Tests by Ohio State and private industry show that common ragweed and giant ragweed are developing resistance to ALS (acetolactate synthase) herbicides

Columbus- Tests by Ohio State and private industry show that common ragweed and giant ragweed are developing resistance to ALS (acetolactate synthase) herbicides.

ALS products include Classic, Canopy, Canopy XL, Synchrony STS, Exceed, Spirit, Beacon, Scepter, Squadron, Detail, Pursuit, Raptor, Lightning and FirstRate.

"It's not something to be alarmist about, but farmers should keep it in mind if they're starting to have control problems," said weed scientist Stachler.

In all but one case, weed samples in OSU's test came from fields planted in soybeans for two to three years, and were treated every year with ALS herbicides, Stachler said.

Weed samples were from all parts of the state, but especially from western areas.

OSU also found weeds were resistant to the three classes of ALS products, so switching to another ALS product isn't likely to guarantee control.

Stachler said the problem has resulted from repeated consecutive use of ALS products in fields planted with only soybeans in consecutive years. The herbicides killed non-resistant weeds, while weeds with some resistance gained ground. The problem could have been lessened with crop rotation and varied herbicides.

ALS herbicides act by interfering with particular enzyme in the weed and disrupting biological processes necesarry for its survival.

"The weeds we tested are resistant because the enzyme keeps functioning even though we put herbicide on it" Stachler said.

When ALS herbicides came to the market, farmers quickly adopted them because they controlled ragweed better with less crop injury than did precursors. Also, they are applied pre-emergence or postemergence, while the older herbicides are only applied postemergence.

Postemergence, non-ALS treatments include Flexstar and Cobra, as well as Roundup Ultra for Roundup Ready soybeans. Apply before common ragweed is 4-6 inches tall, and before giant ragweed is 8-12 inches tall, although Roundup Ultra can be applied on taller weeds using higher, labeled rates.

Report resistance problems to Stachler at 614-292-1393, or by e-mail at: stachler.1@osu.edu. Study the topic by consulting OSU Extension Bulletin 789, "Weed Control Guide for Ohio Field Crops," available at Extension offices or on line.

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