Spraying into a mature crop canopy?
By Carolyn King
For the project’s lab component, the researchers created a canopy of mature wheat plants and a simulated broadleaf canopy, and assessed canopy penetration using different spraying practices. Photo by Tom Wolf, Agrimetrix Research & Training.
A new project to discover the most effective ways to spray fungicides into mature crop canopies is already generating some interesting preliminary results.
The three-year study was initiated in response to emerging information needs.
“Fungicides are the growth area in the crop protection business. More acres and more new products are being sprayed in that part of the business than any other. So we’re seeing a lot of promotion of fungicide use by the crop protection industry. And we’re also seeing quite a bit of interest from the applicators; they have now purchased high-clearance sprayers, and they want to know how to apply fungicides better,” explains Dr. Tom Wolf of Agrimetrix Research & Training. “We realized that we didn’t have a lot of the answers to that because fungicide spraying is a relatively new business.”
Wolf, who is an expert in sprayer and nozzle technologies, is working on this project with two plant pathologists: Dr. Randy Kutcher of the University of Saskatchewan and Dr. Bruce Gossen of Agriculture and Agri-Food Canada. Project funding is from Saskatchewan’s Agriculture Development Fund, the Western Grains Research Foundation, and nozzle companies Hypro, TeeJet Technologies, Greenleaf Technologies, and Wilger Industries. As well, some new funding will be coming from the crop protection industry.
As a first step, the researchers conducted lab experiments in the fall of 2014 to determine where the spray goes in the crop canopy under various treatments. Then starting in 2015, they will evaluate the most promising treatments from the lab study in on-farm research trials.
“Determining where the spray goes is one of the big challenges in the spraying world. It’s very difficult to quantify the amount of spray that ends up in different parts of the canopy, and yet in the plant disease world, that is a very important aspect,” says Wolf. “For example, to control Fusarium head blight (FHB), a prominent cereal disease that is all over Eastern Canada and moving into most parts of Western Canada, the spray needs to land on the wheat head. So we need to know how much of the spray we are actually getting on the wheat head. For a disease like sclerotinia in canola, usually the spray has to hit the flower petal or the flower bud prior to opening. And for diseases like tan spot or septoria in wheat, we need to spray the flag leaf and perhaps the penultimate leaf, so we need to make sure most of the spray goes at least halfway down into the canopy.”
For the lab study, the researchers created two crop canopies. One was composed of mature wheat plants with emerged heads, the crop stage for spraying FHB. The other was a simulated broadleaf canopy made of silk plants that were configured to provide a quantifiable canopy density and a generic look.
For each spray treatment, the researchers placed plastic drinking straws as spray targets at different heights and orientations throughout the canopy. Then the sprayer moved through the canopy and applied a fluorescent dye mixed with water. After each spray pass, the researchers removed the straws, washed them and accurately measured the amount of dye on each straw using fluorimetry.
The sprayer passes were designed to compare a wide range of factors including different travel speeds, boom heights, spray pressures, droplet sizes, nozzle types and nozzle brands.
So the researchers were able to determine what proportion of the total spray applied to the canopy landed on the different places within the canopy, for each application method.
Wolf is now analyzing the data from the lab study. Based on his initial look at the data, he makes several observations.
“The first observation is something we’ve known from other studies, which is that it is very difficult to get much of the spray beyond the top third of the canopy [no matter which application practices are used].
“In the wheat canopy, about 65 per cent of the spray that we applied was intercepted by the wheat head, which is a pretty high percentage. But at the bottom of the canopy, only about 25 per cent of the spray was intercepted.
“In the broadleaf canopy, the picture was bleaker. Typically about 50 per cent was intercepted near the top of the canopy and only about 20 per cent near the bottom.”
Wolf’s second observation is that the answer to the question “Which application practices are most effective?” really depends on where in the canopy the spray needs to go.
“The first and absolutely the most important question to be answered by the applicator is: ‘Where does the spray have to end up?’ The applicator has to know that to make the correct application decision,” he says.
For example, the lab study showed that if the wheat head is the spray target, then the best option is to use an angled spray, a boom height that is relatively close to the wheat head and a relatively fast travel speed. If the target is deeper into the canopy, then the best option is to travel a little slower, spray vertically and keep the boom low.
