Dave Pike, Ph.D.
It seems that every few years Midwest agriculture experiences crop injury resulting from misapplication of herbicides, as growers learn ways to adapt to the changes in technology. This year, numerous cotton fields across the South and soybean fields throughout the Midwest show evidence of dicamba injury. A number of factors appear to have contributed to the reported injury. While some injury incidents appear to be related to using the newer, low-volatile formulation of dicamba, others may be related to misapplication of regular formulations of dicamba in fields with Xtend soybeans that have been modified to be tolerant to the herbicide but have drifted to non-tolerant crops. To understand how those factors contribute to the explosion of reported injury cases in 2017 requires a bit of an explanation.
Dicamba injury to soybean and cotton, from either drift of droplets or drift of dicamba vapors, is most damaging in the late vegetative and early reproductive stages. In past years, dicamba applications were generally applied early in the season and before the soybean and cotton crops reached those sensitive stages. As a result, drift of dicamba typically had little effect on crop yields. However, in 2017, the new Xtend soybeans, which are tolerant to dicamba, now allow applications later in the season. This increases the potential for dicamba to drift to nearby non-tolerant crops in sensitive reproductive stages. The weather we have had in 2017 in many areas may also have contributed to the rash of reported incidents. Late planting and persistent rain and storms have plagued growers, and many have applied herbicides in less-than-ideal wind and weather conditions.
Right now, growers with fields showing injury are scratching their heads trying to figure out the source of their problems. Although the symptoms of dicamba injury to soybeans and cotton are relatively easy to diagnose, the source of the errant dicamba can be difficult to determine. Injury from dicamba can occur as a result of either small spray droplets drifting off target during the initial application, or from volatilization of the herbicide after it has been applied. Determining the origin of droplet drift is often easy. Small spray droplets are typically deposited in a pattern that follows prevailing wind and topography. It will show a reduction in concentration and severity as distance from the point of application increases.
The origin of crop injury from volatile molecules of dicamba is usually harder to identify. Molecules of dicamba can arise from a field within a day or two after it has been applied and drift hundreds of yards before settling over a sensitive crop. Sometimes an entire field shows symptoms and there’s little evidence that would indicate from which direction the volatile components came. A weather phenomena known as inversion, where cooler air is trapped beneath a layer of still, warm air, is also known to facilitate drift of volatile dicamba formulations. Inversions prevent the volatile components from rising into the atmosphere and mixing and diluting itself with the air above. Complicating the identification of the source of the droplets or volatile compounds are two additional factors. The first is that dicamba is systemic (acts within the plant), and it can take 7 to 21 days for symptoms to show up. So, you might have to look into the history of herbicide applications on nearby fields and go back a couple of weeks to identify potential offenders. Additionally, it only takes a very small amount of dicamba, often less than 1/1000th of a pound of active ingredient, to result in significant yield reduction of a crop like soybeans, which are very sensitive to it. This may have us wondering if perhaps the sprayer tank may not have been sufficiently cleaned out.
As of this writing, Arkansas has halted dicamba applications and Missouri and Tennessee have restrictions where applications can only be made within very tight parameters. Other states to the North are sure to follow suit as soybeans advance in maturity in those regions and increase in sensitivity. Growers in these regions would do well to learn from the experience of their southern neighbors: follow the label closely, watch your weather, and scout your fields regularly. If you notice injury symptoms, take careful notes and good photos, as documentation will be critical if you have cause for making a claim for yield loss.
*The Spray Smart feature in Agrible’s Morning Farm Report software provides detailed wind and weather information to help growers keep their products on target. It can also help a grower identify nearby non-tolerant crops that may need special concern when applying products that may drift as droplets or as volatiles. Get this information on the go with the Pocket Spray Smart app. It’s free to download and shows spraying conditions for your current location. See conditions for all your fields when you’re linked to your Morning Farm Report account.