Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 323

opportunities for herbaceous crop production. Agroforestry practices were
probably components of the earliest agricultural systems in the humid tropics
and remain widespread there today (Plucknett & Smith, 1986).
Crop diversity in conventional, traditional, and organic
farming systems
Diversity comprises both the number of species present in a given
area (species richness) and the degree to which each species is equally abun-
dant (evenness) (Pielou, 1977). Large differences in crop species diversity exist
among conventional high-input,traditional, and organic farming systems.As
explained later in this chapter, these differences strongly influence weeds and
weed management.
Over the past half-century,crop diversity has declined precipitously in con-
ventional high-input farming systems used in the USA and other industrial-
ized countries. Large areas are now planted with only one or two annual crop
species. This trend is exemplified by changes in the landscape of Illinois,
where cropland occupies 66% of the state's total land area (United States
Department of Agriculture, 1999).Maize and soybean were planted on 45% of
Illinois' cropland in 1958 (United States Department of Agriculture, 1973),
but on 86% of its cropland in 1997 (United States Department of Agriculture,
1999). Lost from the Illinois landscape during this period were large areas of
pasture, hay, and small-grain cereals (Power & Follett, 1987; Bullock, 1992).
Not surprisingly, crop uniformity across broad landscapes is related to low
diversity in individual fields over time. Clear illustrations of this point are
provided by a 1991 survey of American cropping systems (Economic Research
Service, 1992). In Iowa, where maize was planted on 5.1 million hectares in
1991, maize and soybean were the only two crops planted on 90% of that area
in 1989 and 1990. Similarly, in Kansas, 74% of the 4.8 million hectares sown
with wheat in 1991 was used only for wheat production or fallow in the two
preceding years.Production of the same crop in the same field for at least three
consecutive years was found on 86% of the land used for wheat in Oklahoma,
82% of the land used for cotton in Louisiana, 57% of the land used for soybean
in Mississippi, and 55% of the land used for maize in Nebraska.
Reductions in crop diversity in the USA and other industrialized countries
have been driven by a set of interrelated technical and socioeconomic factors
that have encouraged farmers to seek higher profits through greater econo-
mies of scale, enterprise specialization, and increased substitution of pur-
chased inputs for labor and management time. Cropping systems have
become less diverse as farmers have become more reliant on synthetic