Cover Crops with Allelopathic Potential

Providing weed suppression through the use of allelopathic cover crops is an important method of weed control in organic farming and it is one of the best possibilities of allelopathy application (Sullivan 2003a). Besides, growing of cover crops provide soil protection against erosion and better water infiltration, decrease nutrient losses especially nitrogen, improve soil physical and chemical characteristics, increase soil organic matter and biological diversity and reduce pressures of harmful organisms (Hartwig and Ammon 2002). However, used cover crops are necessary to rotate in the same way as crops due to protection against build-up of weed, pathogen and pest populations, and allelochemicals.

Allelopathy plays an important role in weed suppression by cover crops if other competitive factors are on the same level (Fujii 2003). Roots of the crops or decomposing residues release compounds in the soil that should be toxic to weeds. The weed-suppressive effect of the cover crops is influenced by species, planting date, seeding rate and method, weather, and other factors. There should be sufficient selectivity between the activity of cover crop toxins on weeds and on cash crops that should be relatively insensitive to allelochemicals in the environment. The relative timing and placement of residue relative to crop seeds can be manipulated to reduce the toxicity to emerging crop seedlings. According to Wuest et al. (2000), wheat residue 3 cm below the seed reduced the height and rate of wheat plant development, indicating an inhibitive effect of the wheat residue. Duration of weed suppressiveness provided by decomposing cover crop residues should be in an important consideration. Phytotoxicity of cover crops probably persists in soil from 2 weeks to 60 days (Chou and Patrick 1976; An et al. 1997; Teasdale and Pillai 2005).

Cover crops are possible to use in vineyards, orchards, and common agronomic crops, such as maize, small grains, and forages. A cover crop is a living ground cover planted with or after the main crop and usually killed before growing of the next crop. Annual, biennial, or perennial herbaceous plants in a pure or mixed stand can be grown as cover crops. For example, cool-season legumes as clovers, vetches, medics, and field peas are planted in a mix with winter cereal grains such as oats, rye, or wheat. Successfully established cover crops should develop dense canopies to interfere with the growth of weeds (Hartwig and Ammon 2002).

There are two categories of cover crops:

1. Annuals that are grown during an off-season and that are killed before planting a cash crop.

2. Living mulches are plants interplanted with the annual or perennial cash crop for all or a portion of the growing season.

Examples of highly weed suppressive cover crops are rye, buckwheat, sorghum, or alfalfa. Weed control by legumes is usually lower because nitrogen released from their residues stimulates weed emergence (Blum et al. 1997). Brassica cover crops, including canola (Brassica napus L.), rapeseed (Brassica napus L.), and mustard species (e.g., Indian mustard B. juncea L.; black mustard Brassica nigra L., white mustard Sinapis alba L.) are also often grown. According to Haramoto and Gallandt (2005), incorporated canola (Brassica napus L.), rapeseed (Brassica napus L.) and white mustard cover crops reduced establishment of a wide range of crop and weed bioassay species an average of 29% and the weed emergence was delayed for 1.8

days. Canola was less competitive with weeds than white mustard (S.. alba L.) (Daugovish et al. 2002). Brassica cover crops did not reduce the redroot pigweed growth when they were grown in mixture with green bean (Phaseolus vulgaris L.) (Haramoto and Gallandt 2005). On the contrary, use of winter legume cover crops mixtures resulted in more consistent overall cover crop performance, greater dry matter production, and more effective weed suppression than single cover crop species (Fisk et al. 2001; Linares et al. 2008). Examples of possible use of some cover crops in effective control of some weeds are given in the Table 14.2.

Inhibitive effects are especially influenced by amount of cover crop biomass and soil management. Weed dry matter was reduced when rye residues were greater than 3.7 Mg ha-1 (Crutchfield et al. 1985), and when wheat residues were greater than Mg ha-1 (De Almeida 1985). Fisk et al. (2001) reported that burr medic (Medicago polymorpha L.) and barrel medic (M. truncatula Gaertn.) reduced by 70% weed dry weights while weed density was not affected if were no-till seeded as winter-killed cover crops into winter wheat (Triticum aestivum L.) stubble.

