Intercropping

Another cultural method suitable for weed control strategy is intercropping (growing two or more crops together). Intercropping of clovers and grasses is widely used in pastures or for fodder production, but intercropping (cereals, grain legumes, and oil seeds) for human consumption is not so common. For example, binary grass-alfalfa mixtures for hay production are common in most subhumid to semiarid areas (Berdahl et al. 2001). Italian ryegrass (L. multiflorum Lam.) or oat (Avena sativa L.) was used as a companion crop to establish alfalfa (M. sativa L.) or clover stands

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Fig. 14.3 One of the most common examples of intercropping - oat with red clover (Trifolium pratense L.). Oat provides early competition with weeds while the clover is established; the clover blocking out light to the soil; the oat also takes up excess nitrogen that would otherwise stimulate the weed growth

Fig. 14.3 One of the most common examples of intercropping - oat with red clover (Trifolium pratense L.). Oat provides early competition with weeds while the clover is established; the clover blocking out light to the soil; the oat also takes up excess nitrogen that would otherwise stimulate the weed growth

(Fig. 14.3) in the USA or Europe (Sulc et al. 1993). Alfalfa was the best legume to grow with smooth bromegrass (Bromus inermis Leyss.) (Sleugh et al. 2000). Tall fescue (Festuca arundinacea Schreb.) is compatible with either birdsfoot trefoil (Lotus corniculatus L.) or white clover (T. repens L.); switchgrass (Panicum virgatum L.), indiangrass (Sorghastrum nutans (L.) Nash) or sideoats grama (Bouteloua curtipendula (Michx. Torr.)) could be mixed with purple prairie clover (Petalostemon purpureum (Vent.) Rydb.), roundhead lespedeza (Lespedeza capitata Michx.), leadplant (Amorpha canescens Pursh), Illinois bundleflower (Desmanthus illinoensis (Michx.) MacMill. ex B.L. Rob. & Fernald), catclaw sensitive brier (Schrankia nuttallii (DC.) Standl.) or cicer milkvetch (Astragalus cicer L.) (Posler et al. 1993; Springer 1996; Springer et al. 2001). In the central USA, the grazing season for Bermuda grass (Cynodon dactylon (L.) Pers.) is lengthened with fall-interseeded rye (Moyer and Coffey 2000). Timothy grass (Phleum pratense L.) can be grown together with barley or wheat as companion crops (Jefferson et al. 2000). Interseeding red clover into small grains is one of the most common practices. Johnson et al. (1998) established successful interseeding of rye or oat (Avena fatua L.) into soya bean. Other successful intercrops include: oats and pulses (such as peas, lentils, or beans), flax and wheat, flax and alfalfa, wheat and lentil, flax and lentil, barley and peas, and soya and maize (Wallace 2001).

Intercropping has been an essential production method in tropical regions for hundreds of years (Vandermeer 1989). Small-scale farmers in tropical Africa grow sweetpotato (67 plants ha-1) with groundnut (Arachis villosulicarpa Hoehne) (67 plants ha-1) due to weed suppression and increased land productivity (Ossom 2007). In Philippines, mung bean (Vigna radiata (L.) R. Wilczek ) is grown mixed with maize; the weeds smothering effect of the mung bean protects the easily infested maize. Farmers throughout Central America traditionally grow maize (Zea mays L.), green bean (P. vulgaris L.), and squash (Cucurbita pepo L.) together. Typical crop associations in Swaziland involve grain legumes and sweet potato (Ipomoea batatas (L.) Lam.), but cereals, legumes, sugarcane, maize, and grain legumes have also been intercropped (Vandermeer 1989; Ossom 2007). Intercropping of cotton (Gossypium hirsutum L.) and cowpea (Vigna unguiculata (L.) Walp) improves food security and soil fertility (Rusinamhodzi et al. 2006). Intercropping cereals with some legumes influences positively Striga infestation (see Chapter: Allelopathy in parasite weed management).

There are four possible types of intercropping: mixed intercropping - growing two or more crops (varieties) simultaneously without row arrangement; row intercropping - crops in alternating rows; strip intercropping - crops in alternating strips; and relay intercropping - second crop overseeded into current crop (Wallace 2001). Maize (four seeds m-2) and soya bean (33 seeds m-2) as mixed intercrops are more cost effective than pure stands. Another example of mixed intercrops is forage sorghum into silage maize. Strip intercropping is suitable for maize and soya bean or for cerelas (spring wheat or oats), maize, and soya bean with ridge-till or notill (Sullivan 2003a). Relay intercropping can also be practised with grasses in pasture management.

