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farmer interactions farmer interactions size, and production technology. However, they are similar in aiming for greater land, labor, and capital productivity through flexible management based on the quantification of local weed variability (Table 3.2). In each case, farmers are trying to make better decisions by tailoring practices to weed patchiness rather than using routine uniform practices. Decisions about what practices to use are based on field-to-field and within-field monitoring for timely matching of practices to weed composition and patches.

Precision agriculture employs computerized spatial information for crop management (Lass & Callihan, 1993; Roberts, Rust & Larson, 1995; National Research Council, 1997, pp. 26-43). In this approach, real time yield monitoring on the harvester is connected with satellite-linked global positioning and geographic information systems to produce a detailed yield map that can be overlain on detailed soil maps. Variable-rate seeders and fertilizer applicators make possible the within-field fine-tuning of seed and fertilizer rates according to soil production potential. The more precise use of inputs may be directed to reduced environmental pollution from fertilizers or to further increase yields (Blackmore et al., 1995). Realization of either goal is subject to weather unpredictability. For example, Jaynes & Colvin (1997) showed that yields from specific localities in a field were below average one season and above average in others. This complicates the fine-tuning of input levels, since weather may be difficult to predict.

Weed observations can also be incorporated into the data system to locate problem areas for additional observation from one crop to the next. Variablerate applicators permit fine-tuning herbicide applications to soil type or patches of particular weed species. Sprayer prototypes controlled by weed sensors have also been developed to apply post-emergent herbicides only where weeds are present (Thompson, Stafford & Miller, 1991). Patch spraying has been calculated to save from 9% to 97% in herbicide use, compared to field-wide application for the control of the perennial weed Elymus (Elytrigia) repens in cereal grains in England (Rew et al., 1996). Little saving occurred when weed patches were extensive, a wide buffer was sprayed at the patch edges, and the areas below threshold were sprayed with a lower herbicide dose. High savings resulted when weed patches were few and concentrated, no buffer at the patch edges was sprayed, and the areas below threshold were not sprayed.

The second approach, also in mechanized agriculture, builds on the multiple interactions among diverse living organisms and the physical environment in a crop sequence to minimize the impact of weed variability and uncertainty. Weeds are managed by manipulating a diversity of factors that negatively affect weed population dynamics and favor the crop over the weed in crop-weed interactions. Non-noxious weed complexes are maintained at below-threshold levels through crop rotation, cover crops, timely tillage and cultivation, crop residue management, choice of crop varieties, and other tactics. The ecological approach focuses on the design of multiple-year cropping systems that suppress weeds rather than directed, short-term control of weed patches. This approach is well illustrated by the study of potato-based rotations under three contrasting weed management treatments (conventional, reduced input, and mechanical) and two soil managements (Gallandt et al., 1998) described in Chapter 5. In the southern Brazilian state of Santa Catarina, thousands of farmers routinely use a diversity of green manures either intercropped with main crops or as covers during fallow periods to prevent soil erosion, suppress weeds, reduce weeding costs, and build soil tilth (Bunch, 1993). Green manure management is mechanized with animal-drawn implements, which flatten standing cover crops, conserving the cut biomass on the soil surface, and clear a narrow furrow for planting. Since 1987 the combination of green manures, animal manures, and soil and moisture conservation has produced yield increases of over 65% for maize and soybeans. Labor costs for weeding and plowing have also declined. As both the potato-based system and the minimum-till green manure system show, the mechanized ecological approach is cumulative over seasons and buffered against weed patchiness and uncertainty by crop vigor, low weed levels, and the use of a diversity of practices.

A third approach applies the ecological cropping systems practices mentioned above like green manures, intercropping, and rotations, but uses hand tools, sometimes animal power, and only occasionally, if at all, tractor-drawn implements. This represents an extension of traditional smallholder agricultural systems, which are well suited to the localized management of weed patchiness. The close interaction between mental and manual labor and the speed at which work takes place permits continuous interpretation of field and crop conditions and simultaneous adjustment of how each practice is carried out. Farmers hand-weeding a field can observe local variation in both crop stand and weed severity and simultaneously customize their management plan to patchiness. Roguing primarily noxious weeds before they produce seed, planting cover crops in large gaps in the crop stand, and selectively applying mulch in potentially severe weed patches are examples of mosaic weed management in response to weed patchiness.

Crop production based on the deliberate and opportunistic adaptation to site heterogeneity still characterizes some indigenous agriculture (Richards, 1985; Salick, 1989) and home gardens, but has diminished among many small farmers who frequently use uniform, field-wide practices in spite of high within-field variability. Their ability to innovate has been overloaded in many rural communities by rapid social and economic displacement, shrinking farm size, and accumulated land degradation (Blaikie, 1987, pp. 117-37). In addition, local perspectives on how to innovate in crop production and weed control have been sidetracked by input-linked credit programs and promotion activities of the commercial input sector (van der Ploeg, 1993).

Bean production in Central America in a slash-mulch short-fallow rotation illustrates the management of vegetation heterogeneity for improved cropping. In this system bean seed is thrown into standing one- to three-year fallow vegetation, which is then slashed as mulch to promote bean germination and control weeds. No further weeding is used. This system is low cost, has low labor requirements, and is soil conserving, but has also been criticized as low yielding (Thurston et al., 1994). Farmers using this system readily identified good (e.g., Ageratum conyzoides, Melinis minutiflora, Melanthera aspera) and undesirable (e.g., Rottboellia cochinchinensis, Pteridium aquilinum) fallow species (G. Melendes, unpublished data). A shift to tillage and fertilizer and pesticide inputs increased cropping frequency and yields, but cost more for inputs, required more labor, and was not feasible on sloping lands. Upgrading fallows by planting patches of Tithonia diversifolia or vining legumes like Canavalia ensiformis decreased less desirable vegetation, increased fallow biomass and nutrient content of the mulch, and also increased bean yields up to 50% (G. Melendes, unpublished data). Upgraded fallows of mixed vegetation had higher bean yields with less pest damage than pure improved fallows, suggesting that farmers should introduce certain higher-biomass-producing species in patches without eliminating selected resident fallow species.

Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

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