Soybean foliage residues of dicamba and 2,4-D and correlation to application rates and yield

Plant growth regulator (PGR) herbicides dicamba (3,6-dichloro-2-methyloxybenzoic acid) and 2,4-D [(2,4-dichlorophenoxy)acetic acid] can severely injure soybean [Glycine mar (L.) Merr.] by drift or tank contamination and reduce yield. Often in regulatory disputes, tissue is analyzed for PGR residue. However, relationships between grain yield reduction and foliar residue concentrations at various times after exposure are not well documented. This 2-yr study quantified the amount of dicamba and 2,4-D in soybean foliage 0, 6, 12, 24, and 48 d after treatment (DAT) when treated with 1 to 20% of 0.56 kg a.e. ha(-1) [labeled rate for corn (Zea mays L.)] at the three-leaf (V3) stage of growth and determined if these concentrations were correlated to initial application rate or grain yield. Herbicide concentrations were determined using gas chromatography/mass spectrometry techniques with selective ion monitoring. Visual symptoms were slight (<10%) to severe (90%) and included leaf cupping, epinasty and, in some cases, death of the apical bud. Grain yields from dicamba-treated plants were reduced from 14 to 93% compared with untreated plant yield, whereas only 2,4-D at the highest rate reduced yield. In both years, foliar residue concentrations were correlated with initial application rates and yield reduction up to 24 DAT for dicamba and 12 DAT for 2,4-D, with all treatments having residue amounts similar to untreated plants after these intervals. The data suggest that plant samples should be collected as soon as possible after suspected PGR exposure for accurate detection and quantification of PGR residue.