Dicamba Translocation in Soybean and Accumulation in Seed

Abstract The dicamba-resistant cropping system was developed to be used as a tool to control multiple-resistant weed species, particularly Palmer amaranth (Amaranthus palmeri S. Watson). However, dicamba applications have resulted in off-target movement of the herbicide to susceptible neighboring vegetation, with frequent damage to non–dicamba resistant soybean [Glycine max (L.) Merr.]. Pod malformation and subsequent auxin-like injury to progeny is common when parent soybean plants are exposed to the herbicide post-flowering. Yet no publication to date has conveyed the presence of dicamba in seed. The objective of this study was to determine whether dicamba exists and at what quantities inside soybean seed following a low-dose exposure in the pod-filling stage using radiolabeled herbicide as a tracer. Non–dicamba resistant soybean plants were grown in the greenhouse until the pod-filling growth stage and then treated with 2.8 g ae ha–1 of dicamba (1/200 of the recommended rate of 560 g ae ha–1). Immediately afterward, [14C]dicamba (approximately 6.4 kBq per plant) was applied to the adaxial surface of one trifoliate leaf located in the midportion of each plant. The greatest amount of [14C]dicamba recovered was in seeds and in pods, and these plant parts accumulated 44% and 38% of the total absorbed, respectively. Chromatography results showed that the totality of the [14C]dicamba present in the soybean seeds was in the phytotoxic form, except for a single sample, in which one metabolite was detected (possibly 5-hydroxy dicamba). Precautions should be taken to avoid dicamba exposure to sensitive soybean fields, especially those dedicated to seed production, as this may result in low seed quality and symptomology on progeny plants.

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