Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 325

found in organic farming systems. Diversity in these systems is considered
crucial for encouraging ecological processes that suppress pests, maintain soil
productivity, and reduce dependence on purchased inputs (Lampkin, 1990,
pp. 125-60; Macey, 1992, pp. 80-105). In an area of Ohio where convention-
ally managed farms are dominated by continuous maize or a maize-soybean
rotation, an organic farm uses a four-year rotation composed of maize,
soybean, winter wheat, and red clover (National Research Council, 1989, pp.
253-65). In an area of Saskatchewan where conventional farmers typically
plant spring wheat in alternate years with fallow, an organic farmer uses rota-
tion sequences five or more years in length that include wheat, barley, flax,
mustard, safflower, sweet clover, alfalfa, chickpea, lentil, and field pea
(Matheson et al ., 1991, pp. 46-51). A German organic farm producing both
crop and livestock products uses a 13-year rotation that includes forage
grasses, clovers, alfalfa, maize, potato, carrot, barley, wheat, oat, rapeseed,
mustard, field pea, and vetch (Kaffka, 1985). Intercropping forage grasses or
legumes with cereal crops is common to each of these three organic farms.
Principles guiding crop diversification for weed
management
Cropping systems create stresses, mortality risks, and growth
and reproduction opportunities to which weeds respond. Different crops and
their associated management practices have different effects.Light,water,and
nutrient conditions that affect weed performance vary due to crop-specific dif-
ferences in resource use and specialized fertilization and irrigation practices.
Crop-specific tillage and cultivation practices produce variations in weed mor-
tality, and in the nature and timing of weed germination cues. Damage to
weeds by mowing and grazing occurs with certain types of crops, but not
others.Finally,chemical,physical,and biological characteristics of soil change
through the addition of different crop residues and herbicides. Compared
with simple cropping systems,in which the same crop or type of crop is grown
repeatedly with the same set of management practices, cropping systems that
are diversified through rotation, intercropping, and agroforestry have a
greater variety of factors acting on weed populations. Consequently, diver-
sified cropping increases opportunities for weed management.
Existing information suggests two general principles for managing weeds
through crop diversification:
1. Weeds should be challenged with a broad range of stress and mortality factors
through the use of crop sequences containing dissimilar species and disparate