By Mark Loux and Bill Johnson
You would probably have to be living under a rock to not at this time be aware of the issues with off target dicamba movement affecting soybeans and other plants in the states of Tennessee, Arkansas, and Missouri. The latter two states just banned any additional dicamba applications for the remainder of the growing season to avoid additional problems (subject to change probably), and some changes are coming in Tennessee also apparently. We have seen firsthand examples of this in at least some Indiana and Ohio fields, and have heard about a number of additional ones. It’s somewhat difficult to gauge how widespread the issue is, since there is often reluctance of an affected party to contact regulatory officials and file a complaint, in order to keep good relations with the offending neighbor. This has been a trend over the years where applications of dicamba-containing products to corn have affected nearby soybeans – neighbors tolerating each other – partly based on the knowledge that soybean yield often appears to be unaffected by early-season dicamba exposure(see b).
Our conclusion at this point based the fields we have examined is that the patterns of injury are indicative of both particle drift and volatility. However, an alarmingly high number of fields seem to show that we have more offsite movement due to volatility than we thought would happen based on past experience with dicamba use in corn and the development of lower volatility formulations of dicamba products labeled for use in Xtend beans. This is not to say that spray particle drift is not occurring. It is evident that in many situations, dicamba is being applied in too much wind, or with no buffer left between the treated field and adjacent non-Xtend soybeans. We certainly have had the “perfect storm” of limited days to spray with wet weather delaying field operations. When soil conditions were suitable for sprayer traffic, the winds were often excessive and we likely had herbicide applied during inversions as we rushed to get work done. However, with such an abundance of fields that show uniform symptomology across the entire field, we wanted to raise awareness of this situation and attempt to explain drift vs volatility. There is certainly a lot more to learn about how volatile these new products are under varying conditions. We would most likely expect some changes in how they can be used between now and next year, at least in certain states. The purpose of this article is to discuss particle drift versus volatility, and what history tells us about volatility and symptom development for dicamba products that have some volatility, and also the effect of exposure on soybean yields. This information may be helpful in the assessment of situations where dicamba injury occurs.
In some fields that we have examined, the symptoms of dicamba on sensitive soybeans have occurred at far greater distances, and at much more uniformity, than can be explained simply by spray particle drift. Spray particle drift has a telling pattern, which most anyone in the industry has observed at one time or another for various herbicides. The dosage and symptoms in an adjacent sensitive crop are greatest closest to the treated field, due to the highest frequency of larger spray droplets settling out fairly rapidly. For this reason, one indicator of spray particle drift is herbicide symptomology on weeds growing along an adjacent roadside or in a fencerow between the two fields. The injury then tapers off with distance from the treated area as a decreasing number of smaller droplets continues to settle out, until the point where no injury occurs due to insufficient number of droplets and dosage to cause injury. How sensitive the affected crop is comes into play here also, since it takes a lower dosage to cause injury on a more sensitive crop. Spray droplets can move well into an adjacent field, depending upon wind, temperature, nozzles, pressure, use of drift-reducing agents, etc.
But particle drift does not result in the relative uniformity of dicamba injury over a large adjacent field that has occurred in some cases. This would be more indicative of movement via dicamba volatilization from leaf or soil surfaces, occurring sometime within several days after application. Vapors then move with prevailing air currents, with potential to move far greater distances than spray particles, upwards of a half mile. Movement of vapors does not require much wind. For example, volatilization of dicamba that occurs under relatively still inversion conditions can result in prolonged suspension and movement of vapors with gentle air currents. In one field we looked at, there appeared to be an initial volatilization event from the adjacent dicamba-treated soybeans, with some subsequent soybean recovery. This appeared to followed by a second round of dicamba exposure and injury to the recovering soybeans several weeks later.
Soybeans may not show symptoms of dicamba until 10 to 21 days following exposure, when the injury becomes evident in newest growth. Injury takes the form of leaf wrinkling and cupping, and new leaves trying to expand emerge may remain tightly cupped and small. Higher doses can cause terminal growth inhibition (shorter plants) that are slower to cover the row middles. As soybeans recover, new growth will eventually emerge without symptomology. The ability of soybeans to recover from injury, the rate of recovery, and effect of yield is dependent upon dosage and subsequent environmental conditions, and obviously whether they are exposed to dicamba again while trying to recover. Exposure to dicamba in the vegetative stages has less long-term effect and potential to reduce yield compared with exposure in the reproductive stages. Our experience with injury during the vegetative stages is that it rarely leads to yield loss, unless there is a significant reduction in plant height. This assumption is based on continued suitable environmental conditions for soybean growth and seed fill prior to harvest. With regard to injury from most herbicides, late-planted soybeans can be generally more of a concern since they have less time to develop full yield potential anyway, especially in sub-optimum environments.