Approaching the silver anniversary of weed
resistance in Manitoba We are approaching the silver anniversary (2013) of weed resistance in
Manitoba. Group 3-resistant green foxtail (Setaria viridis) and
Group 2-resistant kochia (Kochia scoparia) were both confirmed
in 1988. The most recent weed resistance survey conducted in Manitoba
in 2008 (300 randomly-selected fields) indicated that 58% of annually-cropped
land (5.41 million acres) was infested with a herbicide-resistant weed
population (in millions of acres): Group 1-resistant wild oat, 2.33; Group
2-resistant wild oat, 0.31; Group 1+2-resistant wild oat, 0.53, Group
8-resistant wild oat, 0.16; Group 1+8, 2+8, or 1+2+8-resistant wild oat,
0.50; Group 1-resistant green foxtail, 1.24, and Group 2-resistant broadleafs,
0.34. Therefore, Group 1 resistance in wild oat and green foxtail is widespread,
at 55 and 44% of fields, respectively. With patents expiring on various
Group 1 products, their increased usage will only be tempered by increasing
incidence of resistance. Growers usually control such resistant grass
weed populations with a Group 2 product, where the incidence of resistance
is relatively low (12% of fields for wild oat). Survey results indicate
that multiple-group resistance in wild oat typically reduces crop yields
by 10 to 20%. With limited in-crop herbicide options in wheat, the outlook
for increased yields in that crop is not good. However, growers are managing
grass weed resistance as best they can by rotating to crops such as canola
(with different mode-of-action herbicides) or inclusion of forages, and
utilizing various non-herbicide weed management practices.
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