Weed Invasions in Western Canada Cropping Systems - Management of Invasive Weeds - page 148

expenditure of billions of dollars in chemical and biological control measures (Pimentel
et al. 2000; Swanton et al. 1993). Besides direct economic costs, invasive plant species
can reduce the stability of ecosystems and threaten biodiversity (White et al. 1993).
Globally, most emphasis on invasive plants has been directed towards species
that impact relatively undisturbed ecosystems. The species considered as threats to
natural Canadian Prairie (western provinces of Alberta, Saskatchewan, Manitoba)
ecosystems are leafy spurge ( Euphorbia esula L.), crested wheatgrass [ Agropyron
cristatum (L.) Gaertn.], and smooth brome ( Bromus inermis Leyss.) (Haber 2002).
All three species are minor, however, when their relative abundance in Prairie crop-
ping systems is considered; the most abundant of these species, smooth brome,
ranks 84th in relative abundance (Thomas and Leeson 2007).
Prior to introduction, most of our agricultural weeds had long been associated
with European crops that are now widely grown in Canada (Clements et al. 2004).
Thus, some considered these weeds to be non-invasive (Thomas and Leeson 2007).
However, according to Pyšek et al. (2004), naturalized (sustain self-replacing popu-
lations for at least 10 years) alien species that spread are invasive. Invasive plants
are naturalized non-native plants (aliens) that, without human assistance, can
spread over large areas and replace native plants (Pyšek et al. 2004). Weeds are
plants that grow where they are not wanted; because they can be self-sustaining
without or in spite of direct human intervention, they have the potential to spread
over large areas and thus can also be classified as invasive (Pyšek et al. 2004). The
interpretation that agricultural ecosystem weeds can be invasive species is consist-
ent with definitions from a 1999 United States Presidential Executive Order
(13112) and the Invasive Species Advisory Council (DiTomaso 2008).
Eighty percent of agricultural weeds in Canada are classified as aliens
(Government of Canada 2004). These weeds annually invade agricultural land and
challenge farmers, extension personnel, and researchers in all major agricultural
cropping regions. Of the 36 weed species considered to be abundant on the
Canadian Prairies, 89% are alien species (Thomas and Leeson 2007). Therefore,
the majority of Prairie crop yield losses due to weed competition are caused by
alien species. Moreover, 99% of herbicide expenditures in western Canada are used
to control alien species (Leeson et al. 2006). Therefore, repeated annual incursions
of native or alien weed cohorts on cropland are invasions that we can ill afford to
ignore (Harker et al. 2005; Thomas and Leeson 2007). Here, we review factors such
as tillage intensity, cropping diversity, and herbicide resistance that influence
repeated annual weed invasions in Canadian Prairie cropping systems. Additionally,
mitigation strategies and tactics to manage such invasions are outlined. All of the
weeds discussed later are invasive aliens of cultivated cropping systems.
8.2 HistoricalPerspective
Before herbicides became available, farmers employed cultural practices such as crop
rotation, delayed seeding, green manure crops, and tillage to manage weeds. The intro-
duction of the first selective herbicide, 2,4-D, in 1946, which was soon followed by