Dicamiba antagonizes grass weed control with imazethapyr by reducing foliar absorption

Greenhouse and laboratory studies were conducted to determine the effects of tank-mixing the sodium salt of dicamba (Na-dicamba) with imazethapyr on the efficacy and foliar absorption of imazethapyr, applied with non-ionic surfactant (MS) or methylated seed oil (MSG), by shattercane, giant foxtail, and large crabgrass. The effects of various salt formulations of dicamba and the addition of ammonium sulfate on efficacy, C-14-absorption and on foliar spray retention by the same species were also evaluated. Na-dicamba antagonized imazethapyr efficacy by reducing C-14-absorption. Using MSO instead of NIS prevented antagonism when Na-dicamba was applied at 70 and 140 g/ha and reduced the severity of the antagonism at greater application rates by greatly increasing C-14-absorption compared to NIS. Reductions in C-14-absorption and spray retention were due to the salt formulations of dicamba rather than the parent acid. The addition of ammonium sulfate prevented dicamba antagonism of imazethapyr toxicity to grassy weeds by maintaining C-14 foliar absorption and spray retention at normal levels.