Biology Reference
In-Depth Information
others, led to dramatic improvements in weed control. Today in western Canada, there
are 30 unique herbicide active ingredients registered for use in wheat (
Triticum
aestivum
L.) alone (Brook 2007). Over the last few decades, there is strong evidence
to suggest that weed management practices employed on the Prairies have substan-
tially reduced weed populations. Post-management weed surveys initiated in the 1970s
indicated that by the early 2000s, the number of weed species detected in average field
samples had been reduced from 7 to 5, while mean plant density had been reduced
from 100 to 31 plants m
−2
(Leeson et al. 2005b). The remainder includes species that
are tolerant of our predominant management methods, and therefore pose a significant
challenge to management systems that have not changed substantially in 50 years.
Unfortunately, our first and often only response to weed infestations is to treat
them with herbicides. We have seldom examined the causes of the perpetual pres-
ence of weeds (Buhler 1999). Herbicidal successes have led to overuse and the
neglect of many useful cultural weed management techniques. With herbicides as
the dominant weed management tool, resistance is now common in some of our
major weeds, such as wild oat (
Avena fatua
L.) (Beckie et al. 1999) and kochia
[
Kochia scoparia
(L.) Schrad.] (Beckie et al. 2001). The combination of weed
resistance to herbicides, herbicide costs, and concerns regarding the non-target
effects of herbicides has led to a resurgence of interest in non-chemical weed con-
trol methods, or at least more integrated crop management (ICM) systems. Such
systems, including some of their components, are detailed later.
8.3
The Crop-Weed Association
The high level of land
disturbance
that accompanies crop production facilitates the
success of weed communities. Froud-Williams (1988) states that: 'Arable land is
characterized by regular, recurrent, and often highly predictable disturbance. The
consequence of this disturbance is that weeds of cultivated land represent the most
ephemeral of plant communities, completing their life cycles within a relatively short
time and producing copious quantities of dormant seed of potentially long life span.'
Indeed, the three habitat traits that favour weed invasion in natural environments are
almost always present in modern agroecosystems; they are as follows: (1) disturbance
(tillage, seeding and harvest operations, grazing), (2) low species richness (crop
monocultures), and (3) high resource availability (bare soil/crop stubble, sunlight,
fertilizer) (Booth et al. 2003). Rapid response to high resource availability is a physi-
ological attribute of most early succession plants, especially weeds (Bazzaz 1979).
8.4 Tillage Intensity
Since the beginning of the twentieth century, the widespread use of tillage in agri-
cultural systems has selected for weeds that thrive in tilled soils (Mohler 2001).
Many weeds have seeds that are relatively small; the fitness of such weeds is