Determining the best application methods for the broadleaf canopy is a bigger challenge for the researchers. “It was very difficult to find one application method that was much better than another one. For spraying the top of the canopy, all the treatments were pretty similar. And we always found very little at the bottom of the canopy, no matter how we sprayed it,” notes Wolf.
“So we’re not comfortable yet saying what growers ought to be doing when spraying broadleaf canopies. The general recommendations about travelling a little slower, using a higher water volume and keeping the boom as low to the canopy as possible are probably true, but the overall benefits of doing it that way over doing it a different way were not as big as we expected.”
The researchers will be releasing their final results from the lab study in the coming months.
They will also be working with some chemical companies, manufacturers, dealers and agronomists to find crop growers who are willing to collaborate in on-farm research trials. They are looking for Saskatchewan field sites because all three researchers are based in that province.
“We want to work with producers who have sprayers and fields that need to be sprayed,” notes Wolf. “It’s a real opportunity for us to speak directly with the applicator and get a very good sense of how they do things. And it could be an opportunity for the applicator to learn, too. For instance, they might already have two or three different nozzles for their sprayer and may not be sure which would be best to use in a specific situation, so we could approach their field trial from that angle. Farmers love to learn, so if we can help them do that learning, usually they can meet us half way.”
The researchers will probably do just two treatments at a site because each site will have to include space for the treatments to be replicated to ensure statistically valid results. So, for instance, they might compare two different nozzles on one farm, and two different travel speeds on another farm.
Kutcher and Gossen will assess the level of crop disease in the different treatments. And crop yield data will be collected for the treatments. Then the researchers will analyze the data to determine which application methods are the most effective for controlling disease under real-life conditions.
Some principles and tips
Understanding the principles of spray management can help with decision-making on fungicide applications. Although it’s too early to formalize such principles for spraying broadleaf canopies, Wolf outlines some principles for spraying grass canopies.
He identifies four principles to use when applicators are aiming for exposed vertical targets, like wheat heads. “The first principle is to use an angled spray to hit the vertical target from the side. If you have a vertical droplet direction, one that just goes straight down to the ground, it is unlikely to hit a wheat head that is also vertical.”
The second principle is to use slightly larger spray droplets. “That angled spray needs to make it all the way from the nozzle to the wheat head, without losing momentum from air resistance and so on, and eventually falling vertically with gravity. So you want to make the droplets bigger to ensure they retain their initial angled direction for as long as possible.”
The third principle is to keep the boom as close to the canopy as the nozzle allows. “There are minimum boom heights that make sure you have good spray patterns, but you want to be as close to those minimum heights as possible. That minimizes the amount of time that a droplet has to travel, so the droplet will likely still be moving at its initial angle [when it gets to the target].”
And the fourth principle is to enhance the horizontal momentum of the spray. “Typically it’s better to have the nozzle pointed forward and not backward. Also, faster travel speeds for the sprayer are usually better in this particular case. Those two practices combined impart greater horizontal velocity to the droplet.”
In contrast, when applicators are aiming for the leaves in the middle of the canopy, the spray droplets should fall directly downward. So, almost the opposite application methods are needed – the applicator should use a vertically oriented spray and travel a little slower, while keeping the boom height as close as possible to the minimum.
Wolf understands why growers are often reluctant to follow advice to go slower and use lower boom heights, so he offers some tips to help make these recommendations more practical.
“At the research end, we are responding to some very powerful market trends. Those trends are that sprayers are bigger and sprayers travel faster, and because of those two factors, sprayer boom heights are generally a little higher than we would like. Farmers are buying these sprayers because they need to do more acres per hour so they can spray everything on time, which is a very important priority. So when we say, ‘To get maximum benefit from this spray, you should slow down, lower your boom and add more water,’ it contradicts some of the productivity gains they are trying to achieve,” he explains.
“Therefore, I think we have to find other ways of achieving efficiencies in the spraying world. One approach is to reduce your downtime, so you can maximize the amount of time you spend actually spraying, which allows you time to do a slightly better job while spraying.”
According to Wolf, examples of ways to reduce downtime include things like using a larger spray tank and/or a higher capacity transfer pump with larger diameter plumbing to allow faster tank filling. Table 1 compares several ways to increase the number of acres sprayed per hour.
“Another time-consuming activity is tank cleanout and waste disposal, which can take as much time as it takes to spray an entire field. By investing in more efficient cleaning equipment like a clean water saddle tank, a wash-down nozzle or boom-end rinse valves…an applicator can win some time back.”