Allelopathy control, weeds

The presence of crop residues has been reported to both increase (5, 6) and decrease crop yields ( 7) and not tilling to increase certain difficult to control weeds (8). However, other reports indicate that the presence of certain mulches can reduce the biomass of certain weeds (9-15) and allow for higher crop yields ( 5, 6). Thus, under certain conditions, mulches can suppress certain weed species, but determining the reason(s) presents many logistical problems, especially under field conditions. To determine the cause(s), the physical and chemical (i.e., allelopathy) effects of the mulch and the role of soil disturbance (or the lack of, as would be the case in a no-till system) must be separated. 

Hence, allelopathy alone can not be a sufficient tool for weed control. Combinations of methods that prevent weed germination and control weeds have to be used (Rasmussen 2004). 

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). 

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). 

These days, allelopathic plants as catch crops or trap crops found utilization in plant protection of tropical regions against parasitic weeds. They do not eliminate the parasite completely but decrease the seed bank in the soil. Other applications of allelopathy for weed control include the use of plant residues as a natural herbicide agent, e.g., water extracts, pellets, flours, by-products of crop processing, etc. The strategies for use of volatile compounds as soil fumigants are developed. 

In we management systems involving allelopathy, crop varieties may be screened or new varieties developed for their potential for controlling weeds. Such varieties may be left as residues in the field, or be incori rated in every rotation system, and/or used as a companion crop. Similarly, if crop varieties allelopathic to pathogens can be found, their residues can be used similarly for disease control. Research groups of Putnam at Michigan State University and of Gliessman at the University of California are two of many that are involved in research of this kind. 

Herbicides. The idea of using allelopathy in weed control was conceived in the late seventies (121. and several workers have considered this possibility, as already mentioned (13-161. However, characterization and possible use of allelochemicals as selective herbicides received attention only recently, and our demonstrations of the selective herbicidal activity of caffeine are among the pioneer ones (17-211. 

In this chapter the different roles that allelopathy can play as a bioregulator tool in agriculture are discussed. A wide spectrum of studies are given on allelopathic plants and other organisms, the chemistry involved in these studies, the mechanisms of action of some allelochemicals, and the use of allelopathy to control weeds, pests (nematodes) and diseases. 

ALLELOPATHY FOR WEED CONTROL IN AQUATIC AND WETLAND SYSTEMS... 

Although much of the published information on allelopathy deals with the toxic effects of compounds, it is also possible that some of the secondary plant products released into the environment possess growth-promoting properties, thereby stimulating the growth of plants. They may help to control weeds, plant diseases, and many pests. It is estimated that in the United States alone the losses caused by weeds, diseases, and pests are about 30% of the potential agricultural production. 

Putnam AR, DeFrank J, Barnes JP (1983) Exploitation of allelopathy for weed control in annual and perennial cropping systems. J Chem Ecol 9 1001-1010 Putnam AR, Tang CS (1986) The science of allelopathy. WUey, New York, NY Qasem IR, Hill TA (1989) On difficulties with allelopathy methodology. Weed Res 29 345-347 Reader RJ (1991) Control of seedling emergence by ground cover a potential mechanism involving seed predation. Can J Bot 69 2084-2087... 

Putnam, A.R., J. Defrank J.P. Barnes. 1983. Exploitation of allelopathy for weed control in annual and perennial cropping systems. J. Chem. Ecol. 9 1001-1010. 

Murphy, S.D. (1999). Is there a role for pollen allelopathy in biological control of weeds. In Allelopathy Update, (Ed., S.S. Narwal) Vol. 2 321-332. Science Publishers, Enfield, Plymouth, USA. 

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... 

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.) R 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). 

Pollen allelopathy can find utilization in field cultivations that could contain pollen of allelopathic crops or weeds. Pollen allelopathy could be an effective method for annual weed control that reproduce, at least in part, via wind pollination and flower concurrently with the allelopathic species. The effects of allelopathy should result in the loss of genetic variation and so in reduction of reproductive ability, but some plants are probably able to detoxify the pollen allelochemicals (Murphy and Aarssen 1995a, b). Murphy and Aarssen (1989) suggested possible delaying of weed flowering at later, less favorable times of the season or diurnal period, so decrease in weed pressure. However, infestation by perennial weeds can worsen due to compensation of pollen allelopathy through increase in the formation of rhizomes. 

Allelopathic compounds act as repellents for herbivorous pests, so the same strategy used in weed control could be effective against pests and pathogens. Only allelopathy is not possible to use the complete control of weeds, pests or diseases it is necessary to combine it with other methods of plant protection. 

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