Except direct use of allelopathic crops as cover crops, smother crops, and intercrops, applications of allelopathy for weed control include the use of allelopathic residues as an herbicide agent, e.g., pellets flours, water extracts, etc. The most common example of crop residue utilization is application of straw on the soil surface (mulching), e.g., rice straw inhibited germination of field bindweed (Convolvulus arvensis L.), winter wild oat (Avena ludoviciana Durieu), and little-seed canarygrass (Phalaris minor Retz.) (Lee et al. 1991; Tamak et al. 1994; Young et al. 1989).
The effect of applied plant residues can be positively influenced by an increase of temperature. Mallek et al. (2007) established that dried and milled crop residues of onion (Allium cepa L.) or garlic (A. sativum L.) were able to reduce seed germination of barnyard grass (E. crus-galli (L.) P. Beauv.), common purslane (Portulaca oleracea L.), london rocket (Sisymbrium irio L.) during their decomposition in soil, but only at the elevated (39°C) soil temperature. It can support combination of methods for weed control, in this case allelopathy with soil heating treatments (e.g., solarization).
Plant material processed into pellets allows easier application and measuring. Alfalfa pellets (commercial forage fodder) were effective as a natural herbicide against Echinochloa oryzicola (Vasinger) Vasinger, Digitaria ciliaris Pers., Cyperus difformis L. and Monochoria vaginalis Kunth in rice paddy fields incorporated at 1-2 Mg ha-1 (Xuan and Tsuzuki 2001). Xuan and Tsuzuki (2004) suggested similar use of buckwheat pellets (at same dosage). The early incorporation of buckwheat pellets into the soil provides great weed control in rice (Xuan and Tsuzuki 2004).
As herbicide agents, by-products of crop processing are possible to use too, e.g., Japanese farmers use rice bran (200 g m-2) for weed control and fertilization on transplanted rice (Kuk et al. 2001). Maize gluten meal, a by-product of maize milling process, has been patented as a natural preemergence herbicide (Christians 1993). The maize gluten meal contains chemicals (five dipeptides, Gln-Gln, Ala-Asn, Ala-Gln, Gly-Ala, and Ala-Ala) that inhibit root growth of germinating weeds but does not damage roots of mature plants (Christians 1993; Liu and Christians 1994).
Medium-grain fatty rice bran was the best material for reducing weed emergence (Palmer amaranth Amaranthuspalmeri S. Wats. and ivyleaf morningglory Ipomoea hederacea Jacq. followed by sicklepod Senna obtusifolia (L.) H. S. Irvin & Barneby, hemp sesbania Sesbania exaltata (Raf.) Cory and prickly sida Sida spinosa L.) in the minimum effective rate 250 g m-2 as preplant incorporated or preemergence in the stand of cotton and maize (Kuk et al. 2001).
Boydston et al. (2008) found that dried distiller maize grains with solubles, a by-product of ethanol production, may be useful for reducing the emergence and growth of common chickweed (Stellaria media (L.) Cirillo) at concentration of 5%, annual bluegrass (Poa annua L.) at concentration of 10% or more, in container-grown ornamentals (Rosa spp., Phlox paniculata L., Coreopsis auriculata L.). Amounts of 800 and 1,600 g m-2 applied to the surface of transplanted ornamentals decreased the number of annual bluegrass (Poa annnua L.) (48%) and common chickweed (Stellaria media (L.) Cirillo) (46%) without injury to ornamentals.
However, not all suggested products are effective. For example, application of Indian mustard (B. juncea L.) and field pennycress (Thlaspi arvense L.) seed meals did not decrease the grassy weed population (Sams et al. 2007).
Extracts are an example of the traditional use of allelopathic plant material. The water extracts of, e.g., sorghum, sunflower, brassica, sesame, eucalyptus, tobacco, etc., contain a number of allelochemicals which are more effective to the weed control (Cheema et al. 1997; Rizvi et al. 1989; Daury 2002; Cheema et al. 2003; Jamil 2004; Anjum and Bajwa 2005). A water extract of mature sorghum plants obtained after 24 h soaking in water, called Sorgaab, is used as a natural herbicide. Cheema and Khaliq (2000) reported that Sorgaab reduced weeds from 35% to 49%. This agent is possible to use alone or in combination with other water extract, e.g., with sunflower water extract or eucalyptus extract. A big challenge is the utilization of the water extracts of medical plants. Aqueous extracts (1-8%, w/v) of the dried powders of terrestrial saururaceae (Houttuynia cordata Thunb.) inhibited the germination and initial growths of Echinochloa spp. and Monochoria sp. in rice paddy field. All the treatments (25-150 g m-2) did not have negative effects on rice, therefore this plant could be used as a natural herbicide to weed control in rice (Lin et al. 2006). Nazir et al. (2007) found that aqueous extracts of rhubarb (Rheum emodi Wall.), Saussurea lappa C.B.Clarke, and Potentilla fulgens Wall. Ex Hook reduced significantly growth of cockscomb (Amaranthus caudatus L.) and finger millet (Eleusine coracana Gartn.). The extracts of lemon balm (Melissa officinalis L.) shoots inhibited the germination and the growth of pendant amaranth (Amaranthus caudatus L.), cress (Lepidium sativum L.), hairy crabgrass (Digitaria sanguinalis (L.) Scop.), timothy grass (Phleum pratense L.), and Italian ryegrass (L. multiflorum Lam.) under laboratory conditions (Kato-Noguchi 2001). Volatile compounds from Eucalyptus exserta F. Muell. and E. urophylla S. T. Blake reduced the seedling growth of Raphanus sativus L., Lactuca sativa L., Leucaena leucocephala (Lam.) de Wit and Acacia mangium Willd.; volatiles from leaves of Eucalyptus citriodora Hook. inhibited weeds like Bidens pilosa L., Digitaria pertenuis Buse, Eragrostis cilianensis (All.) Vignolo ex Jauch., Setaria geniculata (Lam.) P. Beauv (Shiming 2005). Strategies for using the essential oils from plants such as Mentha spp., Satureja montana (Roy) O. Bolos & Vigo, and Ocimum spp. as soil fumigants are developed (Birkett et al. 2001).
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