ASSESSING SENSITIVITY OF HORSERADISH PLANTS TO DICAMBA AND 2,4-D IN NEW SOYBEAN PRODUCTION SYSTEMS

Collinsville, Illinois is the leading producer of horseradish is the nation. The river bottom geography surrounding Collinsville, Illinois near St. Louis, Missouri is a high-production area for horseradish. The development of soybean technologies resistant to dicamba or 2,4-D may allow horseradish growers to gain control of troublesome weeds, such as Palmer amaranth (Amaranthus palmeri) or volunteer horseradish, but could pose risks as well. Drift of these two herbicides or carryover to horseradish could cause severe injury and possible crop loss. While synthetic auxin-tolerant soybean may also allow growers to control volunteer horseradish, herbicide efficacy may differ depending on the volunteer horseradish variety. These risks and benefits could affect the adoption rate of these new soybean technologies in horseradish production areas. A field trial was established in 2015 in Edwardsville, IL and 2016 Medora, IL to simulate drift of both dicamba and 2,4-D onto horseradish. Applications were made in horseradish to mimic drift of a mid-post emergence application in soybean onto the horseradish crop. Plants were monitored for injury and stand, height, and yield reductions throughout the season. Individual roots were evaluated post-harvest. Overall, 2,4-D caused more injury at all rates when compared to dicamba. Horseradish growers may see yield reductions if rates at or greater than 1/1000X of a field rate of 2,4-D drift onto their fields. Not planting horseradish near a 2,4-D-tolerant soybean field, as well as reading the herbicide labels and following application requirements, should help growers prevent serious injury and yield loss. On the other hand, rates of 2,4-D at or above a full field rate offered complete control of all plants; therefore, growers who struggle with persistent volunteer horseradish could rotate to a 2,4-D-tolerant soybean and gain needed control of those plants. Field experiments were conducted in 2014, 2015, and 2016 to investigate the impact of dicamba residues following applications in a dicamba-tolerant soybean crop on horseradish planted the following season. Carryover trials were conducted as two-year rotations of soybean followed by horseradish in Collinsville, Illinois. Multiple rates of dicamba were applied at several timings in dicamba-tolerant soybean and the crop was monitored for injury. The following season horseradish was planted and monitored for injury and stand, height, and yield reductions. No injury or reductions were observed with any treatment in either year, potentially indicating a lack of dicamba remaining in the soil. Horseradish plant stand counts, height as well as yields were not reduced when compared to the nontreated. Results from this experiment suggest that rotating from dicamba-tolerant soybean to horseradish should pose no threat to horseradish. Greenhouse experiments were carried out in 2016 in three separate runs. Each run consisted of three replications of five varieties of horseradish, 604, 788, 9705, Hungarian, and V7E3, and two rates of dicamba, glyphosate, and dicamba plus glyphosate . Plants were sprayed when at least one plant in each pot had reached a height of 17 to 23 cm. Horseradish was then rated for injury at 3, 7, 14, 21, and 28 days after treatment (DAT). Heights were also taken at 0, 14, and 28 DAT. At 28 DAT plants were harvested, weighed and place in a dryer for 72 hours and weighed again. The lowest level of injury at 28 DAT was with variety 604. The control of horseradish roots is critical to ensure the plant is killed completely and does not return the following season as a volunteer. The concerns associated with auxin-tolerant crops can be mitigated with proper management of herbicides and crop locations. While off target movement of 2,4-D may cause damage to a horseradish crop, it could be used as a herbicide to control volunteer horseradish. Additionally, if a grower chooses to use a dicamba-tolerant soybean variety, they may have the choice to use a dicamba plus glyphosate premix which will also give good control of volunteer horseradish with little concerns of dicamba carryover to the subsequent horseradish crop. Capitalizing on the strengths and weaknesses of each technology will help horseradish growers manage many weeds and facilitate the production of this important specialty crop