Last summer brought challenges with weather conditions and off-target movement of pesticides, including dicamba. Three dicamba products — XtendiMax, FeXapan and Engenia — have caused off-target damage to sensitive plants and are now considered restricted-use products.
But were these three products to blame for all of it? Observations by University of Nebraska Extension suggest there was also potential off-target movement from dicamba-based applications in corn in 2017. With the exception of the three RUP dicamba products, application of dicamba does not have as many requirements to manage particle and vapor spray drift.
Rain, cold weather and wind delayed postemergence application of dicamba-based herbicide in corn till the first few weeks of June, particularly in Thayer, Nuckolls, Fillmore and Clay counties. Early June had high heat, some wind and variations in relative humidity. These conditions may have allowed for potential volatility and temperature inversions.
Research at the University of Minnesota in the 1970s showed increased volatility and soybean injury from dicamba with lower relative humidity (70% to 75%) compared to (85% to 95%) and increased temperature (Table 1).
TEMPERATURE’S IMPACT: Table 1. Effect of temperature on volatilization to soybean during six hours of exposure in greenhouse. Source: Behrens, R. and W.E. Lueschen. 1979. Dicamba volatility. Weed Science Journal 27:486-493.
There were also areas of the state with large number of acres of dicamba applied to corn in hopes of controlling Palmer amaranth and weeds not controlled by previous herbicide applications. The above-mentioned counties had hundreds of thousands of acres of dicamba applications within a two-week time period in early June.
When soybean cupping damage began to appear in mid-June, it was difficult to determine where the drift originated. Phone calls to area farmers and co-ops confirmed some correlation of dicamba applications to corn and often dicamba applications in Xtend soybean had not yet occurred.
The plants tell the story
It can take seven to 14 days for leaf damage from dicamba injury to appear on susceptible plants. This is because auxin-like herbicides affect only cell division. So, fully developed leaves (no longer expanding via cell division) are not affected even though they may be expanding by leaf cell enlargement.
Because dicamba is also translocated once inside leaf cells, it impacts cell division of the leaf primordia at the stem apex. Those leaves may not yet be seen because they are still enclosed in the stem apex tissue.
University of Nebraska research shows a soybean will produce a new node every 3.75 days.
Considering this, the local Extension educator used the following method to determine the potential timing of off-target dicamba injury on soybean:
• Ask the farmer when the soybean was planted. (Even better if he or she knows when the soybean emerged.)
• Count the number of nodes from emergence to the first damaged trifoliolate.
• Multiply the number of nodes by 3.75 to estimate how many days old the plant was at that node.
• Count back on the calendar seven to 14 days to determine the potential timing of off-target movement to the soybean. Using this method correlated the majority of the time to a dicamba product applied to nearby corn in the fields examined in Thayer, Nuckolls, Fillmore and Clay counties. Therefore, the first dicamba off-target movement to soybean in this area of the state mostly occurred from dicamba applications in corn.
• Count how many nodes contained damaged trifoliolates. (On average, observation found four to six nodes were damaged with each off-target injury before plants grew out of the injury.)
• Nodes were also counted on the entire plant and multiplied by 3.75 to double-check the age of the plant. This correlated well to determine the time of the first off-target injury. As plants progressed into August, with two to three potential off-target injuries, this method of checking wasn’t as reliable, as the rate of new growth also appeared to be affected. Variation of rainfall or irrigation to help the plants “grow out” of the damage also affected the ability to always determine the timing of the second or third off-target injury.
Nebraska is fortunate to grow indeterminate soybeans, and have irrigation and rainfall that potentially allow soybeans to grow out of dicamba injury.
“Indeterminate” means that the plants continue to develop vegetative growth and nodes once flowering occurs. In states to the south such as Missouri and Arkansas, “determinate” soybean varieties are grown, meaning that upon flowering, growth soon ceases. This can make a difference in yield observed on affected plants (depending on concentration of dicamba and timing of when the plants were injured).
Why bigger problem in 2017?
Dicamba products have been used in corn for decades and haven’t seen landscape-level off-target dicamba injury on sensitive plants in the past. So why the difference in 2017? Perhaps it was a perfect storm of several factors:
• higher number of corn acres were sprayed with dicamba than previously (hundreds of thousands of acres in the area of Nebraska mentioned above)
• late postemergence applications into early and mid-June in a short time period
• highest labeled rates of dicamba used to obtain good control of weeds, including Palmer amaranth
• ability to add ammonium sulfate (AMS) to dicamba applications in corn according to the product label
• high temperatures, wind and fluxes in relative humidity allowing for potential off-target movement via particle drift, volatility and by temperature inversions
Suggestions for 2018
Because of the potential for off-target movement from dicamba applications to corn and soybean, a team of Extension faculty have provided some best management practices in Can We Manage Dicamba Applications in 2018?
This report comes from UNL CropWatch.