Weed management: a need for ecological approaches - Ecological Management of Agricultural Weeds - page 2

of pests (Andow, 1988; Nentwig, Frank & Lethmayer, 1998). Weed species can
reduce soil erosion (Weil,1982),serve as important sources of fodder and med-
icine (Datta & Banerjee, 1979; Chacon & Gliessman, 1982), and provide
habitat for game birds and other desirable wildlife species (Sotherton, Rands
& Moreby,1985; Sotherton,Boatman & Rands,1989).These types of beneficial
effects indicate that weeds are not just agricultural pests, but can also play
beneficial roles in agroecosystems.
In this chapter,we outline the objectives of weed management systems and
then discuss how weeds are managed conventionally.We follow with a discus-
sion of why alternatives to conventional management strategies are needed.
Finally, we suggest how a broad range of ecological processes and farming
practices might be exploited to manage weeds more effectively, while better
protecting human health and environmental quality, and potentially increas-
ing farm profitability. In subsequent chapters, we will examine these ecologi-
cal processes and farming practices in more detail.
Weed management objectives
From the standpoint of crop protection, weed management has three
principal objectives:
(1) Weed density should be reduced to tolerable levels. Experimental studies with a
range ofspecies indicate that the relationship between crop yield loss
and weed density can be described by a rectangular hyperbola (Cousens,
1985; Weaver,Smits & Tan,1987; Norris,1992; Blackshaw,1993;
Knezevic,Weise & Swanton,1994; Chikoye,Weise & Swanton,1995).The
specific parameters ofthis relationship change with differences in
weather and soil conditions,species combinations,and other factors
(Mortensen & Coble,1989; Bauer et al .,1991; Lindquist et al .,1996),but,
in general,reductions in weed density reduce crop yield loss (Figure
1.1a).Although the relationship shown in Figure 1.1a might argue for
total elimination ofweeds from crops,eradication efforts may be exces-
sively expensive,incur unacceptable environmental damage,and
deprive farmers and others ofthe ecological services certain weeds
provide.Thus,with the exceptions ofparticularly noxious or invasive
species,weed management rather than eradication is desirable.
(2) The amount of damage that a given density of weeds inflicts on an associated crop
should be reduced (Figure 1.1b).The negative effect ofweeds on crops can be
limited not only by reducing weed density,but also by minimizing the
resource consumption,growth,and competitive ability ofeach surviving
weed (Mortensen,Dieleman & Johnson,1998).This can be accomplished