Answer: Crop rotations limit the buildup of weed populations and prevent major weed-species shifts. Weeds tend to prosper in crops that have requirements similar to their own. Fields of annual crops favor short-lived annual weeds, whereas maintaining land in perennial crops favors perennial weed species. In a crop rotation, the timing of cultivation, mowing, fertilization, herbicide application, and harvesting changes from year to year. Rotation thus changes the growing conditions from year to year—a situation to which few weed species easily adapt. Rotations that include clean-cultivated annual crops, tightly spaced grain crops, and mowed or grazed perennial sod crops create an unstable environment for weeds. Additional weed control may be obtained by including short-season, weed-smothering cover crops such as sorghum-sudan or buckwheat. Crop rotation has long been recognized for its ability to prevent weeds from developing to serious levels.
Incorporating crops with allelopathic effects into the rotation adds another element of control. Such crops include sunflowers, sorghum, and rapeseed. Allelopathic plants are those that inhibit or slow the growth of other nearby plants by releasing natural toxins, or "allelochemicals." Weed control ability varies among varieties and management practices. Sweet potatoes have been shown to inhibit the growth of yellow nutsedge, velvetleaf, and pigweed. Field trials showed a 90% reduction of yellow nutsedge over two years following sweet potatoes.
There are other beneficial approaches to weed management, as well, such as mulching, competition, crop rotations, and low-toxicity control alternatives. You can learn more about these in the ATTRA publication Sustainable Weed Management for Small and Medium-Scale Farms, which discusses several strategies, both proactive and reactive, as alternatives to conventional tillage systems.
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