Crop Rotation with Allelopathic Crops to Control Weeds

Crop rotation is a system where different plants are grown in a defined sequence. The well-designed crop rotation is the basis of the success in organic farming. Continuous monoculture is unacceptable due to the increased pressure of weeds and pathogens and difficulties with maintaining soil fertility. Diversity of crops in the rotation is the key to a successful crop rotation program that rotate early-seeded, late-seeded and fall-seeded crops; grassy, broadleaf and legume crops; highly competitive crops with less competitive crops; use perennial phases combined with mowing or intensive grazing to control perennials; use cover crops and green manure crops that suppress weeds and disrupt weed life cycles; and provide the frequency of crop growth within a rotation (Wallace 2001). Such diversified rotations create an unstable environment for weeds. Rotation changes the growing conditions from year to year and so, it forms a situation in which only few weeds easily adapt (Sullivan 2003b). Longer rotations with more phenologically diverse crops as well help reduce seedbank populations and abundance of important weeds in organic systems (Teasdale et al. 2004).

An allelopathic crop designed in rotation sequences can suppress weeds in both cultivated and next crops (Mamolos and Kalburtji 2001) through residue decomposition or root exudates. For example, black mustard (Brassica nigra L.) and Indian mustard (Brassica juncea L. Czern.) are ideal following crops for cereals because they improve soil fertility and suppress many weed species. Allelopathic potential has been reported in numerous crops like rice (Dilday et al. 1998), wheat, oats, barley, rye, maize, common buckwheat (Fagopyrum esculentum Moench), millet (Panicum sp.), beets (Beta vulgaris L.), peas, lupine (Lupinus sp.), hairy vetch (Vicia villosa Roth.), sorghum, sunflower, cucumber (Rice 1984), sugarcane (Sampietro et al. 2007), chick pea (Cicer arietinum L.), bitter vetch (V. ervilia Willd.), alfalfa (Medicago sativa L.); velvetbean (Mucuna pruriens DC.), crimson clover (Trifolium incarnatum L.), subterranean clover (Tr. Subterraneum L.), sweet potato (Ipomoea batatas (L.) Lam.) (Batish et al. 2001), tall fescue (Festuca arundinacea Schreb.), creeping red fescue (Festuca rubra L.) and perennial ryegrass (Lolium perenne L.), asparagus (Asparagus officinalis L.), coffee (Coffea spp.), tea (Camellia sinensis (L.) Kuntze) (Khanh et al. 2005), rapeseed (Brassica napus L.), flax (Linum usitatissimum L.) and lentil (Lens culinaris Medik.) (Minorsky 2002), some medicinal plants and others.

If crop rotations include plants that inhibit weed germination, the balance of allelopathic crops is necessary because they can also suppress subsequent crop growth. For example, the wheat growth was depressed by grain sorghum (Sorghum bicolor (L.) Moench) root exudates (Ben-Hammouda et al. 1995). However, tilled sorghum residues delayed the wheat growth but did not affect the grain yield (Roth et al. 2000). In rye (Secale cereale L.) and maize (Zea mays L.) double cropping sequence, maize development was delayed and reduced (Raimbault et al. 1990). Barley (Hordeum vulgare L.) residues negatively influenced durum wheat (Triticum durum L.) and bread wheat (Triticum aestivum L.) (Ben-Hammouda et al. 2001). However in case of balanced allelopathic crops, crop rotation can be helpful for minimizing the toxic effects of allelochemicals on following crops (Mamolos and Kalburtji 2001). According to Conklin et al. (2002), tillage timing and system may modify soil amendment effects on weeds or crops too. When the residues are incorporated, as in strip tillage, allelopathic substances break down relatively quickly.

