Herbicide Residues in Soybean Following Drift

Drift of herbicides onto adjacent fields is a common occurrence in modern agriculture. Most incidences are settled without bringing in outside persons; however, occasionally involved parties are unable to an amicable agreement . In these situations it is not uncommon to have plants analyzed to confirm the presence of herbicide. While these tests can provide useful information, results must be interpreted carefully in order to avoid making improper conclusions.

Researchers at the University of South Dakota investigated the response of soybean to low doses (simulated drift) of dicamba and 2,4-D (Anderson et al. 2004). Soybean were treated at the V3 stage and evaluated for visual injury, persistence of residues in shoots, and yield impact. Partial results of the studies are presented in Table 1. The lowest rate of dicamba evaluated (0.16 oz/A) caused 40% visual injury whereas the highest rate of 2,4-D (3.2 oz/A) caused 35% injury. This injury was described as causing malformation and growth suppression of two terminal leaves. Dicamba caused a 40% reduction in soybean biomass 24 days after application (DAA), whereas 2,4-D caused a 30% reduction. Soybean yields were reduced 14% by dicamba, whereas 2,4-D caused a 32% yield loss.

Six plants from each plot were harvested at the soil line on several dates and kept frozen until analysis for herbicide residue using GC/MS. Herbicide concentrations within the plant decreased fairly rapidly. Within a week of application the concentration of dicamba and 2,4-D declined by 80 and 88% of the levels present immediately after application, respectively. By 24 DAA herbicide residues were 1% of that found on the day of application. The decline in herbicide concentration in the soybean shoot could be due to herbicide degradation, dilution within the plant due to soybean growth, or translocation of the herbicide to the roots.

No injury symptoms were observed on soybean in the control plots, however, these plants contained small amounts of each product (Table 1). Ten foot buffer strips (4 rows) were maintained between plots to reduce the likelihood of movement of the herbicide from the treated plots. The authors did not speculate whether the herbicide residues in the control plants were due to applications used in this study or drift from outside sources.

Table 1. Soybean injury and persistence of herbicide residues in soybean foliage following exposure to simulated drift. 
(DAA = days after application).

10.16 oz dicamba = 1% of 1 pt Clarity; 3.2 oz 2,4-D = 20% of 1 pt 2,4-D.

The results of this study demonstrate some of the difficulties associated with interpreting results of plant analysis used to document herbicide drift. First, residues of both 2,4-D and dicamba were detected in plants from control plots that were not sprayed. No symptoms of growth regulator herbicides were observed on these plants. This illustrates the importance of identifying all possible sources of herbicide trespass. Second, concentration of both herbicides in soybean tissue declined rapidly following exposure. Even though the rates reported here were sufficient to cause significant yield losses, dicamba concentrations in treated and untreated soybean were similar by 12 DAA. This illustrates the importance of collecting plant samples as close to the date of exposure as possible. Finally, most cases of growth regulator herbicide injury involve concentrations that do not impact soybean yield. The low levels of exposure in these situations may be difficult to detect reliably.

Reference:
Anderson, S. M., S.A. Clay, L.J. Wrage and D. Matthees. 2004. Soybean foliage residues of dicamba and 2,4-D and correlation to application rates and yield. Agronomy Journal. 96:750-760.

Related articles:
Plant growth regulator injury to soybean, University of Illinois

Prepared by Bob Hartzler, extension weed management specialist