Agriculture Reference
In-Depth Information
Table 10.3. Effects of crop rotation and intercropping on weed infestation. The number
of studies in which a given effect was observed is listed (data adapted from Liebman
and Dyck, 1993)
(a) Effects of crop rotation
With crop rotation
Weed density
Seed density
Crop yield
Lower than without
21
9
0
Equal than without
5
3
3
Higher than without
1
0
11
(b) Effects of intercropping
Weed biomass in intercropping was
Less than in sole-crop
47
Variable
3
Higher
4
Planting patterns can play an important role particularly if the crop plant has a large
seed size. In this sense, Weiner (1990a,b) used the term 'size-asymmetric
competition' with respect to planting patterns with wheat. Because wheat grains are
considerably larger than many weed seeds, Weiner
et al.
(2001) found in field trials
that regular spacing of wheat seed was more effective than conventional row spacing
in ensuring strong competition of wheat against weeds.
If pathogens, insects or weeds depend on surviving the off-season in the crop
area, a reduction in their severity should reduce the amount of pathogen inoculum,
number of insects or seeds surviving until the next season. This will translate into
lower disease, insect or weed pressure early in the following season and
consequently will delay epidemic development (Kennedy
et al.
, 1987; Wolfe, 1992;
Liebman and Dyck, 1993; see 10.5). If initial inoculum or insect populations migrate
from other regions, however, these benefits can be reaped only if gene deployment
is practised on the appropriate scale (Kennedy
et al.
, 1987; Wolfe, 1992; see 10.4).
10.3.4 Diversity and abiotic stress
There is little information in the literature on the ability of mixtures to reduce abiotic
stress, although this may well be common. Mechanisms could be through niche
differentiation and protection with compensation. For example, we observed such
effects in Poland, where a cold sensitive winter barley variety had markedly
improved survival when mixed with a cold tolerant variety than when grown alone
(Finckh
et al
., 2000). In Pakistan, wheat is intercropped with sugar cane to protect
the latter from cold injury (Aslam, personal communication) and variety mixtures of
wheat are used for the same reason among others by farmers in Oregon, US (Mundt,
personal communication).
