Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 327

Potential benefits of crop rotation depend on the choice of crops grown and
their order within the rotation sequence. Pasture, hay, and cover crops can be
especially useful for improving soil physical characteristics, including bulk
density, aggregate stability, aeration, and water infiltration capacity (MacRae
& Mehuys, 1985; Karlen et al ., 1994). Pasture, hay, and cover crops can also
reduce soil erosion (Gantzer et al ., 1991; Karlen et al ., 1994) and loss of nutri-
ents through leaching (Jackson, Wyland & Stivers, 1993; McLenaghen et al .,
1996).When legumes are included in crop sequences, biologically fixed nitro-
gen can become available to subsequent crops, increasing yields of non-
legume species (Fox & Pielielek, 1988; Frye et al ., 1988). Even when nutrients
are supplied at high levels or N-supplying legumes are not included in the
crop sequence, crops grown in rotations typically yield 5% to 15% more than
crops grown continuously (Karlen et al .,1994).This effect appears to be related
to changes in soil microbiology and biochemistry (Bullock, 1992). Rotation of
host and non-host crops can reduce damage by insect pests and pathogens
that have a narrow host range,limited dispersal ability,and low persistence in
the soil or adjacent land (Sumner,1982; Lashomb & Ng,1984; Sturz & Bernier,
1987; McEwen et al ., 1989).
Continuous production of a single crop and short sequences of crops with
similar management practices promote the increase of weed species adapted
to conditions similar to those used for producing the crops. The resulting
weed flora can be highly competitive and difficult to control, even with chem-
ical technology (Froud-Williams, 1988). In contrast, over the course of a
diverse rotation employing crops with different planting and harvest dates,
different growth habits and residue characteristics, and different tillage and
weed management practices, weeds can be challenged with a wide range of
stresses and mortality risks, and given few consistent opportunities for
unchecked growth and reproduction (Figure 7.1) (Liebman & Dyck, 1993;
Derksen, 1997; Liebman & Ohno, 1998). Consequently, crop rotation can be a
powerful tool for weed management.
Variation in the timing of crop management practices
As noted in Chapter 2, many weed species exhibit characteristic
pulses of germination and growth at particular times of year. Crop manage-
ment practices that affect weeds, such as tillage and seedbed preparation,
planting, fertilization, herbicide application, and cultivation, also have a
marked periodicity. Interactions between the periodicity of weed emergence
and crop-specific farming operations can generate distinct weed communities
in different crops.In a study of weed dynamics during 20 years of arable crop-
ping in England, Chancellor (1985) noted that spring-germinating weed