By Tom Barber, Extension Weed Scientist
Last season the Arkansas State Plant Board received a total of 32 drift complaints resulting from applications of various formulations of dicamba herbicide made to Xtend crops, mostly cotton. This year the release and approval of Engenia herbicide (an improved formulation with reduced volatility) was registered in Arkansas and coupled with more strict application guidelines which included mandatory applicator training, extending 100ft buffer zones around the entire field at application as well as a 0.25 mile downwind buffer to sensitive crops. The thought during the development of these restrictions was an attempt to minimize the amount of injury from off target movement this season. Unfortunately that has not been the case. As of this morning (6/12/17) approximately 40 complaints from off target dicamba movement have been filed with the Arkansas State Plant Board. This is up from the 32 total last year and complaints are increasing daily. To be fair numerous other drift complaints have been filed this year including Paraquat (13) and Roundup (11) but these drift events have been much more localized and at most damaging around 100-200 acres in extreme cases. Dicamba complaints however have been much more widespread and may cover 1000 acres or more each time. Robert Goodson our county agent in Phillips County has estimated approximately 20,000 acres that have been affected in his county. In addition ten other counties across Eastern Arkansas have reported damage. Last Thursday over 7000 acres of drift was reported in Mississippi County which included most of the soybean research at the Northeast Arkansas Research and Extension Center at Keiser. Calls from Jason Bond my counterpart in Mississippi indicate that dicamba damage is also widespread on soybean fields throughout the delta of Mississippi.
So what happened? Many thought that the improved formulations of dicamba would effectively decrease if not eliminate off-target issues that we saw in 2016. This has not been the case. Arkansas regulations required all applicators or anyone that would be spraying dicamba to take and pass an applicators exam that covered all requirements for use in the state of Arkansas. Over 1000 applicators took and passed the required test and were educated to know the application requirements and the risk. The biggest problem is the sensitivity of non-Xtend soybean to dicamba herbicide. As we have seen in our research, a very low rate (1/30,000X) of dicamba can cause soybean leaves to cup. No, there will not be any yield lost at this low of a rate, but you will still see the injury on the beans. Although most growers and applicators took the training, some still decided to spray with the wrong nozzles, high winds, disregarding the buffer zones, or at night during a temperature inversion. This physical drift accounts for at least 80-90% of all the dicamba injured fields that I have personally walked. The other 10-20% is not that easy to figure out. There are some growers/applicators that had the correct sprayer setup, measured off the appropriate boarders, and sprayed Engenia when the wind was blowing away from susceptible soybean, in some cases with a broadcast hooded sprayer. These are the fields that have me scratching my head wondering how on earth the dicamba symptoms appeared 0.25 mile up-wind. Really there are only two conclusions that I can come up with. Either there was a small amount of volatility that occurred, but enough to cause some symptomology, or another theory might be that the dicamba droplets are moving on dust particles after the application is made. I am seeing more and more evidence in the field that points me towards movement with the dust. I don’t know how we can steward a dicamba herbicide moving forward that is so sensitive to non-tolerant soybean cultivars. It is evident that even when applications are made correctly, sometimes that is not good enough.
The main questions after injury is observed are, what can I do? And how much yield is lost? The first thing you should do if you suspect dicamba injury from off-target movement is call the Arkansas State Plant Board. This will provide an official record of the complaint. There is nothing you can do or spray on the soybeans once injury has occurred to speed up metabolism and reduce any potential yield losses. Depending on the temperatures and growth stage, it can take up to 3 weeks to see the full symptoms from the dicamba drift.
Last year I wrote a blog on yield effects based on rate and timing of dicamba injury, it can be found by clicking this highlighted sentence. Most of the injury I have walked appears to be lower rates of dicamba drift at young vegetative growth stages. This should result in minimal if any yield losses. There are some drift cases where soybeans were around R1-R2 and injury suggested higher rates were received. These are the fields that could potentially expect 10-15% yield losses. If the rate of dicamba causes the terminals to turn towards the ground at V6-R2, higher yield losses may be observed. I also put together a blog last year regarding late season dicamba effects on sensitive soybean and offspring, it can be found here.
I have heard some say that all dicamba (Engenia) applications should be banned immediately. The problem with that is there are large acreages of Xtend soybeans purchased and planted in good faith that there would be a herbicide to spray over-the-top for pigweed control. Reports of lower than adequate control with Engenia have been spread this year but when Xtend is planted it is the only option where PPO-resistant pigweed is present.