Agriculture Reference
In-Depth Information
et al. (2006b), trifl uralin was shown to dissipate
faster and provide less effective weed control
under low disturbance tillage systems.
sustainability and profi tability of cropping systems
if new herbicide modes of action were introduced
to replace those failing herbicides. However, the
rate of introduction of new herbicides for world
agriculture has slowed dramatically due to (i) the
diffi culty in discovering new herbicide modes of
action with the necessary environmental proper-
ties and (ii) the strategic and substantial reduction
in herbicide discovery programs by international
agrichemical corporations (Ruegg et al., 2007). In
2005, only 11 companies invested signifi cantly in
crop protection research and development, com-
pared with 35 companies in 1985.
A substantial increase in the cost of herbicide
development, which is partly related to tightening
regulatory systems, has negatively infl uenced
“risk-taking” behavior of the industry. For
example, preapproval costs related to toxicology
and studies on pesticide fate in the environment
have increased from US$28M during 1985-1990
to US$100M during 1990-1996 (Ruegg et al.,
2007). Both these factors have contributed to the
slowdown in discovery and release of new chem-
istries for agriculture. As it is very likely that the
pace of introduction of new herbicide modes of
action capable of controlling herbicide-resistant
weeds will remain slow, there is a strong impera-
tive to use the currently available herbicide
resources in more sustainable ways. Recognition
is growing worldwide that weed management
must be built on a solid foundation of good crop
husbandry. An integrated approach that incorpo-
rates a wide range of weed control methods is
required (Walsh and Powles 2004).
A factor critical to long-term weed manage-
ment is the principle of avoiding heavy reliance
on single control methods such as one highly
effective herbicide. To enable farmers to use
diverse modes of actions and herbicide mixtures,
the pesticide industry adopted a uniform system
of herbicide classifi cation based on the mecha-
nism of herbicide action in the plant (Table 12.6).
Recent simulation modeling research has shown
that herbicide rotation in alternate years is
markedly less effective as a strategy to delay resis-
tance than the use of herbicide mixtures where
both components are used at full rates (Diggle
et al., 2003). However, herbicide combination or
Chemical control
From the beginning of agriculture, farmers have
been controlling weeds by different methods with
varying levels of success. The advent of herbi-
cides has been hailed as one of the most important
advances in agriculture, and herbicides now typi-
cally comprise 20% to 30% of input costs in
North American cropping systems (Derksen
et al., 2002). Herbicides also hold top position
among crop protection chemicals in Australia,
where farmers in all cropping regions spend typi-
cally AUS$30-80 ha
−1
on herbicides for weed
control (Pratley 1996). In Australia the combina-
tion of large cropping areas, expensive labor,
short growing seasons, fragile soils, and the
imperative to minimize production costs have all
contributed toward herbicides remaining a vital
component of cropping systems (Walsh and
Powles 2004). In most grain cropping systems in
the world, herbicides are the dominant method of
weed control.
Despite widespread adoption of herbicides, the
reduced use of herbicides is attracting ever-
increasing interest. Farmers cite crop injury and
herbicide carryover, increased incidence of herbi-
cide-resistant weeds, and public concerns about
the environmental and human health effects of
pesticides as issues forcing reassessment of the
practice of weed management (Blackshaw 2006).
According to Heap (2007), 314 unique herbicide-
resistant weed biotypes belong to 183 species
around the world. For a long time, prospects of
evolution of resistance in weed populations to
glyphosate were considered remote. However,
resistance to glyphosate has now been confi rmed
in 11 weed species worldwide (Heap 2007). With
no doubt the problem of herbicide resistance in
weeds is truly global, is relevant to all cropping
systems, including wheat-based systems, and is
still expanding rapidly.
The widespread evolution of herbicide-resistant
weed populations within intensive crop produc-
tion systems would not be a major threat to the
