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 at www.ag.ohio-state.edu/online/b789/index.html.
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