Characterizing 2,4-D and dicamba residue persistence following tobacco flue-curing
Genetically modified field crops expressing tolerance to auxin herbicides, such as 2,4-D and dicamba, are commonly grown in fields adjacent to flue-cured tobacco in North Carolina. The potential exists for physical drift of spray solutions containing those herbicides onto tobacco. The persistence of 2,4-D and dicamba residues during the flue-curing process is not currently known. To answer this question, a 12.5 % dose of 2,4-D (133 g a.e. ha-1) and dicamba (71 g a.e. ha-1) was applied to flue-cured tobacco 17, 7, 3, and 0 days before harvest. Treated tobacco was flue-cured in independent curing chambers for 144 hr to prevent cross-contamination. 2,4-D residues were not detected in fresh or cured leaves when treatment occurred 17 days before harvest. Tobacco treated with 2,4-D 0 days before harvest resulted in greater 2,4-D residue in fresh leaves (4.13 mg kg-1) compared to three days before harvest (2.08 mg kg-1). Treatments delivered seven days before harvest resulted in a fresh leaf 2,4-D concentration of 0.02 mg kg-1, which was > 100-fold reduction when compared to 0 and 3 days before harvest. 2,4-D concentrations in cured leaf were 0.19 and 0.08 mg kg-1, when treated 0 and three days before harvest, respectively, and 2,4-D was not detected in cured leaves treated seven days before harvest or in fresh- or cured-leaf treated 17 days before harvest. Dicamba residue was not detected in fresh- or cured-leaves treated 7 or 17 days before harvest. Tobacco treated 0 days before harvest with dicamba resulted in a concentration of 1.40 mg kg-1 in fresh leaf which was a two-fold increase compared to tobacco treated three days before harvest (0.69 mg kg-1). Dicamba concentrations in cured leaf were < 0.015 and 0.06 mg kg-1 when treated 3 and 0 days before harvest, respectively. Ultimately, cured leaf residues of 2,4-D and dicamba were reduced by > 20-fold within each treatment interval when compared to fresh leaf samples. Our results suggest that cured leaf residues of 2,4-D or dicamba are unlikely to be detected from drift events occurring > 7 days before harvest. In contrast, physical drift events taking place 0 to 3 days prior to harvest could result in cured leaf levels of 2,4-D or dicamba that are more likely to be detected, depending upon the sensitivity of the instrumentation and analytical equipment used in testing laboratories; however, it is unlikely that they will exceed the current CORESTA Guidance Residue Level (GRL) (0.2 mg kg-1).