What amounts to a few drops in the tank — 1/250,000 of dicamba's labeled rate — that's all it takes to show visual symptoms of off-target dicamba movement.
"We did some work in the greenhouse to see how low of a dose it takes to cause symptoms," says Greg Kruger, Nebraska Extension weed science and application technology specialist. “We went down to 1/250,000 of the labeled rate and were still seeing responses we weren't seeing on the control. That's an extremely small dose."
However, that doesn't mean injury always results in a yield loss. Meta-analyses have shown it takes about 40% injury (in the frequently used 0% to 100% injury scale, with 0% being no injury and 100% being a dead plant) to cause yield loss.
"That's pretty severe injury," Kruger says. "A lot of what we're seeing in Nebraska, we're not anywhere close to that."
While particle drift and temperature inversions often are blamed for injury — and for good reason —tank contamination shouldn't be overlooked, Kruger says.
"I think tank clean-out warrants a good, healthy discussion, because that's one thing we can control," he says. "I can avoid making an application in an inversion, but I can't control whether there's an inversion or not. I can control how clean I get my sprayer when I'm done."
Points of contact
This winter, Kruger highlighted some steps for thorough tank clean-out, and some often-overlooked areas, during Restricted Use Pesticide applicator training workshops held throughout Nebraska and at the 2019 Crop Management Conference in January.
Most applicators are aware of the importance of triple-rinsing after spraying. Kruger notes that while most of the chemical usually is removed after the second rinse, it's important to clean anything that might have come into contact with the chemistry.
"That means we've got to get hoses cleaned out well," he says. "Certain hoses are easier to clean out than others. Generally speaking, the higher quality the hose, the easier it is to clean out."
Oil-resistant hoses generally retain less chemical residue. Research by Dan Reynolds, weed scientist at Mississippi State University, has shown that a low-density, polyethylene blend hose — while the most expensive of the hoses tested — retained the least amount of dicamba residue.
Depending on the operation, the chemical might have come into contact with other tanks as well. Kruger notes one case in Nebraska, where an applicator was running "hot loads" — mixing chemistries in a shop and loading it on a tender or nurse truck before delivering it to the field.
Although they cleaned out the spray tank thoroughly, they forgot to clean out the tender truck, resulting in injury symptoms on about 2,500 acres.
"If I'm reusing chemical jugs, using hot loads, using bulk containers for different chemicals, all those things that are potentially coming into contact with dicamba need to be thoroughly cleaned as well," Kruger says.
Other overlooked areas
It's also important to flush booms — including areas where chemicals might concentrate.
"Chemicals often concentrate in dead space at the end of the boom, particularly dry chemistries,” Kruger says. “They cake at the end of that boom. You need to pull the end cap off and make sure that last few inches of the boom gets completely flushed out. I would recommend pulling the tips and screens off and soaking and washing them to make sure those are cleaned as well."
Another often overlooked area, especially on newer sprayers, is filtration points.
"Some newer sprayers have up to six different filtration points to pull any kind of debris out of that line," Kruger says. "It's really important we pull those filters and get those cleaned. They can be pretty big and hold quite a bit of fluid in them."
Kruger advises cleaning the outside of the sprayer as well. When spraying multiple fields, it doesn't take long for spray particles to stick to the outside.
Tank clean-out isn't a simple task when dicamba is involved, but it will be even more critical as new traits are introduced with resistance to different chemistries.
"The reality is that glyphosate and glyphosate-tolerant crops made things easy for applicators,” Kruger says. “So, we haven't had to think about these things for a long time. Now that we're using more complex postemerge weed control systems, we've got to go back and re-examine whether we are really doing the best we can do."