Situation

Rain, cold weather, and wind delayed post-emergence 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 (Figures 1-4). These conditions may have allowed for potential volatility and temperature inversions.  Research by Behrens and Lueschen, 1979, showed increased volatility and soybean injury from dicamba with lower relative humidity (70-75%) compared to (85-95%) and increased temperature (Figure 5 and Table 1).

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 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 Coops 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 7-14 days for leaf damage from dicamba injury to appear on susceptible plants. This is because auxin-like herbicides affect only cell division. Thus, 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. We may not even see those leaves yet 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 (Specht et. al).

Considering this information, the local Extension educator used the following method to determine the potential timing of off-target dicamba injury on soybean:

1. Ask the farmer when the soybean was planted. (Even better if he/she knows when the soybean emerged.)
2. Count the number of nodes from emergence to the first damaged trifoliolate.
3. Multiply the number of nodes by 3.75 to estimate how many days old the plant was at that node.
4. Count back on the calendar 7-14 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. Thus, the first dicamba off-target movement to soybean in this area of the state mostly occurred from dicamba applications in corn.
5. 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.)
6. 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 and/or irrigation to help the plants “grow out” of the damage also affected the ability to always determine the timing of the second and/or third off-target injury.

*NOTE: The reasoning for a potential second and third off-target injury to the same fields was due to the injury symptomology on the trifoliolates. Plants receiving potential second and third off-target injuries appeared more severely damaged again in the succeeding trifoliolate sets, even after the previous trifoliolate set had “grown out” of the symptoms.

Source: Behrens, R. and W.E. Lueschen. 1979. Dicamba volatility. Weed Science Journal 27:486-493.