Low-growing legumes with dense canopy such as clover and grasses are typically used for living mulches. These secondary intercropped species are often referred to smother crops; species with rapid growth suppress weeds during the critical period, i.e., the period when emerging weeds will cause a loss in the crop yield and they become senescent after this critical period (Buhler and Hartzler 2001). It is important to kill, till in, or otherwise manage the living mulch so that the living mulch does not compete with the actual crop. Kura clover (Trifolium ambiguum M. Bieb.) or other Trifolium species as T. subterraneum L., T. incarna-tum L. can be managed as a living mulch in maize within 12 months without replanting (Zemenchik et al. 2000). This system is possible to combine with mechanical cultivation at about the three-leaf stage of maize. Then maize yield is not reduced and the green cover remained as soil protection till the harvest (Hartwig and Ammon 2002). Other possibility is overseeding hairy vetch (Vicia villosa Roth) into maize at the last cultivation or into sunflower (Teasdale and Daughtry 1993). Vetch can also be seeded into soya bean when their leaves begin to turn yellow. Buckwheat or mixture of rye and buckwheat can be interseeded with maize or sunflowers. The plants can be incorporated into the crop rows to nourish the crop (maize or sunflowers) and suppress weeds when buckwheat reaches a height of 200-300 mm (DeRosier 1998). No-till planting vegetables can be grown into sub-clover, sweetclover, drilled into cereals, or Italian ryegrass (Lolium multiflorum Lam.), or sowed into vegetables (Sullivan 2003a). Trials with the living mulch are provided with underseeding in cabbage (Brassica oleracea L.) and other vegetables (Bellinder et al. 1996). However, this management is difficult to establish, so it is not very acceptable to farmers. A living cover crop is capable of greater weed suppression than killed cover crop residue. The degree of effectiveness depends on factors such as the amount of residue incorporated as well as the timing of incorporation (Fisk et al. 2001).

Living mulches in perennial cropping systems are grasses or legumes planted in the alleyways between rows in orchards and vineyards. Use of the living mulch is a common practice in apple (Malus sylvestris L.) production. Wick and Alleweldt

Table 14.2 Examples of possible use of some cover crops in effective control of some weeds

Crop

Inhibited weed

Reference

Rye (Secale cere ale L.)

Wheat (Triticum aestivum L.)

Winter barley

(Hordeum vulgare L.) Buckwheat (Fagopyrum spp.)

Red clover

{Trifolium pratense L.) Yellow sweetclover

(Melilotus officinalis (L.) Lam.)

Mulch

Cover crop residue

No-till cover crop Shoot residues Mulch

Green manure

Green manure

Residues incorporated or on the soil surface

Common millet Panicum miliaceum L., barnyard grass E. crus-galli (L.) R Beauv., bristly foxtail Setaria verticillata (L.) R Beauv.. redroot pigweed Amaranthus retroflexus L.

Hairy crabgrass Digitaria sanguinalis (L.) Scop, annual rag wed Ambrosia artemisiifolia L.. early broadleaf weeds, carpetweed Mollugo verticillata L., common lambsquarters Chenopodium album L., browntop millet Brachiaria ramosa (L.) Stapf

Goosegrass Eleusine indica (L.) Gaertn., carpetweed Mollugo verticillata L.

Giant foxtail Setaria faberi R. Herrm., wimmera ryegrass Lolium rigidum Gaud.

Barnyard grass E. crus-galli (L.) R Beauv.. bristly foxtail Setaria verticillata (L.) R Beauv.

Canada thistle Cirsium atyense (L.) Scop., quack grass E.s repens (L.) Gould, barnyard grass E. crus-galli (L.) R Beauv.. monochoria Monochoria vaginalis (Burm. F.) C. Presl ex Kunth

Wild mustard Sinapis atrensis L.

Dandelion Taraxacum officinale L. Weber, perennial sowthistle Sonchus arvensis L., Lochia Kochia scoparia (L.) Schrad., flixweed Descurainia sophia (L.) Webb ex Prantl. Russian thistle Salsola iberica Sennen and Pau. downy brome Bromus tectorum L.

Barnes and Putnam 1986; Martens et al. 2001

Barnes and Putnam 1983; Nagabhushana et al. 2001; Reddy 2001; Teasdale et al. 1991

Teasdale et al. 1991

Schreiber 1992;

Wu et al. 2001 Dhima et al. 2006

Xuan and Tsuzuki 2004

Conldin et al. 2002 Blackshaw et al. 2001

Hairy vetch Cover crop Residue

(V. villosa Roth) living mulch

Cover crop residues

Cover crop under no-till management

White mustard Green manure

(Sinapis alba L.)

