Green Foxtail
Cross-Resistance to ALS Inhibitors
Dean Volenberg*, Dave
Stoltenberg, and Chris
Boerboom
Green foxtail (Setaria viridis)
is one of the most widespread grass weeds, and is found
generally throughout the North Central States. Green foxtail,
which has hairless leaves, can be distinguished easily from
giant foxtail (Setaria faberi), which has the upper
surface of leaves covered with short hairs. In addition, green
foxtail seeds are uniformly smaller than giant foxtail seeds,
but seeds of both species are about the same length (1/16 inch).
Green foxtail with suspected
resistance to acetolactate synthase (ALS) inhibiting herbicides
was identified in Chippewa County, Wisconsin in 1999. The
suspected resistant population was identified in a soybean field
in which Raptor, an ALS inhibitor, had been applied. In 1998,
the field was in corn, but an ALS inhibitor was not applied.
Crop and herbicide history for 1997 and earlier are not known at
this time. To help learn more about this potential problem, we
conducted research to confirm and quantify green foxtail
resistance to the sulfonylurea herbicide Accent, and to the
imidazolinone herbicides Raptor and Pursuit.
In greenhouse experiments, green
foxtail plants with suspected resistance were 23-, 55-, and
>1000-fold resistant to Accent, Raptor, and Pursuit,
respectively, compared to susceptible plants. A preliminary ALS
enzyme assay in the laboratory indicated that resistance of
green foxtail plants was associated with an insensitive ALS
enzyme. These results indicted that green foxtail was highly
resistant to Pursuit, and cross-resistant to imidazolinone and
sulfonylurea herbicides.
Although a detailed field history
is currently not known, other weeds species resistant to ALS
inhibitors have typically developed when ALS inhibitors have
been applied annually for several years, but in some instances,
for as few as three years. The occurrence of green foxtail
resistance has several important implications for management,
especially because this weed can be competitive in both corn and
soybean, as well as other crops. Integrated weed management
practices that include alternative herbicide chemistries,
cultural tactics, and mechanical tactics should be implemented
to manage resistant green foxtail populations, and to delay
development of additional resistance problems.
* Graduate Research Assistant |
