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Allelochemicals: Biological Control of Plant Pathogens and Diseases, 31 -78. © 2006 Springer. Printed in the Netherlands.
agriculture, contributing to the dramatic increase in crop yields achieved in recent decades for most major field, fruit, and vegetable crops.
Farmers must contend with approximately 80,000 plant diseases, 30,000 species of weeds, 1,000 species of nematodes, and more than 10,000 species of insects. Today, national and international agricultural organizations estimate that as much as 45 percent of the world's crops continue to be lost to these types of hazards. In the United States alone, about $20 billion worth of crops (one-tenth of production) are lost each year (IFIC 2004).
The word "pesticides" refers to a broad class of crop protection chemicals, including four major groups: insecticides, rodenticides, herbicides, and fungicides. All pesticides must be toxic, or poisonous, to kill the pests they are intended to control. Because pesticides are toxic, they are potentially hazardous to humans and animals. Therefore, people who use pesticides or regularly come in contact with them must understand the relative toxicity and potential health effects of the products they use (Pesticide Education Program. Penn State's College of Agricultural Sciences, 2004). Some pesticides (administered at extremely high dosages) have been found to cause cancer in laboratory animals (Agri 21FAO).
Weeds represent the most serious threat to the sustainability and profitability of agricultural production around the world considering these facts:
■ Weeds represent the most economically serious pest complex reducing world food and fiber production.
■ Controlling weeds costs the United States economy more than $15 billion annually, surpassing the combined cost of controlling disease and insect pests. U.S. herbicide expenditures account for 85% of all pesticide purchases and 62% of the total amount of active ingredient applied.
■ Nearly all (greater than 95%) of the corn and soybean acreage in the U.S. receives herbicide applications. In developing countries, the cost of weed control in terms of labor and loss of yields is even greater, proportionally, than in the U.S. Worldwide herbicide purchases ($16.9 billion in 1997) constitute 58% of all pesticides bought and 53% of the amount of active ingredient applied (1 billion kg a.i.) worldwide.
Crop yield losses from weed competition can be substantial. The degree of loss depends on, crop and weed species present; timing and duration of competitive interactions; and resource availability (Agri 21 FAO*, IFIC 2004**).
Worldwide, the competitive effect of weeds causes a 10% loss in agricultural production. Yield losses in rice and other grass crops in West Africa have been reported to range from 28-100% if weeds such as witchweed (Striga hermonthica) -a parasitic weed-are not controlled; the greatest reductions occur on nutrient-poor soils. Left unchecked, weeds cause dramatic reductions in food production that eventually can
* Agri 21 FAO: Agriculture 21. IPM and Weed Management. Food and Agriculture Organization of the United Nations (FAO). Agriculture Department. 2004. http://www.fao.org/WAICENT/FAOINFO/AGRICULT/ Default.htm.
** IFIC 2004 : International Food Information Council. 2004. Agriculture & Food Production. Background on Agriculture & Food Production. http://www.ific.org/food/agriculture/index.cfm.
destabilize economic and social systems. Hence, there is an urgent need to develop and refine weed management strategies in crop production that are effective, safe, and economically viable.
Regardless of cropping system or agricultural region of the world, effective weed management strategies are needed continually to maintain crop yields and crop quality as well as to reduce the negative impact of weeds in future years. This ongoing quest to optimize weed management strategies is largely because of the ability of agricultural weeds to adapt to many of our most important crop production systems (Agri 21 FAO, IFIC 2004).
Integrated Pest Management (IPM) is a system that works in partnership with nature to produce foods efficiently (Upadhyay et al., 1996). The concept began in U.S.A. in the 1950s, and recently resurfaced in popularity. Although many definitions of IPM have been advanced, two elements are critical:
■ using multiple control tactics;
■ integrating a knowledge of pest biology into the management system.
IPM involves the carefully managed use of an array of pest control techniques including biological, cultural, and appropriate chemical methods to achieve the best results with the least disruption of the environment. With IPM, growers are adopting less chemically intensive methods of farming, which may include pest-resistant plant varieties, adjustments in planting times, low tillage, and other non-chemical techniques. The objectives of IPM are:
■ Appreciate the importance of controlling weeds within an integrated pest management program.
■ Understand the key biological differences that make IPM for weeds more difficult than IPM for insects.
■ Become familiar with integrated weed management (IWM) strategies.
The United States Department of Agriculture (USDA) proposed national standards for organic farming and handling in 1997.The federal regulations for organic standards were finalized in 2001 and began full implementation in 2002. Generally, organic food is produced by farmers who emphasize the use of renewable resources and the conservation of soil and water to enhance environmental quality for future generations (IFIC 2004, Agri 21 FAO).
Barberi (2002), assessed that despite the serious threat which weeds offer to organic crop production, relatively little attention has so far been paid to research on weed management in organic agriculture, an issue that is often approached from a reductionist perspective. Compared with conventional agriculture, in organic agriculture the effects of cultural practices (e.g. fertilization and direct weed control) on crop-weed interactions usually manifest themselves more slowly. Weed management should be tackled in an extended time domain and needs deep integration with the other cultural practices, aiming to optimize the whole cropping system rather than weed control per se. Many organic farmers are aware that successful weed management implies putting into practice the concept of maximum diversification of their cropping system. However, this task is often difficult to achieve, because practical solutions have to pass through local filters (soil and climate conditions, availability of and accessibility to external inputs -seeds, crop cultivars, machinery, etc.) and socioeconomic constraints (market, tenure status, attitude towards entrepreneurial risk, etc.). The use of cover crops and organic amendments, via the promotion of diversity in insect, fungal, bacterial or mychorrhyzal communities, may alter antagonist or competitive effects to the benefit of crops and to the detriment of weeds. Once factors driving these effects are better understood, it might be possible to use this knowledge to improve organic weed management systems locally. It would also be helpful to find indicators of "functional biodiversity", where weed species abundance is assessed on the role that they have in the agroecosystem (e.g. strong /weak competitors, promoters of the presence of beneficial arthropods, etc.). Management of allelopathy is another potential tool in the arsenal of the organic farmer (Barberi, 2002). In the United States, the rate of increase of organic growers was estimated at 12% in 2000. However, many producers are reluctant to undertake the organic transition because of uncertainty of how organic production will affect weed population dynamics and management. The organic transition has a profound impact on the agroecosystem. Changes in soil physical and chemical properties during the transition often impact indirectly insect, disease, and weed dynamics. Greater weed species richness is usually found in organic farms but total weed density and biomass are often smaller under the organic system compared with the conventional system. The improved weed suppression of organic agriculture is probably the result of combined effects of several factors including weed seed predation by soil microorganisms, seedling predation by phytophagus insects, and the physical and allelopathic effects of cover crops (Ngouajio and McGiffen, 2002).
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