Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 333

maize in rotation than for continuous maize.Weed seed densities were as low
in the wheat-maize rotation without herbicides as with herbicides.Although
the continuous maize and wheat-maize systems contained equal numbers of
weed species,they differed in the relative importance of the species.In contin-
uous maize, the most abundant weed, Echinochloa crus-galli (a C 4 grass like
maize), accounted for 71% of the total number of weed seedlings. In contrast,
in the wheat-maize rotation, the most abundant species, Amaranthus
retroflexus , accounted for only 39% of the total weed density, and E. crus-galli
accounted for only 26% of the total.Impacts of crop rotation practices on weed
species diversity are discussed in more detail in Chapter 10.
Effects of lengthening rotations and increasing diversity from two to three
crops can be seen in a long-term study conducted by Schreiber (1992). The
experiment included a zero-tillage, minimum weed management treatment
that received herbicides at concentrations lower than commercially recom-
mended. Subplots were left untreated with herbicides to allow fuller expres-
sion of possible crop rotation effects on weed dynamics.Density of the annual
grass weed Setaria faberi was measured in continuous maize, maize rotated
with soybean, and maize following winter wheat in a soybean-wheat-maize
rotation. Over a seven-year period, mean density of S. faberi plants was highest
in continuous maize, lowest in the three-year rotation, and intermediate in
the two-year rotation. Thus, S. faberi density decreased as crop diversity
increased. Perhaps more important from a farmer's viewpoint is that variance
in the weed's density between years decreased as crop diversity increased.
Consequently farmers using the three-year rotation could feel more confident
that S. faberi density would not become unexpectedly high.
Schreiber (1992) attributed the marked reductions in S. faberi density in the
three-year rotation to allelopathic effects of wheat straw.However,differences
among the fall-sown crop (wheat) and the two spring-sown crops (maize and
soybean) with regard to planting and harvest dates and other management
practices may also have affected weed dynamics. Results of Schreiber's (1992)
study do not resolve questions of how many and what types of crops should be
included in a rotation for maximum weed suppression, but they indicate that
further research addressing those questions would be valuable.
Perennial forage crops
Perennial forage crops constitute important components of many
crop rotation systems and offer opportunities to suppress weeds through
competition (Risser, 1969), mowing (Hodgson, 1958; Norris & Ayres, 1991),
and grazing (Chapter 9) (Heard, 1963; Dowling & Wong, 1993). Sunlight
filtered through a canopy of well-established forage plants inhibits the