Biology Reference
In-Depth Information
culmination of numerous decisions over a long time period. Highly simplified crop-
ping systems with heavy dependence on herbicide tools are susceptible to the selec-
tion of herbicide-resistant weed biotypes (described later).
8.8 Herbicide Resistance
8.8.1 Abundance
The relatively recent phenomenon of herbicide resistance can potentially influence
weed invasions in agroecosystems
.
Herbicide-resistant weed populations are
found in over 10% of cultivated land (ca. 5 million ha) in the Canadian Prairies
(Beckie et al. 2008). Incidence and economic impact of herbicide resistance are
much greater in grass than in broadleaf weed species. Wild oat resistant to acetyl-
coA carboxylase (ACCase) or acetolactate synthase (ALS) inhibitors account for
80% of these infestations. Green foxtail resistant to ACCase inhibitors is common
in the central and eastern Prairies. Cases of ALS inhibitor resistance in a number
of broadleaf weed species, such as wild mustard (
Sinapis arvensis
L.), stinkweed,
chickweed [
Stellaria media
(L.) Vill.], and spiny annual sowthistle [
Sonchus asper
(L.) Hill], are less common than the two grass weeds but are increasing steadily
(Beckie et al. 2008).
Because green foxtail and wild oat rank first and second, respectively, in relative
abundance among weed species in the Prairies (Leeson et al. 2005b), herbicides are
frequently used to control them. Since 1996, ACCase inhibitors have been applied
to nearly 50% of cropped land annually, resulting in high and sustained selection
pressure (Beckie et al. 2008); ALS inhibitors are routinely applied to one-third or
more of cropped fields (Leeson et al. 2005a). Where the mechanism of resistance
has been ascertained, most cases of ACCase inhibitor resistance in grass weed spe-
cies and ALS inhibitor resistance in grass or broadleaf weed species in the Prairies
are due to target-site mutation selected by herbicides applied at rates giving high
efficacy (>90% control).
As in other species, ALS inhibitor resistance in kochia is due to target-site resist-
ance; some plants can contain multiple target-site mutations (Warwick et al. 2008).
In contrast to resistant biotypes of wild oat (Beckie et al. 2005) or other species, the
incidence or spread of ALS inhibitor-resistant kochia is increasing the fastest.
8.8.2
Has Herbicide Resistance in Kochia Accelerated
Its Invasiveness? A Case Study
Kochia, an invasive alien, is highly variable in morphology, growth, and develop-
ment as influenced by environment (Bell et al. 1972; Eberlein and Fore 1984). The
biological characteristics of kochia that enhance its spread or invasive capability
