Successful production of full-season, no-till soybeans and double-crop, no-till soybeans depends on control of existing vegetation (cover crops and weeds) and broadleaf and grass weeds that emerge after planting. A diversity of herbicides and cover crop/residue situations makes it impossible to utilize a single program to efficiently control weeds and grasses in all situations. Herbicide selection based on weed histories of each farm or field is necessary to achieve weed-free high-yield soybeans.
Existing vegetation is traditionally controlled by the nonselective herbicides, glyphosate (Roundup Weather Max, Touchdown Total, Touchdown HiTech, and others) or paraquat (Gramoxone Inteon) in no-till soybeans. If in-crop applications of glyphosate are planned, consider use of an alterative burn down program to reduce the potential for development of tolerant or resistant species. Two paraquat applications may be required for satisfactory control of some species. More recently, 2,4-D, Authority MTZ, Harmony Extra, Canopy, Canopy EX, Sonic/Authority First, Sencor and Valor XLT were labeled for control of broadleaf weeds prior to planting full-season, no-till soybeans. In double-crop, no-till soybeans planted into barley or wheat stubble, Linex/Lorox or Sencor plus adjuvants have controlled small broadleaf weeds without the addition of nonselective herbicides. Annual grass control in no-till soybeans can be obtained with preemergence applications of MicroTech, Outlook, Dual II Magnum/Cinch, Define, Prowl and Prowl H2O, or with any of several postemergence grass herbicides. Stem brittleness and lodging can be associated with Prowl when soil conditions are cool and wet; these conditions can occur most frequently in early planted, full-season, no-till soybeans.
Annual broadleaf weed control can be achieved with preemergence or postemergence broadleaf herbicides. Because of the diversity of species in many fields, it is frequently necessary to apply postemergence herbicides to control weeds that escape preemergence herbicides.
Escalating herbicide costs and our interests in keeping herbicide use to a minimum are encouraging many people to consider use of only postemergence herbicides. Our experience leads us to suggest that existing vegetation always be controlled prior to planting. Preemergence or postemergence herbicides can then be selected that will control weeds that emerge after planting. Reducing or eliminating all or some of the preemergence herbicides can reduce costs in fields that historically have needed postemergence herbicides. This approach might first be tried in double-crop, no-till soybeans where weed populations have traditionally been low and then expanded to full-season, no-till soybeans.
