Knowledge and technology for weed management an historical perspective

Humans have been managing weeds for over 10000 years. The distinction between crops and weeds was one of the earliest human concepts distinguishing the beginning of agriculture (Rindos, 1984, pp. 137-43). Crops such as wheat and squash were among the first plants to be cultivated rather than simply gathered. Other species such as rye and maize were selected for deliberate planting and weeding somewhat later (Minc & Vandermeer, 1990).

Certain species, after undergoing initial domestication, subsequently lost favor as food sources and lapsed back into the weed complex or into the collected, but not cultivated, category. For example, Setaria in Mesoamerica (Minc & Vandermeer, 1990) and Chenopodium (Smith, 1992, pp. 103-32) in North America were domesticated early, but were abandoned as crops with the domestication of maize. Today species in these genera are important weeds.

Over thousands of years since the first planted fields and in diverse climatic zones, human society has continued to evolve techniques for crop production and weed control. This development has resulted in widening landscape disturbance and management, driven by increasing human population and changes in technology from stone, bronze, and iron through steel, petroleum, and computers.

Farmer and community learning has been central to the development of technology for crop production and weed control. Recent studies of farmer experimentation (e.g., Scoones & Thompson, 1994; Sumberg & Okali, 1995) indicate three important components in the farmer development of technology.

First, farmers cultivating the land year after year under diverse conditions of soils, crops, weeds, and weather accumulate a vast range of data on the effectiveness of their agricultural practices. In each field in each new planting season, farmers observe and adjust local crop production practices, although they are not conducting experiments. Although the observations in a single crop cycle only rarely lead to major changes in crop production or weed control techniques, farmers' gradual adjustments in cropping techniques over time have been the major force in the evolution of agricultural technology.

Second, farmers conduct tests or experiments in which they compare something new with their normal practices. In these experiments, farmers commonly test new physical inputs to crop production like crop species or varieties, cover crops, tools and equipment, or soil amendments. For such tests, farmers use the kitchen garden, a corner or strip offield close to their residence, or even an entire field. They may also test variations in management such as timing, spacing, or quantities of their usual inputs in a single or several seasons.

A third area in farmer technology development is the multiyear organization of the cropping system and the mix of activities that make up the farm operation. Whereas the testing of new physical inputs is primarily suited to planned experimentation, farmers use both planned testing and experiential learning under variable weather and market conditions to develop their cropping system organization and home consumption or income-generating strategies.

The rapid change in agriculture and weed control methods in Great Britain from the 17th to 19th century illustrates the central role played by farmers in the development of farming methods (Elliot et al., 1977; Pretty, 1991, 1995, pp. 181-3). These centuries were characterized by increasing private tenancy of rural lands, urbanization, population growth, and industrialization. These factors influenced agricultural practices. During this period key technologies with weed control implications contributed to the intensification of crop production. Farmers developed crop rotations that included legumes like red clover and alternated cereals with "cleaning" crops such as turnip, potato, and forage beet. Cleaning crops planted in wide rows could be cultivated more easily during the crop cycle than the traditional small-grain cereals such as wheat and barley, which were categorized as "fouling" crops. Iron and steel parts for plows, harrows, and cultivators increased both the degree and precision of soil disturbance. In the 18th century, cultivation with horses improved labor efficiency in weed control. Improved seed cleaning equipment virtually eliminated Agrostemma githago and Lolium temulentum, weeds that had been extremely difficult to remove from small grains with prior methods (Elliot et al., 1977). During this period new weeds continued to be introduced, as they had been in previous centuries. Weeds introduced in this period included Cardaria draba, Veronica persica, and Galinsoga parviflora (Godwin, 1960). Floristic composition also shifted in response to both changing weed control practices and other crop production factors. The effect of soil fertility on weed floristic composition, for example, was clearly shown in experiments at Rothamsted, England, started in 1843. In low fertility plots, species like Equisetum arvensis and Aphanes arvensis were found, whereas at intermediate and high fertility Ranunculus arvensis and Stellaria media were more frequent (Cousens & Mortimer, 1995, pp. 181-2). This suggests that the increasing use of animal manure, lime, ashes, bone meal, and green manure, as well as field drainage, also contributed to changing weed complexes.

