Dicamba and 2,4-D Utilization in Growth Regulator Resistant Soybean

Research was conducted to evaluate weed management practices in 2,4-D resistant and dicamba resistant soybean. Dicamba resistant and 2,4-D resistant soybean will allow for the use of an additional herbicide site of action for control of troublesome broadleaves such as Palmer amaranth, tall waterhemp, horseweed, and giant ragweed. The longevity and utility of 2,4-D resistant and dicamba resistant soybean will depend on how the crops and their corresponding herbicides are managed and integrated into a weed management strategy. The movement of herbicides from the application site to sensitive vegetation as well as selection of herbicide resistant weeds from repeated herbicide use are two factors that are likely to decrease the longevity and utility of these crops. Studies were conducted to investigate the influence of broadcast spray nozzle design on the deposition, absorption, and efficacy of postemergence applied herbicides. When evaluating droplet deposition in a narrow row soybean canopy, nozzle design did not influence herbicide coverage at the ideal height of weeds. Coverage in a narrow row soybean canopy was influenced by spray volume with the 140 l ha-1 volume resulting in greater coverage than the 94 l ha-1 volume. Deposition of glyphosate plus 2,4-D or dicamba postemergence onto target weeds resulted in equivalent depositions on the leaf surfaces between two traditional flat fan nozzles and two air induction drift reduction nozzles. Herbicide efficacy was only influenced by nozzle design when applications were made to high-density stands of weeds or when the weeds exceeded the recommended heights for postemergence application. Results from these investigations showed applications of 2,4-D and dicamba herbicides with the required low drift nozzles can provide control of troublesome broadleaf weeds when making applications following a residual herbicide to appropriately sized weeds with spray volumes greater than 94 l ha-1 . Research experiments were conducted over four growing seasons to evaluate the influence of herbicide use patterns on weed species density and diversity and herbicide resistance selection pressure. Herbicide Strategies that relied on glyphosate plus dicamba or 2,4-D with minimal use of residual herbicides resulted in greater weed densities and number of weed species at postemergence herbicide applications, indicating a significant increase in herbicide resistance selection pressure. The dicamba or 2,4-D reliant strategies also resulted in shifts towards monocot species both in field observations as well as soil seed bank evaluations with fall panicum becoming the predominate species. Integrated strategies that included soil residual herbicides reduced both dicot and monocot species abundance and a predominate weed species was not apparent after four years. The ultimate longevity of the 2,4-D resistant and dicamba resistant soybean systems will depend on the implementation of best management practices. The use of air induction nozzles for herbicide applications in this system will reduce the potential for off-site movement thus retaining or building a positive rapport amongst the agriculture community and general public. When used appropriately the air induction nozzles provided effective delivery of the herbicide to the target weeds and thus efficacy of the herbicides were not reduced. The useful life of these technologies can be lengthened by implementing herbicide strategies that contain multiple herbicide sites of action, soil residual herbicides, and incorporate cultural weed management practices to supplement herbicides