Green manure

Pigweed Amaranthus spp., foxtail Setaria spp., velvetleaf Abutilón theophrasti Medikus Carpetweed Mollugo verticillata L., common lambsquarters Chenopodium album L., browntop millet Brachiaria ramosa (L.) Stapf , hairy crabgrass Digit aria sanguinalis (L.) Scop. Goosegrass Eleusine indica (L.) Gaertn., stinkgrass Eragrostis cilianensis Vignolo ex Janch.. carpetweed Mollugo verticillata L.. Hemp sesbania Sesbania exalt at a (Raf.) Cory, common lambsquarters Chenopodium album L.. redroot pigweed Amaranthus retroflexus L.. barnyard grass E. crus-galli (L.) P. Beauv., hairy nightshade Solanum sarrachoides Sendtner. Kochia Kochia scoparia (L.) Schrad. shepherd's-purse. Capsella bursa-pastoris (L.), green foxtail Setaria viridis (L.) P. Beauv. Hemp sesbania Sesbania exalta (Raf.) Cory, hairy nightshade Solanum sarrachoides Sendtner). longspine sandbur Cenchrus longispinus (Hack.) Fern.. Kochia Kochia scoparia (L.) Schrad.. shepherd's-purse. Capsella bursa-pastoris (L.), green foxtail Setaria viridis (L.) P. Beauv.

Teasdale and Daughtry 1993

Reddy 2001;

Teasdale et al. 1991

Teasdale et al. 1991

Vaughn and Boydston 1997; Boydston and Hang 1995; Krishnan et al. 1998; Martens et al. 2001

Boydston and Hang 1995; Vaughn and Boydston 1997; Krishnan et al. 1998

(1983) found subterranean clover (T. Subterraneum L.) or white clover (T. repens L.) as suitable cover crops for vineyard cv. "Daliak." According to Fujii (2003), hairy vetch is the most promising cover plant for orchards, but as well vegetable and rice production in Japan.

Naturally occurring weeds could be used as the living mulch too. For example, common chickweed (Stellaria media L.), one of the widespread uncontrolled weeds, was tested in vineyard; grape yields were not lower and soil erosion was reduced (Hartwig and Ammon 2002). Possible influencing of the seedbed preparation for following crops, a possible source of infection to cash crops and in some cases developing a high carbon to nitrogen ratio that could reduce the yield of following crops are the main disadvantages of using cover crops (Peel 1998).

14.5 Mulching

Annual cover crops are usually killed before planting a cash crop and then either incorporated as a cover crop residue into the soil or left as a mulch on the surface of the soil. A trend towards more reduced weed growth was observed where residues were not incorporated into soil but they were retained on the soil surface (Jones et al. 1999).

Mulching of the soil surface prevents weed germination by blocking light transmission, acting as a physical barrier, decrease soil temperature, and other physical properties (Teasdale and Mohler 2000). Allelopathic chemicals in the mulch also help to suppress weed emergence. Surface residues with a large number of layers and a small amount of empty internal space will be most suppressive and can reduce weed emergence up to 90% (Teasdale 2003). From barley, canola, chickpea, field pea, mung bean, and sorghum mulch, barley residues were found to be the most inhibitive (47% of the fallow treatment) (Jones et al. 1999).

Weed suppression correlate with the amount of residues. So, cover crops that produce high amounts of biomass will enhance weed suppression by leaving high amounts of suppressive residues. For example, a mixture of hairy vetch plus crimson clover (T. incarnatum L.) and rye produced higher amount of biomass and suppressed weeds more than each species in monoculture (Teasdale and Abdul-Baki 1998). Decomposition time of plant residues is another important factor of weed control by mulching. Cover crop residue that decomposes slowly will extend the period of weed suppression. The hairy vetch mulch is decomposed more rapid than rye. Therefore, rye provides longer-lasting mulch that blocks weed growth longer (Teasdale and Mohler 1993).

Weed control by mulching is effective in growing seasons without high rainfall. Under high rainfall regimes, supplemental weed management practices are needed (Barker and Bhowmik 2001). Perennial and selected large-seeded annual weeds, that are able to reproduce in cover crop mulch, should become problematic by the management. The mulches can play important role especially in weed control in no-till cropping systems (Sullivan 2003a).

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Responses

  • Albert
    Which cool season cover crops are most allelopathic?
    6 months ago
  • callum
    Is millet allelopathic?
    1 month ago

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