moving forward

The dicamba/soybean tank mix provided very good control of glyphosate-resistant giant ragweed and Canada fleabane in Roundup Ready 2 Xtend soybean. Photo courtesy of Monsanto Canada.

Soybean growers will have to wait another year or two before they have the option of using a soybean cultivar that is resistant to both glyphosate and dicamba. While Monsanto Canada has received full regulatory approval in Canada for Genuity Roundup Ready 2 Xtend soybean, the industry’s first biotech product with herbicide tolerance to both glyphosate and dicamba, more registration work and export approvals are required before the new cultivars can be commercialized.

Mark Lawton, technology development lead with Monsanto Canada at Guelph, Ont., explains that the regulators must still approve dicamba use on soybean. In addition, importer approvals are required before the biotech product can be shipped to countries such as China and the U.S.

“We have a basket of export approvals that we are working on. China isn’t finished, and about one year ago the U.S. decided to do an Environmental Impact Study on Monsanto’s Xtend and Dow’s Enlist systems,” says Lawton. “These are still under review in the U.S.”

Lawton says Monsanto has historically sought all necessary export approvals prior to commercializing a product. He says this approach isn’t any different for any technology, chemistry or biotech innovation, and that the last thing the company would want to do is cause marketing issues for Canadian farmers. The company is hoping that all approvals will be ready for the 2015 growing season.

The stacked technology will offer soybean growers additional weed control options and the potential to better manage glyphosate resistant weeds. Dicamba is a Group 4 herbicide, and this group has not been previously available to soybean growers as an in-crop application.   

Dicamba controls over 95 annual and biennial broadleaf weed species and provides suppression of over 100 perennial broadleaf and woody species worldwide. Combining both dicamba and glyphosate tolerance in soybean would give farmers the option of applying Roundup WeatherMAX herbicide and low-volatility formulations of dicamba, separately or as a tank mix.

Of special interest for Ontario soybean growers is the ability to control glyphosate resistant giant ragweed, common ragweed and Canada fleabane. Peter Sikkema, a professor and weed scientist at the University of Guelph, conducted several years of trials on the Xtend system and found it very effective for the control of glyphosate-resistant giant ragweed and Canada fleabane. Glyphosate applied alone as a post-emergent treatment in the Xtend system provided less than 50 per cent control of the giant ragweed populations, indicating a significant proportion of glyphosate-resistant biotypes were present in the plots. With the addition of dicamba to glyphosate, giant ragweed control increased to 87 to 94 per cent.  

“In our trials, glyphosate plus dicamba provided very good control of glyphosate-resistant giant ragweed and Canada fleabane in Roundup Ready 2 Xtend soybean,” says Sikkema. “I would say the system is a strong performer but I really think it has to be used in an integrated program to maintain the benefits of the technology.”

The Xtend system could also provide soybean growers with the option of applying dicamba in a pre-seed burndown with glyphosate, prior to an in-crop application of glyphosate. This approach would provide residual control to help keep a field cleaner early in the season.

Stewardship will be important
As with any new technology, prudent use will help preserve the long-term sustainability of the system. Rotating herbicide groups and applying multiple modes of action are two components of resistance management.

In Western Canada, this new dicamba option in soybeans will also help growers manage herbicide resistance. Glyphosate-resistant kochia, which is also resistant to Group 2 herbicides, has been identified in southern Alberta and Saskatchewan. While glyphosate-resistant kochia hasn’t been confirmed in the main soybean growing areas of Manitoba and Saskatchewan, the option of applying multiple modes-of-action can help manage glyphosate resistance.

In an ideal world, Sikkema would like a weed control system in soybean where a soil-applied, pre-plant herbicide other than a Group 4 was applied to control grass and broadleaf weeds. A post-emergent application of glyphosate and dicamba in the Xtend system would control later emerging weeds.

“There would be excellent weed control early in the season and the number of weeds exposed to the dicamba/glyphosate tank mix would be greatly reduced, which would reduce the selection intensity for glyphosate resistant biotypes,” says Sikkema. “With our crop rotations, this approach would have three or four different modes of action on every acre in every year.”

Sikkema cautions that soil residual herbicides would have to be properly planned in the rotation so that residues wouldn’t hurt subsequent sensitive crops. He says there isn’t a one size fits all, but that the Xtend system could help manage herbicide resistance if planned properly.

Monsanto is also working on stewardship initiatives to help ensure off-target dicamba drift is minimized during application.  The company is developing next-generation glyphosate and lower-volatility dicamba formulations to complement the new crop system. They will provide specific recommendations on nozzle types and application guidelines. Tank cleanout to prevent cross contamination when moving from crop to crop will also be an important message coming from Monsanto.

“What the Xtend system can bring farmers is pretty exciting with better weed control and expanded weed resistance management strategies, but we also understand that we need to communicate how to use the technology in a sustainable manner,” says Lawton.

February 18, 2014  By Bruce Barker