ROCKVILLE, Md. (DTN) -- Just like most families, herbicides don't always bring out the best in each other.

In the human world, it's called "sibling rivalry" or "personality differences." In the chemical world, scientists call it "herbicide antagonism," a phenomenon wherein two or more herbicides in a tank mix produce poorer weed control than the individual herbicide components would supply alone.

The primary victim of herbicide antagonism tends to be glyphosate, still a staple of many farmers' tank mixes. Clethodim (Select) is another herbicide often on the losing end of this phenomenon.

And the chief antagonists? Well, like most feuds, it can vary depending on factors such as environment and targeted weed species. But recently, auxin herbicides such as 2,4-D and dicamba are showing their antagonistic side. Contact herbicides, such as glufosinate, can cause problems, too.

The proliferation of dicamba and 2,4-D-tolerant crops on the landscape in recent years is bringing new attention to the phenomenon, said Larry Steckel, University of Tennessee Extension weed scientist. In Tennessee, for example, dicamba-tolerant Xtend soybeans account for more than 95% of the state's soybean acreage.

"Herbicide antagonism is not new," he explained. "But what is new is that we're mixing so much dicamba and glyphosate together and using it in season now on bigger weeds. Dicamba is hindering the grass control of glyphosate, and if you put another grass herbicide in like clethodim (Select), it's antagonizing that as well."

AUXINS DON'T PLAY NICE

Herbicides have different ways of killing weeds. Glyphosate, for example, depends on the plant moving the chemical quickly down to its growing point, a process called translocation. For translocation of glyphosate to happen, a plant must be actively growing, explained Bryan Young, a Purdue University weed scientist.

"Glyphosate needs a living plant and roughly three days of active growth for absorption and translocation to occur," he said. The chemical has a longer trip in grassy weeds than broadleaves, since the growing point of a grass is generally found at the base of the plant, he added.

Auxin herbicides are plant-growth regulators, which means they tinker with a plant's growth and development, including its photosynthetic process, Young said.

"When an auxin herbicide is applied, one of the first things that we see is the downregulation of photosynthesis within two to four hours of application," he noted. This may be part of the reason dicamba and 2,4-D hinder the full potency of a glyphosate application, although scientists aren't sure of the exact mechanism yet.

"Somehow it's muting the glyphosate," said Steckel. "So antagonism is a big issue with some species like junglerice, barnyardgrass, fall panicum and goosegrass. They're surviving even multiple applications of dicamba and glyphosate -- but if you separate them out, the glyphosate will still kill them."

Another common grass herbicide, clethodim (Select), faces the same problem with auxin antagonism.

As a Group 1 (ACCase) herbicide, clethodim also requires rapid translocation, and both 2,4-D and dicamba may indirectly interrupt that process. Upping the rate of clethodim and using an oil-based adjuvant to favor greater absorption of the grass herbicide into the plant can overcome the problem sometimes, Young said. Steckel is recommending growers simply separate the auxin herbicides from their grass herbicides in the tank.

"So far, splitting up applications is the only way to make glyphosate and clethodim work reliably here," he said. "I have had some farmers even come back and remark that they had no idea glyphosate still worked that well on their grasses after they split out the application."

CONTACT BURNS ARE A PROBLEM TOO

Contact herbicides are another common antagonistic culprit, the weed scientists added. Here, the mechanism is better understood.

"If you apply a contact herbicide like glufosinate along with glyphosate, it might start to reduce the amount of [glyphosate] translocation because the contact herbicide will start to kill the leaves, which are the primary source for pumping glyphosate down to the meristem," Young explained.

Applying glyphosate with a contact herbicide in cooler conditions (when translocation is slower), or without optimal adjuvants like ammonium sulfate (AMS) can increase antagonism, he added.

POTENTIAL HERBICIDE FEUDS AHEAD

With more stacked herbicide-tolerant traits in the future, growers will have to become better acquainted with these inner feuds of the chemical world.

For example, the XtendFlex soybean, expected to be commercialized early this decade, will be tolerant to over-the-top applications of three herbicides -- glyphosate, dicamba and glufosinate. While the chemicals might work individually, between differing droplet size requirements, ammonium salt components and antagonism, they're unlikely to play well together in a tank, Young said.

Scientists are racing to figure out the nuances of herbicide antagonism in time to giver growers the right recommendations in the years ahead, Steckel said.

"Growers will need research and fine-tuning to know how to handle these applications," he said. "Which herbicide needs to be applied first? How much time is needed between applications? And we are doing that research."

This year's weed escapes in the field may be growers' primary frustration with antagonism, but Young is more worried about the future consequences of weak weed control on the same grass species year after year.

"The greater long-term problem is the potential that repeated herbicide failure from antagonistic herbicide interactions could progressively lead towards the evolution of herbicide resistance," he warned.

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee

(PS/AG)

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