Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 326

management practices. Diverse crop sequences repeatedly change the envi-
ronment to which a weed community is adjusting.Over time,diverse
sequences reduce weed density by creating inhospitable or fatal condi-
tions throughout the life history ofeach weed species present.Sequences
that include crops suited to different seasons ofthe year,and that alter-
nate annual and perennial crops,can be especially effective for weed sup-
pression.
2. Crop mixtures and sequences should be designed to maximize capture of light,
water, and nutrients by crops and preempt resources used by weeds. Production of
a single crop generally does not exhaust the resources necessary to
support plant growth.Weeds are well adapted for rapid establishment at
microsites where resources are available and,once established,compete
with crops for resources.Because annual crop mixtures often exploit a
greater range and quantity ofresources than sole crops,they can be more
effective for suppressing weeds through resource preemption.Weed
establishment and growth also can be reduced by sowing cover crops
that compete for resources during seasons when main crops are absent or
dormant.In agroforestry systems,trees and shrubs can suppress infesta-
tions ofboth perennial and annual herbaceous weeds through greater
capture ofresources,especially light.
In the following sections of this chapter, we examine applications of these
principles in a range of farming systems.The reader should note that rotation,
intercropping, and agroforestry are discussed separately for ease of presenta-
tion, but that farmers often use these practices in various combinations to
address weed management and other concerns.
Crop rotation
Why farmers use crop rotation
Because crop rotation can improve soil characteristics and reduce pest
pressures, yields from rotation systems are often higher and more stable than
those from continuous monocultures. Although market forces and govern-
ment policies can make one- or two-crop systems profitable and attractive to
farmers, at least in the short term (National Research Council, 1989, pp.
233-40), a growing number of analysts consider the environmental and eco-
nomic costs of such systems to be unsustainable (Power & Follet, 1987;
Reganold,Papendick & Parr,1990; Brummer,1998).Agronomic and biological
advantages of crop rotation are likely to become increasingly important if pro-
tecting soil and water quality,minimizing agrichemical use,and reducing crop
subsidy payments become policy priorities (Faeth et al ., 1991; Faeth, 1993).