Utility and Stewardship of Dicamba In Dicamba Tolerant Cropping Systems.

New weed control options are needed to manage a growing weed resistance problem that is limiting control tactics and in some areas cropping options.  Glyphosate is an important herbicide in many cropping systems, but problematic weeds like Palmer amaranth (Amaranthus palmeri), waterhemp (Amaranthus tuberculatus), giant ragweed (Ambrosia trifida), and horseweed (Conyza canadensis) have been confirmed resistant to it in at least 24 states.  And many of these populations are also resistant to more than one herbicide mode of action.  Given the limited herbicide options in many cropping systems, these weeds present significant management problems for producers.  The dicamba tolerant cropping system will offer growers a new weed management option in cotton (Gossypium hirsutum) and soybean (Glycine max).  Dicamba complements the weed control spectrum of glyphosate and controls many broadleaf weeds that have been reported to be resistant to glyphosate.  However, proper implementation of the dicamba tolerant cropping system is required to ensure its long term sustainability.  As part of an integrated strategy, one should consider several stewardship tactics to address weed resistance management and on-target deposition.  Weed management programs should consider an integrated system using multiple herbicide modes of action, effective rates and timings, and site monitoring as well as mechanical weed control when necessary.  Maximizing on-target deposition can be addressed with formulation and application techniques including nozzle selection, boom height, and spray pressure.  Environmental conditions such as wind and inversions also have significant influence on the level of on-target deposition and need to be considered before application.  The goal of such a stewardship program is to allow growers to maintain flexibility and control of their farming operation.  A training and education program can assist growers in achieving this goal.  An improved formulation, optimized application techniques, and integration of other effective weed control tactics like alternate modes of action, tillage, and crop rotation will ultimately provide the most sustainable production system.