Agriculture Reference
In-Depth Information
tions.Covarelli & Tei (1988) found that the number of weed species was equiv-
alent in various maize-winter wheat rotations and continuous maize.
However, equitability in the maize phase of the rotations was greater than in
continuous maize.
Thus, diverse crop rotations appear to favor a mix of generalist weeds
whereas continuous cropping of a single species tends to favor one or a few
weeds that are well adapted to the control measures that are possible in that
crop. Although generalists may pose substantial problems, they are usually
easier to manage across a rotation than are specialists in a continuous culture
of the crop to which they are specialized.Managing for equitable distribution
in the relative abundance of weed species in a field is therefore reasonable,
because high equitability generally indicates a low degree of specialization in
the community.
In contrast, high weed species richness is not desirable for the reasons dis-
cussed in the preceding section. Presence of a diversity of plant species in a
field often improves management of insects and disease (Risch, Andow &
Altieri, 1983; Altieri & Liebman, 1986; Andow, 1991). However, better man-
agement of the pests can be accomplished with less risk through intercrop-
ping of species that are easy to control and have properties that effectively
inhibit the particular pest organisms, rather than with weeds that can poten-
tially decrease crop yield. Similarly, attempts to conserve rare weeds by
increasing weed diversity in farm fields are probably misguided (Hebden et al .,
1998); a better approach may be to favor particular rare species in some fields
by appropriate management based on their biology. Moreover, the higher
weed species richness that is often observed in integrated weed management
systems and organic agriculture (Clements, Weise & Swanton, 1994;
Rasmussen & Ascard,1995; Doll,1997) is not desirable from the standpoint of
weed management. Fortunately, high weed species richness may not be an
inevitable consequence of these systems.
Although more work on this subject is needed,intensification of crop com-
petition appears to offer a means for simultaneously increasing equitability
while decreasing species richness. Palmer & Maurer (1997) examined weed
diversity in five sole crops and the corresponding five-species intercrop,all at a
single density.The intercrop had the highest weed species richness.The cause
of this pattern was unclear, but it was apparently not due to different crop
species favoring different sets of weeds. Crop biomass, however, had a signifi-
cant negative effect on weed species richness in this experiment. Chapter 7
discusses other intercropping experiments that showed lower species richness
and greater equitability as the height, density, and biomass of the crops
increased. In a sole-cropping experiment, Lawson & Topham (1985) similarly
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