Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 331

Figure 7.2 Density of Bromus tectorum plants in winter wheat grown in rotation
with rapeseed or in continuous monoculture,with or without tillage,in an
experiment conducted in Alberta,Canada.(Adapted from Blackshaw,1994.)
benefit from rotating crops managed with different herbicides, as well as
contrasting tillage and planting dates. Over a six-year period, densities of the
winter annual grass Bromus tectorum were substantially lower in winter wheat
grown in rotation with spring-sown rapeseed than in continuous winter
wheat (Figure 7.2). Wheat yield was inversely proportional to B. tectorum
density.Available herbicides were ineffective against B. tectorum in continuous
wheat because of the weed's phenological and physiological similarity to the
crop. Control of B. tectorum in the wheat-rapeseed rotation was attributed to
spring cultivation and pre-plant herbicides,which killed the fall-germinating
weed before rapeseed planting, and to selective post-emergence herbicides,
which were tolerated by rapeseed, but effective against B. tectorum. Because
seeds of B.tectorum have little dormancy and generally germinate in one to two
years, suppression of the weed during the rapeseed phase of the rotation
averted a problem in the subsequent wheat phase.
Although rotation of crops in conventionally managed cropping systems
typically implies a rotation of herbicides as well, changes in weed density and