Diversifying crop rotations with temporary grasslands - Université de ...

term considerations (Munier-Jolain et al., 2002). In reality, farmers have to control weeds not
only to avoid yield losses in the current crop, but also to limit weed seed production for
reducing weed infestations in future crops.
A.II.2 Weed control & environment
Since about 1950, herbicides have become the main technique of weed control in arable field
crops (cereals, rape, beets, maize…) of industrialized cropping systems largely replacing other
techniques and principles such as cultural and mechanical control (see Ch. A.III.2 below).
Herbicides may be applied rather easily on large surfaces and are generally very efficient in
killing the plants of most species. Therefore, herbicides may contribute to the maintenance of
crop yields and to the economic profitability of the farms. The development of different
herbicides was one important factor that enabled the simplification of cropping systems
including shorter crop rotations and monocultures (and, more recently, no-till practices).
However, the reliance on herbicides may have several agronomic, economic and
environmental drawbacks that will be summarized in the following.
Efficiency and selectivity
Despite the intensive use of herbicides and other curative weed control techniques for many
decades, the ‘weed problem’ could not be solved. Weeds did not disappear from arable fields,
which was a widespread hope when herbicides became available. Intensified curative weed
control as well as changed agronomic practices (including simplified crop rotations dominated
by annual winter-sown crops) rather lead to reduced weed species numbers and community
shifts. While many species showed strong population declines or even got extinct from entire
regions (see Ch. A.II.3), some other less sensitive species showed increasing abundances and
distributions. In some cases, the repeated use of herbicides with the same mode of action
during many consecutive years selected herbicide resistant biotypes of weed species that are
normally killed. Resistances are observed for an increasing number of herbicide molecules
and weed species worldwide, but particularly in industrialized countries (as documented by
Heap, 2009). In some situations, herbicides may also damage the crops and reduce crop yields
which may lower the economic profitability. These problems of herbicide efficiency and
selectivity show that weed management should not be based on one single principle.
Economic costs
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