Successful control of weeds emerged at planting is a critical component of weed management in no-till systems. A variety of products are available for this task; selection should be based primarily on weeds present (species and size) and cost.
Residual herbicides with foliar activity. Triazine herbicides (atrazine, Bladex, Canopy, Lexone/Sencor and Preview) and urea herbicides (Lorox and Lorox Plus) have sufficient foliar activity to control small weeds present at planting. These products generally will control broadleaf weeds less than 3 inchews in height and grasses less than 1 inch. Addition of a crop oil concentrate or surfactant, or use of liquid nitrogen as a carrier will improve foliar activity. If larger weeds are present, Roundup or Gramoxone Extra should be included.
Roundup. Roundup rates are determined by species to be controlled, weed size, and application parameters (carrier, tank mix partners). Application alone in water allows the use of lower rates on larger weeds. Roundup is more active on grasses; thus, 1 pt/A 2,4-D is typically added to improve broadleaf control at the lower range of Roundup rates (16 oz). Although the Roundup label does not specify a delay between application and planting, ISU recommends separating these operations by at least one day to avoid lack of performance due to the mechanical stress placed on weeds by the planter. The potential impact will be greater with a no tillage soybean drill than if soybeans are planted in wider rows. If 1 pt 2,4-D LVE is used with Roundup for burndown in soybeans, a 7-day delay between application and planting is required. Longer intervals are required with higher 2,4-D rates or if amine formulations are used.
Gramoxone Extra. Rates are determined primarily by weed size: 1.5 to 2.0 pts/A are used for most weeds 1 to 3 inches in height. The addition of a triazine will improve control of smartweed, horseweed and larger annual weeds. A minimum of 20 GPA carrier is required; do not use on weeds larger than 6 inches. As with Roundup, no time delay is specified between application and planting. The quicker action of Gramoxone Extra may somewhat reduce the risk of decreased weed control due to disturbance caused by the planter, but ISU recommends separation of these practices, if possible.