Tommy Butts, University of Arkansas System Division of Agriculture, reminded farmers not to forget the application process when controlling weed, disease and insect pests.
Butts addressed application technologies and coverage issues at the recent Mississippi State University Row Crop Short Course.
"To get our end product, we need to control weeds, insects, and fungi. It all starts with the application process, which is often overlooked," Butts said. "We can improve our overall control just by focusing on the process. It's a balancing act when it comes to physical drift particle movement. We're trying to reduce drift, but we still need to maintain control.
"We can either give up some drift reduction and get better coverage, or we can get better drift reduction with larger droplet sizes, but we reduce coverage."
If you don't have the right application setup, a pest will roll through the improper coverage and droplet size. The wrong nozzle choice, application setup errors, bad boom/flight heights, and bad application speeds can mean poor pest control.
"Let's say we're spraying at a fixed volume of 10 gallons per acre. If we're spraying with a thousand-micron droplet at that spray volume, we have one droplet to hit our target," he said. "Now, this exponential relationship kicks in, and if we cut our droplet size in half, we get eight times the number of droplets available to impact our pest. Instead of one droplet at 1000 microns, if we go to 500 microns, we have eight droplets to try and hit the target. If we cut it in half again, we have 64 droplets to hit the target and get some coverage.
"This can go the opposite way too. When we're trying to reduce drift and we have a larger droplet size, we go from 64 droplets to eight droplets to one droplet, trying to hit our target and get some control. That's a challenge."
If forced to use larger droplet sizes for drift control, the only way to get more droplets to hit the target is by changing the spray volume.
"Everybody cringes when they hear that we have to increase our spray volume. That's the only way to get more droplets out there to hit our target. It's a numbers game," Butts said.
The XR flat fan nozzle is a medium spray quality with an average droplet size of about 300 microns.
"You can see at 5 gallons per acre we got about 20 percent coverage," he said. "If we go up to 15 gallons per acre, we're at 65 percent coverage. That's a huge difference, going from 5 to 15 gallons per acre."
The TTI nozzle has a huge droplet size at 800 microns, and it is an Ultra Coarse spray.
"At 5 gallons per acre, we only had about 10 percent coverage," Butts said. "At 15 gallons per acre, we're going to have 30 percent coverage, an improvement. We need to be at a higher spray volume with the larger droplet size, but we're still losing coverage compared to smaller droplet sizes.”
"As a final comparison, with the XR flat fan droplet size, we can get equivalent coverage pretty much with 5 gallons per acre compared to 10 gallons per acre with the TTI nozzle."
Liberty herbicide is considered a contact herbicide.
"We got our best weed control with Liberty herbicide when we were around 300 to 450 microns which was a medium to coarse spray quality," he said.
Using dicamba with a TTI nozzle, droplet size mattered but not as much as it did with Liberty.
"From 300 microns to 750 microns, it didn't necessarily matter," Butts said. "We got about the same level of control, so why not use the 750 microns which is an extremely coarse spray? We could still maintain control, but we can reduce that physical particle drift. The 900-micron treatment wasn't good. In fact, that treatment at 5 GPA was not statistically different from our non-treated control group. We could have sprayed absolutely nothing and done just as well at controlling the Palmer amaranth as spraying dicamba with a 900-micron droplet at 5 GPA."
Nozzle arrangement is rarely considered, and there are a lot of directional nozzles available today such as the 3D and TTI.
"We wanted to see if the way we put these nozzles on our boom mattered. For example, we put the directional nozzles on all facing forward, backward, or alternated across our boom," he said.
The 3D and AIXR nozzles got better coverage than the ULD and the TTI due to their smaller droplet size. The smaller and more numerous the droplets mean better coverage overall.
"If the directional nozzles all face forward, we get good coverage on the front of the plant but not the back," Butts said. "If all the nozzles face backward, you get barely anything on the front of the plant, almost zero coverage, but you get coverage on the back of the plants. If we alternate them, we get a pretty good happy medium coverage, similar to the straight down nozzles like the AIXR and ULD."
Aerial applications use a smaller droplet size typically than what is used on a ground application. A smaller spray droplet size provides better coverage at lower volumes.
"From a ground application, we want to decrease our pressure, which gives us a bigger droplet size. Aerial applications are a whole other animal," he said. "We need to increase our pressure to increase our droplet size because wind shear is affecting our droplet size. If we shoot a spray into that airstream at a higher velocity with more pressure, it has less of a chance for the wind to break those droplets up into smaller droplet sizes."
Airspeed affects droplet size in aerial applications.
"Wind shear is the driving force, so if we're flying faster, we have more of a wind shear underneath that plane, which creates finer droplets. The solution should be fly slower and increase our pressure, right? It's not that simple with a plane, especially with our current agricultural aircraft which carry a lot of weight," Butts said.
Since agricultural aircraft carry so much weight, they must fly faster to stay in the air.
"Pressure is also driven a lot by power and airspeed, so if we tried to decrease our airspeed, normally, pressure is going to go down too," he said. "If we keep that airspeed up but we're still using a lot of power to keep that plane in the air, we're taking power away that could be used to boost our pressure. Based on the weight distribution factor, it lends itself to difficulty maintaining a high pressure and lowering our airspeed, so it's a big challenge trying to manage the droplet size from an airplane."
Wind direction is another factor.
"Depending on the wind direction, it can be hard to get the right application overlap," Butts said. "There is also the potential to cause crop injury from improper overlap in aerial applications simply because of the way the wind was blowing that day.
"The nozzle arrangement, spray volume, droplet size, and boom/flight height are small details that can make a big difference when it comes to coverage and pest control."