Agriculture Reference
In-Depth Information
only at a particular growth stage and a higher degree of variability within an oat crop
could allow for escape from attack and subsequent compensation. Varietal mixtures
have been tested in many systems and results from such experiments have been
variable with reductions in pest attacks in some cases and increases in others (see
Andow, 1991).
At larger scales among insect pests, regional deployment strategies can also be
considered. For example, the brown planthopper (
Nilaparvata lugens
) cannot
survive the winter in Japan and temperate Korea and it re-immigrates into these
areas annually from subtropical and tropical Asia. Despite this migration pattern,
regional deployment would be unlikely to be successful, however, because the
migrating populations are extremely diverse and able to adapt to the host genetic
backgrounds they encounter within one cropping season (Roderick, 1994). This
underlines the dependence of regional deployment strategies on relatively low
diversity and consequent slow build-up of the migrating pest population in each
season.
10.3.3 Diversity and weed control
Species mixtures have been shown often to reduce weeds both when the mixtures are
intended to reduce weeds and as a side-effect in intercropping systems (Altieri and
Liebman, 1986; Bulson
et al.
, 1990; Liebman, 1995; Liebmann and Gallandt, 1997).
Variation in patterns of competition, allelopathic interference, soil disturbance and
mechanical damage due to crop rotation make the environment generally less
hospitable for weeds (Table 10.3; Liebman and Dyck, 1993). One of the most
important factors affecting weed suppression is crop density (Liebman, 1995) which is
generally greater in polyculture than in monoculture. In addition, advantage can be
taken of different times at which plants may be competitive against weeds. In peas,
there has been a trend towards semi-leafless or leafless varieties to improve the
microclimate and thus reduce disease and lodging. However, this leaves the ground
bare and open to weeds early in the season. In organic farming, where no herbicides
are used, a new system is now being tested using
Camelina sativa
,
a brassica that
develops early as a dense rosette suppressing all other plants around it. Later in
spring, however, only thin stalks are sent up so there is no competition for light with
the developing peas. The seed of the brassica and peas can be harvested together and
easily separated as the peas are much larger. The brassica is an oil seed that can be
used for the production of biodiesel, for example. First experiments have shown no
yield reductions in peas but almost complete suppression of weeds and an additional
yield of oil seeds (Ackermann
et al.,
2005).
An example of pre-emptive weed
control is the sequential cropping system of winter peas and maize (Grass, 2003;
Grass and Scheffer, 2003). By using leafy winter pea varieties, early in the spring
the field is covered with dense growth suppressing all weeds. Just before the maize
is sown, the biomass is harvested for silage or biogas, leaving most of the fixed
nitrogen in the soil. Maize can be sown without tillage in a clean soil which in
addition is protected from erosion through the residual pea roots that cannot
regrow.