Allelopathic crops can inhibit the growth of the same species through the release of toxic chemicals into the environment too. This phenomenon is called autotoxicity, it is a type of intraspecific allelopathy (Rice 1984). This phenomenon probably regulates seed germination and defends against phytopathogenic agents but from an agricultural point of view it is one of the causes of "soil sickness." Therefore the knowledge of autotoxic effects would avoid unexpected harvest losses (Macias et al. 2003). Autotoxicity is common in fields where sole cropping under reduced or no-tillage system is practiced (Batish et al. 2001) e.g., in continuous monoculture rice fields and especially in fields with poor water drainage (Chou 1990), when wheat straw was retained on the soil surface (Wu et al. 2001) or by reseeding of alfalfa. This phenomenon was described in many common crops (Table 14.1) but in many weeds and other plants too. For example, autotoxic effects have been demonstrated in a tropical herb Wedelia chinensis Merrill (Luo et al. 1995).

The size of the autotoxic zone was about 20 cm around of the old alfalfa (Medicago sativa L.) plant (Jennings and Nelson 1998). Older stands caused greater inhibition than newly seeded (Peel 1998). The autotoxicity may be more severe in sandy soils, but the autotoxic factor is leached out of the root zone more easily in the sandy soils than in soils of heavier texture. Autotoxicity can be limited by proper crop rotation (Batish et al. 2001), proper soil and plant residues management, as well as microbial degrading (Yu 2001). Recommended interval for reseeding alfalfa ranges from two weeks (after ploughing or tilling) to 24 months but the most common recommendation is after rotation with a non-legume crop grown for one or more seasons (Tesar 1993). Irrigation on light-textured soils may help dilute the autotoxic chemical making it possible to shorten the rotation interval (Jennings and Nelson 1998). Autotoxicity could be overcome by selecting resistant

Table 14.1 Common crops with described autotoxicity



Soya bean (Glycine max Merrill L.)

Xiao et al. 2006

Maize (Zea mays L.)

Yakle and Cruse 1984

Asparagus (Asparagus officinalis L.)

Shafer and Garrison 1986

Alfalfa (Medicago sativa L.)

Hegde and Miller 1992;

Chung and Miller 1995

Cucumber (Cucumis sativus L.)

Yu and Masui 1997

Rice (Oryza sativa L.)

Chou 1990, 1995

Barley (Hordeum vulgare L.)

Ben-Hammouda et al. 2001

Pearl millet (Pennisetum glaucum (L.) R. Br.)

Saxena et al. 1996

Sorghum (Sorghum bicolor (L.) Moench)

Hegde and Miller 1990

Tomato (Lycopersicon lycopersicum (L.) Farw.

Shiming 2005

Wheat (Triticum aestivum L.)

Wu et al. 2007

Watermelon (Citrullus lanatus (Thunb.) Mansf.)

Hao et al. 2006, 2007

Mugwort (Artemisia vulgaris L.)

Onen 2007

Strawberry (Fragaria x ananassa Duchesne)

Cao and Wang 2007

Tea plant (Camellia sinensis (L.) Kuntze)

Luo et al. 1995

Weeds Effects
Fig. 14.2 Well established stand of common buckwheat as a cover crop has great weed suppressive effects. Buckwheat emits allelochemicals, thus controlling weeds

varieties because significant varietal differences exist in tolerance to the autotoxin (Chon et al. 2002; Queslati et al. 2005). Therefore careful selection of suitable varieties is necessary in a continuous cropping system to minimize the negative impacts of autotoxicity (Wu et al. 2007).

Additional weed control may be obtained by including allelopathic plants as cover crops, green manure, or so called smothering crops (a living mulch) such as sudan grass (Sorghum sudanense L.) or common buckwheat (Fagopyrum esculen-tum Moench) (Fig. 14.2) and others in the crop rotation (Sullivan 2003b). For example, the allelopathic activity of buckwheat was an effective tool for inhibiting the growth and development of quack grass (E. repens (L.) Gould), field pennycress (Thlaspi arvense L.), Canada thistle (Cirsium arvense (L.) Scop.), ribwort plantain (Plantago lanceolata L.), barnyard grass (E. crus-galli (L.) P. Beauv.), common purslane (Portulaca oleracea L.) (Tominaga and Uezu 1995; Golisz et al. 2004; Kalinova 2006). The allelopathic constituents as gallic acid and their derivative, catechin, rutin, palmitic acid, long-chain fatty acids, fagomine, 4-piperidone, and 2-piperidine were identified (Iqbal et al. 2003; Kalinova et al. 2007).

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