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Montana State University Communications Services

"Make it Count" on Kochia; Herbicide Resistance Threatens

04/23/98 BOZEMAN -- Innumerable old movies gave the same advice: when you're getting low on ammunition, you have to "make it count."

One weed where the ammo is running low is kochia, says Bill Dyer, a researcher in Montana State University's Department of Plant Sciences. Most kochia in Montana already is resistant to sulfonylurea herbicides such as Glean, Ally, Amber, Express, Finesse, Harmony Extra and Peak.

So we have to be very smart about herbicide use or we'll be "out of ammo" entirely.

Dyer and other MSU weed scientists are studying kochia plants from a few fields near Fort Benton and Miles City that are no longer controlled by label rates of Banvel herbicide.

To date, Banvel-resistant kochia has been detected in only a few fields and is present at very low levels, says Dyer.

Rosebud County Weed Supervisor Vince Thomas says he has noticed a problem with kochia in the Howard Flats area, along county roads and railroad rights of way. He is working with a representative of the company that produces Banvel to ensure that the problem remains limited.

Controlling weeds, especially kochia, by crop and chemical rotation is urgent, says Dyer.

"It is extremely important to prevent the spread of Banvel-resistant kochia now before it becomes widespread," he added. And there is still time. Banvel is still an economical and effective herbicide for managing kochia and other broadleaf weeds in Montana. Dyer estimates that certain fields may have only five of 100 kochia plants that now are resistant to Banvel.

Herbicide resistance is thought to start from one or a few weeds present in the field before spraying that already are resistant to a herbicide. Because there are so few, they usually aren't noticed during the first years a herbicide is used, says Dyer. However, since resistant weeds are the only ones left growing after a herbicide treatment, they propagate and their population grows. Each succeeding year a grower uses a herbicide that kills weeds the same way, the more resistant weeds grow in the field.

To prevent spread of Banvel-resistant kochia, farmers should follow label directions and use crop and herbicide rotations and cultural control practices. By herbicide rotations, Dyer says he means herbicides that use different ways of killing weeds.

Russell Davis, a herbicide applicator for Mountain View Co-op in Power, says such crop and chemical rotations seem to be working on wild oats that had become resistant to Far-Go.

"We're getting rid of the problem by using chemicals with different modes of action and with crop rotations," says Davis.

Dyer adds that if this spring is as dry as some forecasters predict, it will be critical to treat kochia at the earliest recommended growth stage with full rates and the proper spray volumes. (See also a longer article on Herbicide Resistant Weeds.)

Send questions or comments to Dyer and Carol Flaherty, MSU Communications Services, Bozeman, MT 59717 or to Dyer and Flaherty with this link:

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