Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 330

Crop rotation and variations in soil conditions
As crops vary within a rotation sequence, changes occur in soil mois-
ture and fertility conditions, residue cover, microtopography, and other soil
properties that influence weed dynamics (see Chapter 5). Generally, a particu-
lar set of soil conditions will be favorable for some weed species,but less favor-
able or inhibitory for others. Crop sequences that involve large changes in soil
conditions offer opportunities to disrupt selection for adapted weed species
by destabilizing their environment.
This concept is exemplified by sequences that include both irrigated and
rainfed crops. Rotation of wetland rice with dryland crops, such as maize,
soybean, peanut, mungbean, sweet potato, and pasture species, reduces infes-
tation by water-tolerant weeds. In an experiment in which no herbicides, cul-
tivation, or hand-weeding were used during four succeeding cropping
seasons, density of the perennial, water-tolerant weed Scirpus maritimus was
36% to 49% lower when wetland rice was grown in rotation with a dryland
maize/mungbean intercrop than when wetland rice was grown continuously
(Moody & Drost, 1983). Reductions in S. maritimus density were greatest
during the dryland phase of the rotation, when soil moisture conditions were
least favorable for the weed. Nonetheless, rotation of wetland and dryland
crops in this experiment failed to fully suppress S. maritimus and did not ade-
quately reduce annual weed densities, indicating the need for other weed
management tactics to complement soil moisture management.
As discussed in Chapter 5,residues of various crop species have allelopathic
properties that can influence weed germination, growth, and competitive
ability. Integration of these effects into crop rotation strategies has not been
addressed systematically, but opportunities appear to exist. For example,
Einhellig & Rasmussen (1989) found that weed cover and biomass were lower
following sorghum than following soybean, and attributed this effect to allel-
opathic properties of sorghum residue.Roder et al .(1989) obtained an average
of 9% more soybean in rotation with sorghum than in continuous monocul-
ture, and suggested that the yield advantage was due to greater soil water
content early in the growing season. Thus, sequencing sorghum before
soybean might provide both agronomic and weed management benefits.
Other rotation sequences showing analogous crop yield and weed suppression
benefits should be identified and investigated,with attention directed toward
understanding the mechanisms and management of allelopathic interactions.
Crop rotation with and without herbicides
Results of an experiment conducted in Alberta by Blackshaw (1994)
provide an exceptionally clear illustration of how weed management can