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prickly lettuce (Lactuca serriola L.), and Russian-thistle (Salsola tragus L.) bio-
types resistant to chlorsulfuron were reported from the U.S. Great Plains and the
Western U.S.A. (Heap 2016 ). Chlorsulfuron inhibits acetolactase synthase (ALS)
and this site of action is common for many of the herbicides used in wheat produc-
tion in North America. Consequently, weed biotypes resistant to this site of action
are plentiful throughout the Western U.S.A. and Prairie Provinces of Canada (Table
1 ).
As of 2016, there were 72 resistant weed biotypes reported in wheat from the
western U.S.A. and Canada (Heap 2016 ). Amongst these biotypes, resistance to
seven different sites of action were reported (Table 1 ). Eight of the biotypes were
resistant to two or more different sites action, i.e., they exhibited multiple resis-
tance. Varanasi et al. ( 2015 ) confirmed the first case of resistance to four herbicide
sites of action (ALS inhibitors, synthetic auxins, photosystem II inhibitors, and
EPSP synthase inhibitors) in a single kochia population from western Kansas.
Multiple resistance in weeds is of growing concern to weed scientists and growers.
No-till production systems in North America are heavily reliant on herbicides
and no herbicide has played a larger role in no-till production than glyphosate.
Monsanto’s patent for glyphosate ended in 2000 and generic glyphosate quickly
followed. Generic glyphosate played a major role in the adoption of no-till practices
throughout the dryland farming regions of North America because it was cheap and
effective, particularly at higher use rates that were unaffordable for many prior to
generic glyphosate. However, after more than a decade of reliance on this single
active ingredient, weed resistance to glyphosate is threatening to stall, and possibly
reverse, the gains in no-till management adoption (CAST 2012 ). Glyphosate-
resistant kochia was first documented in several fallow fields in Kansas in 2007
(Waite et al. 2013 ). By 2014, more than five million hectares from Texas into the
Prairie Provinces of Canada were infested with glyphosate-resistant kochia
(Stahlman, personal communication). In 2015, glyphosate-resistant Russian-thistle
was documented in Montana and in 2016 another population was identified in
Washington (Heap 2016 ). Kochia and Russian-thistle are troublesome weeds in dry-
land agriculture and both species have a history of developing herbicide-resistant
biotypes (Table 1 ).
2.2 Integrated Weed Management concepts
In order for dryland agriculture to remain economically and environmentally sus-
tainable, production systems will need to rely less on herbicides and more on an
integrated approach to weed management. Integrated weed management (IWM)
involves the sustainable use of all available methods to reduce weed pressure,
including sanitation, mechanical, cultural, chemical, and biological methods. The
three general principles of integrated weed management are: (1) use agronomic
practices that limit the introduction and spread of weeds, (2) help the crop compete
with weeds, and (3) use a diverse set of practices, altered over time, to prevent weed
N.C. Hansen et al.