Increasing crop specialization and intensification in developed countries have virtually eliminated the use of rotations for vegetable Brassica production, resulting in undesirable side effects such as soil compaction, loss of soil structure and decreased organic matter content (Nicholson and Wien, 1983).
Research indicates that intercropping with rows of plants whose purpose is soil improvement, placed between the economic crop, restores soil structure with minimal deleterious effects on the efficiency of the cash crop. Intercropping is still a common cultural practice in tropical areas, but is much less frequently used in temperate regions (Theunissen et al., 1995). Intercropping can be part of an ICM strategy contributing to ecologically and economically acceptable forms of sustainable horticulture. Favourable effects of intercropping particularly include suppressing or delaying pest population expansion and crop colonization.
The requirement that intercropping reduces pest insect populations is not fulfilled in all cases. Some species of pest seem to be unaffected by either the presence or absence of intercrops or only react in specialized conditions, such as, for instance, the small white butterfly Pieris rapae (Risch, 1981).
Intercropping may affect the morphology and development of brassicas. For example, cabbage heads taken from intercropped plots were smaller but more compact, and this in turn can affect their sensitivity to pest attack. It is also suggested that stresses induced by polyculture may alter the physiology of the crop plant, rendering it less attractive or nutritious, or even more toxic to its pests. Results summarized in Table 6.7 compare the effects of mono- and polycropping on the yield and value of white cabbage grown in The Netherlands.
The successfulness of intercropping is increased where chemical suppression of the 'live mulch' prevents excessive competition. The most promising living mulches were the shorter and less vigorous turf grasses and clover. Chewing's fescue (Festuca rubra var. commutata), Kentucky bluegrass (smooth meadow grass) (Poa pratensis) and Kent wild white clover (Trifolium repens) did not affect the yield of cabbage. Some compromise is required between improving soil structure and limiting the competition with the Brassica crop when growth of the live mulch is not controlled. In principle, interseeded cover crops can be chosen to complement the main crop in resource use while directly or indirectly interfering with weed growth, thereby suppressing weeds but not the crop (Barnes and Putnam, 1983; van der Meer, 1989). In practice, however, success has been limited because interseeded crops often suppress neither the crop nor the weeds, or suppress both. Two basic designs for comparing the performance of intercropping with monoculture are advocated, the additive and substitutive formats.
Table 6.7. Yield and financial data from mono- and polycropping systems applied to white cabbage (Brassica oleracea var. capitata).
Mean weight of % Marketable marketable cabbages cabbage heads (kg) Gross income (Dfl/ha)
Treatment 12 12 12 3*
Monocrop 52 29 1.98a 1.97a 9,062 4,643 4,647
raneum 84 70 1.45b 1.71b 10,656 9,306 9,307
^Including marketable class 3; 1 = Experiment 1 (1990); 2 = Experiment 2 (1991).
Figures followed by a different letter differ significantly at P = 0.05.
After Theunissen et al. (1995); data are retained in Dutch guilders rather than conversion to euros.
Advantages are gained from intercropping where there is limited competition between the species or where one species provides a benefit to the other. A low level of competition occurs when two or more crops use different components of the ecosystem or use the same components in varying ways, or exploit alternative ecological niches. This is the Principle of Competitive Production as identified by van der Meer (1989). Low levels of competition can result from differing times of resource interception, rates of resource requirement, specificity for key resources and varying tolerances to biotic and abiotic stresses. These factors may interact with population densities of the components of the intercropping system. As yet, the impact of population density on the successfulness of intercropping has received only limited attention from researchers. In studies of broccoli and cauliflower intercropped together and with other crops such as potatoes and oats, Santos et al. (2002) concluded that successful combinations would exploit factors such as asynchrony of growth and maturity periods, and variations in canopy height and flexibility.
In seeking to use intercrops for transplanted broccoli production in New York State, USA, Brainard and Bellinder (2004) suggest that winter rye (Secale cereale) will be successful where it is sown at high densities, in localities or seasons with low initial temperatures and in combination with other weed management tools. This exploits the cold adaptation C3 attributes of winter rye with germination requirements of 0-5°C compared with those of Powell's amaranth (green pigweed, Amaranthus powelli) which is a C4 species adapted to warmer environments germinating at 10-15°C.
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