Agriculture Reference
In-Depth Information
7
Crop diversification for weed management
Introduction
One of the defining characteristics of an ecosystem is the diversity of
plant species it contains. In agricultural systems, diversity of the dominant
plant species - crops - can vary in both spatial and temporal dimensions.
Crops can be sown in pure stands (
sole crops
), but can also be sown in multispe-
cies mixtures (
intercrops
or
polycultures
),a practice that probably began with the
development of tropical agriculture (Plucknett & Smith, 1986). Temporally, a
crop can be sown continuously in the same field (
continuous monoculture
) or
sown only intermittently, in sequence with other crops (
rotation
), a practice
known from ancient Greece, Rome, and China (Karlen
et al
., 1994). Rotation
sequences often contain only food, feed, and fiber crops, but may also include
cover crops to improve and conserve soil during seasons when “main” crops
are absent. In temperate areas, rotation cycles typically extend over several
years, with only annual changes of crops, but in areas with long or continuous
growing seasons,farmers may plant a sequence of several crops within a single
year (
multiple cropping
), or overlap the late growth period of one crop with the
interplanting and early development of another (
relay cropping
).
Spatial and temporal diversity in agricultural systems may also result from
growing trees and shrubs with herbaceous species (
agroforestry
). Orchard trees
with a cover crop understory form one type of agroforestry system. Another
type of agroforestry mimics the ecological process of succession: woody peren-
nials are planted with short-lived herbaceous crops and after the herbaceous
species senesce, the woody species persist, grow larger, and dominate the
system until removed by disturbance or harvest (Hart,1980).Mixtures of trees
can be tended in a manner that resembles late stages of succession and merges
agricultural crop production with forestry.Alternatively, trees in agroforestry
systems can be pruned cyclically to arrest succession and create periodic
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