Introduction

Agriculture is the process of managing plant communities to obtain useful materials from the small set of species we call crops. Weeds comprise the "other" set of plant species found in agroecosystems. Although they are not intentionally sown, weed species are well adapted to environments dominated by humans and have been associated with crop production since the origins of agriculture (Harlan, 1992, pp. 83-99).

The ecological role of weeds can be seen in very different ways, depending on one's perspective. Most commonly, weeds are perceived as unwanted intruders into agroecosystems that compete for limited resources, reduce crop yields, and force the use of large amounts of human labor and technology to prevent even greater crop losses. In developing countries, farmers may spend 25 to 120 days hand-weeding a hectare of cropland (Akobundu, 1991), yet still lose a quarter of the potential yield to weed competition (Parker & Fryer, 1975). In the USA, where farmers annually spend $6 billion on herbicides, tillage, and cultivation for weed control (Chandler, 1991), crop losses due to weed infestation currently exceed $4 billion per year (Bridges & Anderson, 1992).

At the other end of the spectrum, weeds can be viewed as valuable agroeco-system components that provide services complementing those obtained from crops. In India (Alstrom, 1990, pp. 25-9) and Mexico (Bye, 1981; Mapes, Basurto & Bye, 1997), farmers consume Amaranthus, Brassica, and Chenopodium species as nutritious foods before crop species are ready to harvest. In western Rajasthan, yields of sesame and pearl millet can be increased by allowing the crops to grow in association with the leguminous weed Indigofera cordifolia (Bhandari & Sen, 1979). Certain weeds may limit insect damage to crops by interfering with pest movement or by providing habitat for natural enemies of pests (Andow, 1988; Nentwig, Frank & Lethmayer, 1998). Weed species can reduce soil erosion (Weil, 1982), serve as important sources of fodder and medicine (Datta & Banerjee, 1979; Chacon & Gliessman, 1982), and provide habitat for game birds and other desirable wildlife species (Sotherton, Rands & Moreby, 1985; Sotherton, Boatman & Rands, 1989). These types of beneficial effects indicate that weeds are not just agricultural pests, but can also play beneficial roles in agroecosystems.

In this chapter, we outline the objectives of weed management systems and then discuss how weeds are managed conventionally. We follow with a discussion of why alternatives to conventional management strategies are needed. Finally, we suggest how a broad range of ecological processes and farming practices might be exploited to manage weeds more effectively, while better protecting human health and environmental quality, and potentially increasing farm profitability. In subsequent chapters, we will examine these ecological processes and farming practices in more detail.

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