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Off-Target Dicamba Movement Is Occurring in Illinois, Too

Physical Drift, Temperature Inversions, Dicamba Residues in Sprayers Are Suspects in Off-Target Dicamba Movement.

Most articles dealing with dicamba damage focus on off-target movement in Missouri and Mid-South states like Arkansas and Tennessee. Damage is popping up in more northern states like Illinois, though. 

“Instances of soybeans demonstrating symptoms of exposure to dicamba have greatly increased over the past two weeks, and it’s nearly certain the number of affected acres will continue to rise,” writes Aaron Hager in a column appearing in this week’s edition of The Bulletin, a growing-season web newsletter from the University of Illinois. 

Below are some thoughts Hager has on the subject. For the full column, go to http://bulletin.ipm.illinois.edu/?p=3942.

It’s Been Around for Nearly 50 Years

Some might be surprised to learn that instances of soybean exposure to dicamba have been an annual occurrence in Illinois since dicamba was first commercialized almost 50 years ago. One of the first experiments that described soybeans’ sensitivity to dicamba was conducted by Loyd Wax at the University of Illinois in 1966-1967. The stated objective of the experiments “. . . was to determine the response of soybeans to soil and foliar applications of dicamba, picloram, and 2,4-D to assess the potential hazard of using these herbicides in crops in rotation with soybeans and in areas adjacent to soybean fields.” 

The symptoms of soybean exposure to dicamba described by these researchers 50 years ago are nearly identical to those currently being observed.

Increased plantings of glyphosate-resistant corn hybrids in Illinois during the first decade of the 21st century was accompanied by decreasing dicamba use in corn. This led to relatively few complaints of soybean exposure during the last 10 years. 

With more dicamba currently being applied, it’s not surprising the instances of soybean exposure have increased. Whether applied in corn or dicamba-resistant soybeans, the fact remains that few dicot species in the Illinois landscape are more sensitive to dicamba than soybeans.

Symptoms of Exposure

Confusion exists regarding exposure symptoms to dicamba compared with leaf symptoms caused by nondicamba factors. Wax and his colleagues described the effects of dicamba on soybean leaves as “. . . cupped and crinkled,” which are terms still commonly used today. Other factors can cause leaf distortions, but Hager says he’s not aware of anything other than dicamba that causes the following symptoms collectively:

  • Extreme cupping of trifoliolate leaves. These are most pronounced on the upper trifoliolates. 
  • Veins of affected leaves tend to assume a parallel orientation, instead of the usual net venation pattern. These plants may sometimes remain stunted the remainder of the season.
  • Depending on time and dose of exposure, pod development can be adversely affected.
  • Tips of cupped leaves with parallel veins are often brown or cream color.
  • Plants are stunted as compared with plants not demonstrating the aforementioned symptoms.

Explanations? Everything/Everyone Except the Russians – So Far

While the symptoms of exposure to dicamba are apparent, identifying how the exposure occurred is not always obvious. Speculations and explanations from some industry personnel have included almost everything except the Russians. (Stay tuned – that one might be next.) 

Soybeans’ extreme sensitivity to dicamba sometimes complicates accurately identifying the source of exposure. Recent research published by Kevin Bradley, University of Missouri Extension weed scientist, indicated symptoms of exposure to dicamba could be induced at 1/20,000 of a 1x (0.5 lb. ae/A) field-use rate. Additionally, symptoms generally do not develop immediately after exposure. We have observed instances where 21 days elapsed between exposure and symptom development.

Possible routes of exposure include:

  • Physical drift of spray particles during the actual application. This route of exposure might be the easiest to identify based on field observations. Symptoms are usually most pronounced along the edge of the field adjacent to the drift source and lessen as the distance from the source increases. Remember, the symptoms of exposure to dicamba depend largely on the dose. Symptoms are different on soybeans directly sprayed with dicamba (often dead plants) compared with soybeans exposed to a very low concentration (leaf cupping, etc.) farther from the source. Exposure from physical drift has been observed this year, but it does not appear to account for the majority of off-target exposure instances to date.
  • Residues remaining in/on the spray equipment from previous applications are applied at low concentrations with the postemergence soybean herbicide. These symptoms are often most pronounced around the perimeter of the field and along the edge where the applicator began spraying the remainder of the field. Symptoms often become less pronounced as the sprayer moves farther across the field away from the side where the application began. Contamination has been touted by some as an explanation for cupping of Liberty Link varieties, but it seems odd that it hasn’t been mentioned much as an explanation for cupping of Roundup Ready varieties.
  • Herbicide vapors on the plant or soil surface move out of the treated field (vapor drift). The volatility of a herbicide (i.e., tendency to change from a liquid to a gas) is a function of several factors related to the formulation of the herbicide and to prevailing environmental conditions. Vapor pressure is a measure of the tendency of a herbicide to volatilize. As the vapor pressure of a herbicide increases, the potential for volatility also increases. Volatility tends to increase as soil moisture and temperature increase. As soil moisture decreases, the amount of herbicide adsorbed to soil colloids can increase and reduce the amount of herbicide available to volatilize. All commercially available formulations of dicamba have the potential to volatilize.
  • Applications made during temperature inversion conditions. Small droplets can remain suspended in the air when pesticides are applied during temperature inversion conditions. These particles then move out of the target area when winds begin to move the following morning. Where and how far they move depend primarily on wind direction and speed. Labels of dicamba-containing products approved for in-crop application to dicamba-resistant soybeans restrict applications during temperature inversions. Some have speculated that applications made at night (when inversions occur) have been responsible for off-target damage, but does anyone have actual data on how many acres are treated when headlights are needed on the applicators?
  • Leaf distortions. There has been much conversation about leaf symptoms that likely were not caused by exposure to dicamba. As mentioned previously, symptoms of dicamba exposure can vary according to the dose of exposure and stage of soybean development. However, the symptoms of low-dose exposure tend to be fairly consistent. Can other herbicides cause leaf distortions? Yes, but these symptoms are different from those caused by dicamba. Foliar-applied PPO inhibitors can cause leaf distortions, but the degree of cupping is generally much less than that caused by dicamba, and the symptoms appear on leaves treated with the application. In contrast, cupping caused by dicamba is generally seen on leaves that emerge after the exposure occurred. We also have observed distorted leaves following postemergence application of soil-residual herbicides, but these symptoms are very different from those caused by dicamba.

Effects on soybean yield

If cupped soybean plants were actually exposed to dicamba, will yield be adversely effected?   

It’s absolutely impossible, at this point of the season, to predict whether or not yield will be impacted. Published literature suggests this injury does not always result in soybean yield loss, but several factors are involved in determining if yield loss will occur.  

In particular, soybean growth stage at the time of exposure, dosage of exposure, and growing conditions for the remainder of the growing season are important factors that determine if yield loss does or does not occur.  

Much of the available literature suggests that if minor exposure occurs during early vegetative development, yield loss is less likely to occur than if exposure occurs when soybeans have entered reproductive development. However, there are no data that describe yield effects on soybeans exposed to dicamba more than once.
 

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