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The Impact of Genetically Engineered Crops on Farm Sustainability in the United States Appendix B Tillage Systems Below is an outline of general tillage and weed-management practices for corn, soybean, and cotton. Tillage and weed-control practices, however, vary greatly across regions of the United States and within a region based upon grower preference, soil texture, structure, and erosion potential for each individual farm. Conventional or Conservation Tillage Pretillage – may include shredding of cornstalks, usually in the fall, shortly after harvest. Primary Tillage – types of equipment include moldboard plow, chisel plow, field cultivator, and tandem disk depending upon preference by the grower and soil erosion potential. For example, moldboard plow would be avoided if the fields are designated as Highly Erodible Land. Consequently, the chisel plow, field cultivator, or tandem disk are primary tillage implements in conservation tillage systems. Primary tillage can occur in the fall or the spring, depending on soil conditions and erosion potential of the soil. Furthermore, primary tillage may be done as a zone over the row where crops will ultimately be planted (strip tillage). Secondary Tillage – types of equipment include tandem disk, field cultivator, chisel disk, disk harrow, harrow, and other implements. The type of equipment used depends on grower preferences, machinery complement, the region of the country, soil structure and texture, soil erosion potential, and environmental conditions during the season. Secondary tillage opera-
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The Impact of Genetically Engineered Crops on Farm Sustainability in the United States tions typically occur in the spring. Secondary tillage can involve multiple passes but quite often a series of implements are pulled in tandem, making it a one-pass operation. If growers are practicing conservation tillage, more than 30 percent of residue is present on the soil surface after planting. Weed management – may or may not include a half rate or full rate of soil-applied herbicides with residual activity before or after planting (preemergence or early postemergence to the crop). Modern sprayers are typically 90 feet in width or more, and this step requires very little time and fuel use. Some growers, however, have tanks mounted on their planters and spray the residual herbicides at planting, thus saving an operation. Herbicide-resistant crops are typically treated with glyphosate postemergence to the crop and weeds and may include other herbicides with different mechanisms of action. A second (and possibly a third) application of glyphosate is typically applied, especially in cotton and soybean. If one or more weeds have become resistant to glyphosate or the farmer’s herbicide of choice, then the grower may apply a substitute herbicide or use a tillage operation to control the weed. Organic growers substitute tillage (cultivation) operations, typically three or more, for herbicides for weed control. The first operation is usually performed just after corn or soybean has emerged, using a rotary hoe. This step is usually followed by two cultivations, the first occurring when corn or soybean is quite small. No-Till Pretillage – some growers may shred cornstalks. Weed management – some growers will use a nonselective ”burn-down” herbicide, such as glyphosate, to kill existing weeds if they are present before planting. Farmers may also wait until after planting for the initial herbicide application. This treatment may include a combination of herbicides that provide residual control of weeds that emerge later in the season. Postemergence applications of herbicides may be used later in the season depending on the weed infestation. Planting – involves more sophisticated versions of the planters used for conventional tillage. Typically, the planters have heavy coulters and other attachments to clear the residue from the previous crop in the planting row. Not all soils are suitable for no-till, especially wet and heavy soils in northern latitudes, and no-till may lead to increased pest occurrence because conventional tillage may reduce insect, pathogen, and weed occurrence. On the other hand, no-till is well adapted to well-drained soils in warm regions because no-till improves soil-water infiltration and reduces soil evaporation, thereby providing more soil water to the crop.