Soybean response to sublethal doses of 2,4-D and dicamba

The rise of glyphosate resistant weeds across the Cornbelt and Southeast US has increased interest in the next generation of herbicide resistant crops - those resistant to the Group 4 herbicides (growth regulators). One valid concern with these crops is the potential for injury to non-target plants via off-target movement or contaminated sprayers. This paper will briefly review results of recent research investigating the response of soybean to low concentrations of 2,4-D and dicamba. One of the more interesting papers was written by Dave Mortensen’s group at Penn State (Egan et al. 2014). They conducted a metaanalysis using earlier research to generate dose response curves for the two herbicides. As one would expect, soybean was more sensitive to dicamba than 2,4-D (Table 1). The authors used rates of 5.6 and 56 g/ha to represent 1 and 10% exposure from an application of 0.5 lb ae/A of each product {100 g ae/ha is equivalent to approximately 3 oz of a 4 lb/gal formulation of 2,4-D or dicamba (Clarity)}. There was little yield response to 2,4-D at either rate, whereas dicamba caused nearly 50% yield loss at the 10% rate. The yield loss from exposure to 5.6 g dicamba was twice as great when the soybean were exposed during flowering compared to vegetative stages, whereas there was little difference in yield loss when soybeans were exposed at flowering.

Table 1. Predicted soybean yield loss from 2,4-D and dicamba. Egan et al. 2014.

Researchers in North Carolina (Johnson et al. 2012) evaluated the yield response of soybean to 2,4-D and dicamba at rates ranging from 0.5X to 0.002X the label rate. A total of four experiments were conducted. Dicamba caused a significant yield loss at half of the locations when applied at 8 g/ha (0.03X), whereas this frequency of response was seen with 270 g/ha (0.5X rate) of 2,4-D (Table 2). The maximum yield loss observed with 2,4-D was 65%, whereas yield losses up to 100% were reported with dicamba.

Table 2. Response of soybean to different doses of 2,4-D and dicamba. Johnson et al. 2012.

Finally, researchers in Indiana and Illinois (Robinson et al. 2013) evaluated the response of soybean to an amine formulation of 2,4-D at five locations. They found that soybean were more sensitive to 2,4-D at the V5 growth stage than at either the V2 or R2 stage (Table 3). Regression analysis predicted a 5% yield loss at 87 g 2,4-D/ha at the V5 stage, whereas 116 g/ha would be required to cause a similar yield loss at the earlier or later growth stage.

Table 3. Response of soybean yield to 2,4-D exposure. Robinson et al. 2013.

They reported that visual injury was correlated with yield loss. For example, 35% visual injury 14 days after exposure resulted in a 10% yield loss. However, Johnson et al. (2012) stated that injury symptoms due to dicamba were a moderate predictor of yield response. Visible injury symptoms caused by 2,4-D or glufosinate were poor indicators of yield effects due to exposure to those herbicides. Using injury symptoms to estimate yield loss is complicated by several factors: 1) the relationship between visual injury and yield loss varies depending on crop growth stage at exposure, 2) the relationship varies depending on the time interval between exposure and injury evaluation, 3) visual estimates of injury are highly subjective and likely to vary among persons evaluating the damage, and 4) exposure of the crop is rarely uniform across a field.

This research documents the sensitivity of soybean to Group 4 herbicides, especially dicamba. While not included in this discussion, visible injury symptoms occurred at much lower rates than required to reduce yields. It is important to remember that the sensitivity of plant species varies to the two herbicides. While soybean are more sensitive to dicamba than 2,4-D, other plants in the landscape may be more susceptible to 2,4-D (e.g. grapes). Thus, similar precautions to avoid off-target movement should be taken regardless of the herbicide being used. The labels of products registered for use on these crops will have specific recommendations designed to reduce off-target movement of these herbicides. Good stewardship and common sense will be required to minimize non-target injury associated with the increased use of these products following introduction of Enlist and Roundup Ready Xtend crops.

References
Egan, F.E., K.M. Barlow, and D.A. Mortensen. 2014. A meta-analysis on the effects of 2,4-D and dicamba drift on soybean and cotton. Weed Sci. 62:193-206.

Johnson, V.A., L.R. Fisher, D.L. Jordan, K.E. Edmisten, A.M. Stewart and A.C. York. 2012. Cotton, peanut, and soybean response to sublethal rates of dicamba, glufosinate, and 2,4-D. Weed Technol. 26:195-2006.

Robinson, A.P., V.M. Davis, D.M. Simpson, and W.G. Johnson. 2013. Response of soybean yield components to 2,4-D. Weed Sci. 61:68-76.

Prepared by Bob Hartzler