Calls for caution after 2,4-D-resistant weed identified in U.S.

overuse Researchers fear farmers will go for short-term solutions, 
creating longer-term problems

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The discovery of 2,4-D-resistant waterhemp in the U.S. has sparked calls for new rules governing how farmers use herbicide-tolerant crops.

The researchers who documented 2,4-D-resistant population of waterhemp say new crops stacked with glyphosate and Group 4 herbicide tolerance must be managed carefully to avoid selecting for super weeds.

“The commercialization of soybean, cotton and corn resistant to 2,4-D and dicamba should be accompanied by mandatory stewardship practices that will minimize the selection pressure imposed on other waterhemp populations to evolve resistance to the synthetic auxin herbicides,” they wrote in the latest issue of the Weed Science of America’s journal.

“The fact that resistance to 2,4-D has evolved in at least one waterhemp population (in Nebraska) should be emphasized to corn, soybean, and cotton producers to show that proper stewardship of these new technologies is critical for maintaining their effectiveness.”

Adding 2,4-D or dicamba to glyphosate-tolerant crops is one way to combat glyphosate-tolerant weeds, University of Manitoba plant science instructor Gary Martens said in an email. If the glyphosate doesn’t kill it, the 2,4-D or dicamba will.

“But it is only a short-term solution,” he said. “Adding 2,4-D-resistant crops will dramatically increase the use of 2,4-D.”

And that could result in more 2,4-D-resistant weeds, he wrote.

“(T)here are concerns that we’re going to overuse that group of herbicides and just create a big mess down the road,” said Hugh Beckie, a Saskatoon-based Agriculture and Agri-Food Canada (AAFC) research scientist who studies herbicide-resistant weeds. “And that’s certainly a valid concern. Stewardship of those crops — say Roundup plus 2,4-D — will be key to try not to create that big mess down the road.”

Other Group 4s affected

Weeds that become resistant to 2,4-D could also develop resistance to other Group 4 herbicides such as dicamba, Beckie said.

2,4-D and other Group 4 herbicides are important to Manitoba farmers because they are so widely used and are relatively inexpensive. “We do not have a readily available alternative, inexpensive broadleaf herbicide should 2,4-D fail,” Martens wrote.

2,4-D, developed during the Second World War, is one of the oldest herbicides still in common use. While some resistant weed populations have developed, they have so far remained small and isolated. There are 17, 2,4-D-resistant weed populations in the world, including an isolated population of wild mustard discovered in Manitoba in the early 1990s.

Earlier this year, AAFC confirmed glyphosate-resistant kochia in Alberta. Ontario also has populations of glyphosate-resistant Canada fleabane and giant ragweed.

Dow AgroSciences and Monsanto are expected to release glyphosate-2,4-D- and glyphosate-dicamba-tolerant crops by 2014. Dow’s program is called the Enlist Weed Control System, while Monsanto’s is named Roundup Ready 2 Xtend.

Officials from both companies said in separate interviews they’ll stress good stewardship to avoid creating new herbicide-resistant weeds.

“We really want to encourage mixing up modes of actions and good rotation practices,” John Foran, Dow AgroSciences Canada’s, market development specialist for Enlist Weed Control System said in an interview. “In Canada it won’t be mandatory, but we will recommend a soil-applied, foundation treatment (along with applications of glyphosate and 2,4-D).”

That additional herbicide application could be done at a reduced rate to make it more affordable for farmers, he added.


But some agronomists doubt farmers will be able to avoid the temptation of simple, inexpensive weed control. For example, the resistant waterhemp population in Nebraska was treated with 2,4-D for 10 years consecutively.

As well, rotating away from those two products is easier said than done. Cereal crops such as wheat and oats often follow soybeans. Group 4 herbicides are commonly applied in those crops. After soybeans farmers shouldn’t plant canola or sunflowers — crops that 2,4-D isn’t used on — because of the disease risk.

Edible beans should be avoided too because soybean volunteers can reduce edible bean quality.

The agriculture industry has learned important lessons about herbicide resistance weeds from the experience with Roundup Ready crops, said Mark Lawton, Monsanto’s technology lead for Canada.

“Where we’ve had glyphosate weed resistance occur there was probably an overly enthusiastic use of the technology,” he said.

In some instances farmers grew nothing but Roundup Ready soybeans or cotton on the same land many years in a row.

“It was effective and was at a good price point, but too much of a good thing turned out to be a problem,” he said. “The world of biology and nature kind of reminds you of that.”

He stressed the importance of rotating crops, herbicide groups and good agronomic practices such as proper timing for applications.

Applying a tank mix of more than one herbicide group is also a good way to delay weed resistance, Beckie said. It’s more effective than rotating herbicide groups.

“I encourage growers to plan out their rotations in advance,” Beckie said. “Don’t plan year to year what crop to grow. If they can, think ahead and try to avoid using the same herbicide resistant crops year after year. Again, if you rely too heavily on one technology you’re probably going to run into a problem. So moderation, I guess, would be my advice.”

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