No-till production systems are more dependent upon herbicides for weed control, compared to conventional or reduced-tillage systems where soil is tilled in the spring prior to planting. In no-till, those weeds that emerge from the previous fall through spring must be controlled at the time of planting with burndown herbicides that are capable of doing so, such as glyphosate and paraquat (Gramoxone Extra). The weed population in the spring in no-till fields can consist of perennials (e.g. Canada thistle, dandelion), winter annuals (e.g. mustards, pennycress, chick weed), and early-emerging summer annuals (e.g. giant ragweed, Pennsylvania smartweed). If not killed at the time of planting, these weeds often become too large to be controlled by postemergence herbicides that are typically applied about three weeks after planting, and may reduce crop yield.
Broadleaf weeds emerge earlier in the spring than grasses, and application of 2,4-D with a preplant herbicide program may provide adequate burndown where grasses are not present. Several preplant herbicides have foliar activity in addition to soil activity, and can control or help control small broadleaf weeds. These herbicides include atrazine, Canopy XL, Hornet, Balance, Guantlet, Steel, Python, and Sencor. Where annual grasses more than a few inches tall are present, the burndown herbicide program should include Gramoxone Extra or glyphosate. Glyphosate should be applied in the spring if quackgrass is present. Combinations of glyphosate or Gramoxone plus 2,4-D may provide more complete control of a mixed population of weeds than either herbicide alone.
Applying preplant herbicides earlier in the spring when weeds have not emerged or are very small will minimize the need to include glyphosate or Gramoxone Extra. Early application allows more opportunity for herbicide to receive adequate rain and move into the upper few inches of soil, compared to application at planting. However, early application can result in poor late-season control of some weeds, especially giant foxtail and fall panicum, when crop growth is slow and rain is abundant in the early part of the growing season. Moving herbicide application closer to planting can prevent this problem to some extent. Preemergence herbicides applied at or after planting can provide acceptable control in no-till, but more rainfall will be needed than in a conventional tillage seedbed for activity, since herbicide must move through crop residue to reach the soil surface.
Postemergence herbicide programs seem to fit well into no-till production systems. There can be a reduction in the population of annual broadleaf weeds in no-till over time, possibly reducing the need for a broad-spectrum preplant herbicide program. The application of postemergence herbicides on an as-needed basis may ultimately result in a reduction in herbicide inputs and costs in no-till. Giant foxtail often increases in population in no-till during the first several years, but effective control will eventually reduce the foxtail population, due to a lack of seed return to the soil surface. When planning a postemergence herbicide program in no-till, use of a burndown herbicide at planting is critical for success. Do not count on postemergence herbicides to control weeds that have already emerged at planting.
A major change in weed management programs that must occur in no-till is increased attention toward control of perennial weeds, which become more prevalent and difficult to control as tillage is reduced. These can include hemp dogbane, bindweeds, milkweeds, dandelion, and other weeds. Most of these emerge fairly late in the season, so that they cannot be killed by application of glyphosate at planting. Likewise, most postemergence herbicides only suppress perennials, and this is more easily accomplished in corn than soybeans. The main key to control of perennial weeds is application of glyphosate, dicamba (Banvel), and/or 2,4-D when they are are in the bud to bloom stage, or as late in the fall as possible before the weeds senesce or growth ceases due to frost or freeze. At this growth stage, the weeds will move herbicide throughout the plant and into the roots, resulting in maximum kill of the entire plant. The best opportunity for making this type of application is during the late- summer through fall after wheat harvest when plants have grown undisturbed for several months. Including wheat in a rotation to allow fall herbicide applications will aid greatly in management of perennial weeds. Throughout the rest of the rotation, apply burndown or postemergence herbicides as necessary to at least suppress perennials, since this can keep infestations in check until a fall application can be made.
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