Agriculture Reference
In-Depth Information
extensionists to take a long view of weed problems, and to suggest to govern-
ment and the other segments of the agricultural community general strate-
gies for their solution. In particular, researchers need (i) to construct theories
and models that predict how and why new weed problems develop, (ii) to
monitor the introduction of new crops, technologies, and cropping systems
for the emergence of novel weed problems, and (iii) to consider how regional
problems can be managed before rather than after they have a major economic
impact. The public health style approach suggested here for the control of
new weed problems is a substantial extension of conventional weed science.It
nevertheless represents only a small first step toward long-term management
of the slow but persistent increase in local,regional,and global weed diversity.
Conclusions
As argued in the preceding sections, agricultural ecosystems continu-
ously generate new weed problems. Recent advances in control measures
appear to have reduced losses due to weeds relative to historical levels
(Pimentel et al ., 1978). However, economic losses from weeds continue to be
large (Bridges, 1992). Continuous advances in both weed biology and weed
control technology may be necessary just to maintain the current levels of loss
in the face of the continuing spread of introduced species,shifts in the compo-
sition of weed communities, and the evolution of resistance to herbicides and
other management practices.
Two overall strategies for management of changes in weed communities
are apparent.One is to continue the present essentially responsive approach in
which shifts in weed composition and development of herbicide resistance are
attacked with newly developed herbicides and complex mixtures of existing
materials. This approach guarantees a continuing market for new chemical
technologies, but leaves the grower with a generally increasing bill for weed
control.
The alternative is to take a more methodical approach in which principles
are elucidated that predict the response of weeds to control measures, and
strategies are developed to intercept problems before they become severe.The
growing interest among weed scientists in modeling the dynamics and
genetic composition of weed populations is a first step in implementing this
alternative approach to the management of incipient weed problems.
However,new categories of higher-level models are needed to understand and
predict phenomena like species shifts, the spread of weeds within and
between regions,and the evolution of herbicide resistance in taxa that are cur-
rently susceptible. Such phenomena occur at spatial and temporal scales that
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