The initial ideas for all these innovations originated with farmers and in local workshops (Pretty, 1991). Neighboring farmers and farmers from other regions observed the ideas in practice or learned of them through farmer clubs, books, and newspapers, all of which became increasingly common during the period. The number of books on agriculture, written primarily by farmers, increased from 2-10 in the last half of the 16th century to 150-400 by the beginning of the 19th century. Other farmers adapted the new ideas and reported their experiences and experiments at fairs, in printed material, and at farmer club meetings.

In the later half of the 19th century in the British Isles, Europe, and the USA, a new sector emerged in the development of agricultural knowledge. In

1843 Lawes and Gilbert, among the earliest proponents of the scientific method in agriculture, set up the first of their field experiments in Rothamsted (Johnston, 1994). Early agricultural scientists studied the effectiveness of mechanical weed control, rotations, and cover crops (Karlen et al., 1994). These concepts had been developed by farmers centuries earlier, and much of the early scientific agronomic knowledge was drawn from farmer practice.

The advent of the experiment station and the agricultural scientist brought about an important change in the development and spread of crop production technology. Whereas previously farmers were both the principal generators and users of technology, with the development of formal experimental science, a large proportion of technology generation moved off-farm. Thus, the generators and the users of technology separated into two different sectors (Busch & Lacy, 1983, pp. 5-36). Initially experiment stations maintained close links to the farm sector. However, with the emergence of disciplines within agricultural science, scientists distanced themselves from regular contact with farmers. They developed professional networks and journals to systematically document their work for their own use (Lockeretz & Anderson, 1993, pp. 26-7). In 1914 in the USA the separation in the generation and use of crop production technology was formally addressed through the establishment of the cooperative extension service under the Smith-Lever Act.

The development of herbicides, beginning in 1896 in a French vineyard with the chance discovery of the selective effects of copper sulfate on plants, further altered the relationship between weed technology generation and use. Advances in the laboratory sciences of chemistry and plant physiology led to the near simultaneous discovery in USA, England, and France of hormonal herbicides in the 1940s. This technological innovation was made with no input from farmers. Although farmers now have the choice of hundreds of different herbicides for a wide variety of crops, weed control has become a consumable, off-farm input in crop production. Herbicides must be purchased for each crop cycle. The separation between technology generation and use in the case of herbicides has been addressed by the public extension service, field sales representatives, and private crop consultants.

Farmers' intuitive understanding of weed ecology has formed part of crop production technology from the beginnings of agriculture. However, formal studies in weed ecology originated just in the past 50-75 years. Studies in weed ecology have only recently begun to affect the development of technologies for weed management. These studies have focused on the minimum weed-free period for different crops, the crop loss effects of different weed species, the dynamics of weed seed banks, and the physiology of vegetatively propagated weeds. In many cases, these concepts have been applied principally for managing herbicides. Weed ecology has also been useful in understanding why practices such as rotations, cover crops, and intercropping are effective in weed management, as other chapters in this book demonstrate.

Despite the expansion of the off-farm generation of knowledge about weed management in the past 50-75 years, farmers in temperate and tropical agriculture continue to experiment with machinery, crops, cropping systems, and farm organization for better weed control.

Much of the equipment for reducing herbicide applications, such as band applicators, wicks, and recirculating sprayers, or for combining spraying with other operations originated in farm workshops. During the 1970s and early 1980s more than 30% of the entries in the Ideas Competition at the Royal Norfolk Agricultural Show related to spraying (Sumberg & Okali, 1995, pp. 142-3).

In areas of the world where mechanization is less common, farmers face different weed problems and experiment with other methods. In Manya Krobo, a dry forest transition zone in Ghana, farmers in the past 60 years have gone from cocoa and long fallow cropping to bush fallow food cropping. More recently they have faced land and labor shortages, less reliable rainfall, and the spread of new herbaceous and woody weeds (Amanor, 1993). These include Digitaria and Panicum grasses, Chromolaena odorata, and Leucaena spp. They are currently experimenting with the conservation of tree seedlings and sprouts during weeding to reduce the invasion of savanna grasses, the use of cowpea and short-cycle cassava in different rotations with current crops to maintain productivity, and selective fallow management to promote native tree and shrub species.

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