Weed management: a need for ecological approaches - Ecological Management of Agricultural Weeds - page 5

mixtures of different crops can also be used to preempt resources from weeds
(see Chapter 7).Allelochemicals released from live crops and crop residues (see
Chapters 5, 6, and 7), biological control agents (see Chapter 8), grazing live-
stock (see Chapter 9), and herbicides may be used to damage weeds and
improve crop performance.
Desirable shifts in weed species composition can be promoted by tillage
practices (see Chapter 4), grazing practices (see Chapter 9), and manipulations
of soil conditions (see Chapter 5) and crop canopy characteristics (see Chapters
6 and 7).Selective herbicides can also be applied to alter weed species composi-
tion.
Currently, herbicides are the primary method for managing weeds in
industrialized countries and are becoming more widely used in developing
countries. Although we do not believe that they should be excluded from the
weed management tool kit, we have given them relatively little attention in
this topic.There are four reasons for our orientation.
First, a large amount of information about herbicides and their effects on
weeds and crops already exists, whereas much less information is available
about other management tactics.We hope this topic contributes to the closure
of that information gap. Second, we believe that, over time, heavy reliance on
herbicides reduces their efficacy by selecting for resistant or tolerant weed
species and genotypes. To maintain the effectiveness of herbicides as weed
management tools, weeds should be exposed to them as infrequently as pos-
sible. Third, we believe that certain herbicides can jeopardize environmental
quality and human health. To minimize the potential for damage, effective
weed management systems that are less reliant on herbicides are needed.
Finally, herbicides constitute a rising proportion of crop value at a time when
farmers are challenged by serious economic pressures. To promote farm
profitability, there is an important need to develop effective weed manage-
ment strategies that maximize opportunities for farmers to reduce input costs
and increase the value of the crop and livestock products they sell.
We examine these points in more detail in the following sections.
Herbicide sales and use
Herbicides dominate the world market for pesticides and pervade the
production of staple crops.Worldwide in 1997,$16.9 billion was spent for 1.0
billion kg of herbicide active ingredients, compared with $11.6 billion for 0.7
billion kg of insecticides and $6.0 billion for 0.2 billion kg of fungicides
(Aspelin & Grube, 1999). Global herbicide sales are greatest for materials used
for maize, soybean, wheat, and rice (Figure 1.2).