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Troubling Tumbleweed
Monday, October 15, 2018 6:08AM CDT
By Emily Unglesbee
DTN Staff Reporter

ROCKVILLE, Md. (DTN) -- The tumbling tumbleweed just got a little bit harder to control.

Weed scientists at Kansas State University have confirmed populations of kochia in western Kansas that show cross resistance to both dicamba and fluroxypyr (Starane Ultra).

Dicamba resistance in kochia isn't new -- it was first documented in Montana in the mid-90s, and isolated populations have since been found in other Great Plains states like Nebraska, Colorado, Idaho and North Dakota, according to KSU weed scientist Vipan Kumar. Glyphosate resistance is also common in this troublesome annual weed.

"But this cross resistance is new for Kansas because fluroxypyr was working pretty well here and has been used a lot, especially in wheat, to take care of kochia," said Kumar, who led the KSU research. Fluroxypyr is a Group 4 herbicide that is a common ingredient in many herbicide premixes in wheat and corn.

The discovery could seriously complicate kochia control, especially if dicamba use continues to grow with the use of Xtend crops, Kumar said.

The good news is that the resistant weeds, which were identified near Garden City and Hays, Kansas, could have a "fitness cost," since that has been the case with other dicamba and fluroxypyr-resistant kochia populations identified in other states in the past, Kumar said.

That means the weeds could produce less seed and grow more slowly and eventually be out-competed by other susceptible weeds -- but only if dicamba and fluroxypyr aren't used on them for several generations.

That's unlikely, Kumar said. Since glyphosate resistance was identified in kochia in Kansas 11 years ago, dicamba and fluroxypyr use in the state has risen dramatically.

"Growers have relied heavily on dicamba here," he said. "I believe there will be a shift from dicamba and fluroxypyr-susceptible weeds to more tolerant ones in western Kansas, especially in wheat-based fields," he said.

Wheat fallow fields commonly get three to four applications of glyphosate with dicamba or 2,4-D added in, he said. Dicamba and fluroxypyr are also used in both pre and postemergence applications for kochia control in wheat and corn.

The growth of dicamba-tolerant Xtend crop acreage in Kansas and other states will increase dicamba use even further, Kumar said. In 2018, Kansas farmers grew Xtend soybeans on about half of the state's nearly 5 million soybean acres, as well as some Xtend cotton acres, Kumar said.

"While the soybeans are mostly limited to the eastern half of the state, dicamba-tolerant cotton will be grown in the southwestern counties of the state that struggle with kochia," added Kansas State University weed scientist Randall Currie, who first identified some of the cross-resistant kochia populations for Kumar to test.

"The only way we have a chance of controlling kochia in cotton is with dicamba or 2,4-D, but I don't know how long we can pull that off now," he said.

Farmers in Kansas and beyond should also be on alert for dicamba-resistant Palmer amaranth, given the rise in dicamba use in Xtend crops, Currie added.

In the meantime, Kumar will be doing a survey of western Kansas kochia, to see how prevalent the dicamba and fluroxypyr cross resistance is.

"We are not sure about the mechanism behind this resistance," he added. "That is another thing we need to find out."

You can find more details on the discovery from Kansas State here:

https://webapp.agron.ksu.edu/…

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

Follow her on Twitter @Emily_Unglesbee

(PS/BAS)


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