The agroecosystem redesign approach is characteristic of ecological weed management and involves a shift from linear, one-to-one relationships between target weeds and a particular weed management tactic, to webs of relationships between weeds, multiple weed management tactics, and other farming practices (Liebman & Gallandt, 1997). Emphasis is placed on preventing weed problems and reducing requirements for purchased inputs through better use of ecological factors that stress and kill weeds. Emphasis is also placed on integrating weed management activities with other farming practices that maintain soil productivity and crop health, minimize the impacts of other pests and unfavorable weather, and reduce financial risks (Swanton & Murphy, 1996).
Exner, Thompson & Thompson (1996) have described an Iowa crop and livestock farm that uses an agroecosystem approach to manage weeds successfully. Most of the 125-ha farm is in a five-year rotation sequence (maize-soybean-maize-oat + clover-and-forage-grasses-hay) that challenges weeds with varying patterns of soil disturbance and resource competition. A rye cover crop, planted between the maize and soybean phases of the rotation, is used to allelopathically suppress weed emergence and growth. The ridge tillage system used for maize and soybean production kills weeds within the crop row at planting, but also minimizes soil disturbance and stimulation of weed germination before the crops are sown. A rotary hoe and an inter-row cultivator designed for high-residue conditions are also used to control weeds mechanically. Crops are planted at higher than conventional rates to increase crop competitive ability.
The owner-operators of this farm are willing to apply post-emergence herbicides if other tactics fail to provide sufficient weed control, but generally have not needed to do so. They monitor weeds in their fields, tinker with and improve machinery and other components of their farming system, conduct field day tours, exchange information with other farmers, and participate in collaborative research projects with scientists at Iowa State University and other institutions (Chapter 3) (Harp, 1996; Thompson, Thompson & Thompson, 1998). Experiments conducted on the farm have shown that its weed management system protects maize and soybean from weed competition as effectively as conventional herbicide-based systems (Exner, Thompson & Thompson, 1996; Thompson, Thompson & Thompson, 1998). Production costs on the farm are lower than for conventional farms in the area, but crop yields are as high (National Research Council, 1989, pp. 308-23).
Combinations of ecologically based weed management tactics are also used effectively by certain farmers in the Canadian prairie provinces and the American northern plains states. Matheson et al. (1991) and Hilander (1997) described crop and livestock farms in the region that range from 400 to 1100 ha and operate profitably with little or no herbicide use. Insect herbivores and mixed stocking of sheep with cattle are used to suppress perennial weeds in grazing lands. In arable fields, weeds are managed through the use of higher seeding rates, competitive crop varieties, pre- and post-emergence cultivation, and crop rotation. Various annual crops (e.g., wheat, barley, oat, lentil, pea, flax, buckwheat, and sunflower) are grown in sequence or in mixture with perennial or biennial forage legumes (e.g., alfalfa and sweet clover). In some cases, short-duration legumes (e.g., pea and lentil) are included in rotations as green manures. Where possible, fall-sown crops are alternated with spring-sown crops. As discussed in Chapter 7, sequences and mixtures of diverse crops can help to prevent the proliferation of adapted weed species by challenging them with complex sets of stress and mortality factors.
Additional options have been proposed for ecological weed management in the Canadian prairies and American northern plains. Derksen, Blackshaw & Boyetchko (1996) suggested that increased use of residue-conserving tillage techniques in concert with moderate use of herbicides may improve habitat for insects, fungi, and bacteria that attack weed seeds and seedlings. The investigators noted that there is considerable potential to minimize herbicide use in conservation tillage systems through improved use of crop rotations, competitive cultivars, and crop densities and fertilizer placement strategies that enhance crop competitive ability against weeds.
Lightfoot et al. (1989) described the use of the agroecosystem redesign approach by a group of farmers and scientists in the Philippines seeking to manage the perennial grass Imperata cylindrica. To begin the process, group meetings and individual farm visits were used to facilitate discussions and identify impacts of different cropping systems, soil fertility practices, burning and cultivation regimes, and various socioeconomic issues, such as land tenure, cash requirements, labor availability, and family health. These discussions led to recognition of several interrelated factors that favored the proliferation of I. cylindrical (i) there was minimal soil cover on many fields because of residue burning and intensive tillage; (ii) the lack of soil cover was exacerbated by low fertility due to continuous cropping and erosion; and (iii) I. cylindrica seeds blew in from surrounding fallow areas, and germinated easily and grew well in bare soil.
In further discussions, plowing and herbicides were deemed inappropriate options for dealing with I. cylindrica because cash reserves were insufficient to hire draught animals or purchase chemical inputs. Labor availability was also identified as a key constraint. Several of the farmers had observed, however, that the weed was effectively suppressed when shaded by vigorously growing vines.
Farmers in this group then visited field experiments and demonstration sites where rapidly growing legume cover crops were being tested by research and extension workers for their ability to improve soil fertility and provide erosion protection. The farmers discussed the weed-suppression potential of the various legume species they saw and chose several (Pueraria, Centrosema, and Desmodium spp.) with which to conduct trials on their own farms. Seven months after the initial discussions, 31 farmers in the group had begun experiments testing legume cover crops for soil improvement and I. cylindrica suppression. The farmers, with some assistance from researchers, made measurements of weed and cover crop performance, and visited experiments on other farms. The scientists collected information on labor inputs. All the information collected was discussed by group participants.
The story is incomplete, in that Lightfoot et al. (1989) did not describe how introduction of cover crops ultimately affected I. cylindrica management. What does emerge, however, is that by participating in the problem-solving process as partners, both the farmers and scientists improved their capacity for decision-making and future interactions. The farmers gained a better idea ofhow their farming systems functioned, what they wanted, and what their options were for achieving their goals. The scientists were better able to produce and refine a relevant research and extension agenda. These types of interactions are as important in industrial countries as they are in developing countries and are examined more thoroughly in Chapter 3.
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