Mixture intercropping is possible as well with different varieties. The primary reason in implementing variety mixtures would be to reduce pathogen and weed populations but phenotypic variation in varieties may allow choosing those which will suppress the specific pathogen or weed problems by the crop (Mundt 2002). Crop mixtures have also advantages under certain unfavorable conditions such as frost or lodging. Frost-resistant varieties have been found to protect less resistant ones. Similarly, cereal varieties which do not lodge can be a support for components with weak stems (Lastuvka et al. 2007).

Intercropping favors utilization of water, nutrients, cropping area and productivity of cultivated plants, increase the ecological diversity in a field, and may also contribute to the prevention of nitrogen leaching risks. At the same time intercropped plants form a potentially absorptive barrier against pest and fungi to movement between those plants and they can reduce occurrence of weeds and insects (Baumann et al. 2000). These advantageous effects are attributed partially to allelo-pathic interaction between crops and other organism living in the field. Weeds are controlled by increasing shade and increasing crop competition with weeds through tighter crop spacing during all or part of the crop growth.

Intercropping is most successful when the two crops have the complementary growth and resource needs, e.g., Italian ryegrass (L. multiflorum Lam.) was too competitive with the timothy grass (Phleum pratense L.), therefore it should not be recommended as a companion crop for timothy establishment (Jefferson et al. 2000).

Allelopathic effects of a selected crop on the component crops must be considered. Strong allelopathic and competitive effects were for example observed in tall fescue (Festuca arundinacea Schreb.), white clover (Trifolium repens L.) or creeping red fescue (Festuca rubra L.) (Weston 1996; Sanderson and Elwinger 1999). Therefore a companion plant, that is selectively allelopathic against certain weeds and does not interfere with the crop growth, should be used. In oat - peas mixture, oat provides early competition with weeds while the peas is established; the peas then climbs on the oat, blocking out light to the soil; the oat competes more with grassy weeds for nutrients and the peas competes with the broadleaf weeds; the oat also takes up excess nitrogen that would otherwise stimulate the weed growth (Wallace 2001).

Intercropping may facilitate weed control if intercrops are more weed competitive than sole crops or are able to suppress weed growth through allelopathy (the weed species is more susceptible to such phytotoxin than crops). If intercrops do not suppress weeds more than sole crops, they should provide yield advantages due to better utilization of resources or by converting resources than in case of sole crops (Liebman and Dyck 1993).

An example, when intercropping suppressed the weed growth more than sole, is a leek (Allium ampeloprasum var. porrum (L.) J.Gay) - celery (Apium graveo-lens L.); intercrop sown in a row-by-row layout decreased relative soil cover of weeds by 41%, reduced the density and biomass of groundsel (Senecio vulgaris L.) by 58% and 98%, respectively, and increased the total crop yield by 10% (Baumann et al. 2000). Increased weed suppression and the crop yield were also demonstrated in cereal-legume intercrops in many different environments (Ofori et al. 1987).

However many factors such as planting date, presence of weeds, spatial arrangement, varieties, relative proportion of component crops, fertility, the crop growth in the given environment, etc., affect successfulness of intercropping (Altieri and Liebman 1986; Ofori et al. 1987). In practice, it is necessary to optimize them as much as possible. Seeding of each crop at two-thirds of its normal rate provided good results. Early-heading varieties of perennial ryegrass (Lolium perenne L.) or orchardgrass (Dactylis glomerata L.) are more compatible with white clover (T repens L.) (Sleugh et al. 2000). Tall fescue (Festuca arundinacea Schreb.) varieties differ in their compatibility with white clover (Pedersen and Brink 1988). Orchardgrass lines with later maturity are more compatible with birdsfoot trefoil (Lotus corniculatus L.) (Short and Carlson 1989).

More compatible components for mixtures are being actively sought for cocoa, tea, rubber, grasses mixtures, and tree-based intercropping systems with Persian walnut (Juglans regia L.) (Ercisli et al. 2005). One of the possibilities for walnut intercropping is muskmelon (Cucumis melo L.) because seedling growth of this crop was increased by juglone (the allelochemical of walnuts) treatments (Kocacaliskan and Terzi 2001). Research is as well oriented to exploiting the allelopathic effects of different healing herbs, e.g., Mentha spp., to repress weeds.

The interaction of weeds with crops may be positive too. In a study, controlled densities of wild mustard (Brassica campestris L. var. italica) interplanted with broccoli (Brassica oleracea var. italica Premium Crop) increased the yield by 50% (Jimenez-Osornio and Gliessman 1987).

The main disadvantages of intercropping are: competition between crops; possible damage of the other crop during the harvest of one crop component; complication of mechanization and cultivation; decrease of symbiotic nitrogen fixation in legumes caused by plant competition for resources (Soon et al. 2004).

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