In 2013 we ran a sprayer coverage demonstration in a ginseng garden in Norfolk County, Ontario. The goal was to encourage growers to reconsider their spray operation with an eye to coverage. We performed a down-and-dirty comparison between simple disc-core nozzles and the considerably more expensive Arag Microjets. Opinions were mixed, but we were confident the humble disc-core could do the job.
One grower took the day to heart.
Having experienced Alternaria infection (likely due to frost damage) in the outer rows, he decided to buy a few packages of water sensitive paper and put his spray boom to the test. Multiple ground speeds, nozzle choices, pressures, spray volumes and even nozzle orientations were tested. This led him to what we will call “ideal coverage” from what may be the perfect ginseng boom.
Possibly the “perfect” ginseng spray boom. 25 hollow cones and four drop arms sporting 2 full cones apiece.
On June 15th, the temperature was about 22 °C, winds were light and humidity was about 40%. The nozzle arrangement was 24 D4-45’s (hollow cones) on the horizontal booms, spaced every 50 cm (20 inches). The grower built four drop arms, hung over each alley (not just behind the wheels) with twin bodies that each held two D5-35’s (full cones), for a total of eight dropped nozzles.
His output was ~1,000 L/ha (115 US gallons per acre) and he sprayed at ~14 bars (200 psi) and he was travelling at ~7.2 kilometers per hour (4.5 miles per hour).
Compared to traditional methods, that’s low pressure and low volume for ginseng. The ground speed was reasonable given the art of negotiating a sprayer under a shade structure. Collectively, this is a savings in fuel, water and pesticide.
Positions for water sensitive papers.
Water sensitive papers were placed in seven positions (see image below) in a three-year old garden. In each position, the papers were folded so the paper wrapped around the stems and could show coverage facing each alley. They were placed on the stems just above the ground and just below the canopy on three plants. The seventh card was folded over the uppermost leaf, to show coverage on the adaxial (top), and abaxial (underside) of the leaf.
Water sensitive papers corresponding to the numbered positions in the earlier illustration. Cards were folded around the stems to face each alley (Cards 1-6) and around the top leaf for surface and under-leaf coverage (Card 7). There are some drenches, but no misses.
The coverage was excellent. A completely blue card represents a drench, which isn’t necessary but can be difficult to avoid when trying to spray all surfaces in a dense canopy. The rest of the papers show a high droplet density which tends to lead to an effective application. Ideally, hope to see 10-15% coverage and >85 droplets per cm2. This is a difficult or even impossible prospect for abaxial coverage, but we achieved it (note the lower half of card 7).
The trick, you ask? The full cones on the drop arms are aimed so the bottom of the cone is parallel with the ground (essentially, aimed up about 30°). That creates a cloud of spray moving under the canopy, improving the odds of contact on all surfaces. It is important to not spray the cone into the ground or the raised mound, and to spray in from both sides.
The improved drop arm
The drops themselves have been modified so they are flexible enough to move through an overgrown 3rd or 4th year garden (yes, there will be some leaf damage), but are also stiff enough not to sway. This was accomplished by sliding a sheath of electrical conduit over the drop arm and using a metal stabilizing arm that terminates in a ring around the conduit.
With the right timing and product choice this method of spraying will be hard to beat. And it’s cheap! It’s going to save fuel and wear because of lower pressures, and save spray mix because he can go a lot farther on a tank spraying only 1,000 L/ha.
For more information, check out the OMAFA research article describing the original research that set us on the path of drop leg technology.
Special thanks to Richard Klosler of Michael Klosler Farms Ltd. for sharing his great boom design.
Pulse-width Modulated flow control allows you to change travel speed by a factor of about five without a change in spray pressure. This chart shows which nozzle flow rates to use. Note the significant pressure drop across the Capstan solenoid. This value must be added to the cab spray pressure, as explained here.
All sprayers experience a drop in pressure as the solution moves further away from the pump. Here’s why that’s important, and how to measure it.
Optimal nozzle operation in terms of spray quality and fan angle is closely tied to spray pressure. As we try to maximize travel speed range with a modern sprayer, we often push spray pressure to its limits on the low and high side. For many air-induced nozzles, spray quality and fan angle become critical at about 30 psi. We need to be sure about the exact nozzle spray pressure to prevent problems.
Pressure drop is caused by the friction that the spray solution experiences as it moves from the pump to the spray nozzles. It’s caused by a number of factors, including length of tubing, elbows, valves, screens, and other flow obstructions.
Plumbing components add friction to liquid flow. If the pressure gauge is installed before these components, the nozzle pressure is unknown but will be lower than the gauge reading.
The pressure transducer that reports pressure to the cab is usually located between the pump and the manifold that divides the spray into the various boom sections. At this point, the spray liquid hasn’t experienced any significant flow restrictions. The transducer basically reports pump pressure.
Once the spray mixture starts moving through boom sections towards the nozzles, it encounters those restrictions, and pressure at the nozzle will therefore be lower than the cab reading indicates. The higher the liquid flow, the greater the friction, and therefore, pressure loss.
Even older sprayers with only two boom sections (left and right) and few elbows and reducers, will see pressure losses due to the narrow and long boom pipe that feeds up to 60′ on each side.
The nozzle pressure can be measured with a gauge placed on a nozzle body. Simply purchase a quality gauge and a threaded nozzle cap, combine the two and install in place of a nozzle.
A pressure gauge threaded into a nozzle cap can measure boom pressure.
Operate the sprayer at your expected spray pressure (say, 60 psi) with all boom sections on. Install the portable pressure gauge on an open turret position and turn into place, noting its reading. If both gauges are accurate, the boom pressure will likely be below 60 psi.
The difference between the cab gauge pressure and the boom gauge pressure two is the pressure drop. Repeat the measurement for each boom section. Also repeat at your lowest, as well as your highest expected flow rates. Higher flow rates cause greater pressure drops.
Now, use this information to adjust your interpretation of the cab pressure reading. For example, if you want to spray at 60 psi and your pressure drop is 10 psi, then the cab pressure should read 70 psi.
If your boom pressure is higher than your cab pressure, and you’ve checked the accuracy of your new boom gauge, then don’t be too mystified. Your pressure transducer is malfunctioning.
This exercise is important if you’re trying to compare your nozzle flow to the expected nominal flow of the nozzle – perhaps you’re trying to determine nozzle wear. The nominal flow of agricultural nozzles is determined at 40 psi, so it will be important to measure the flow at exactly that pressure.
By measuring pressure drop on all your boom sections, you also get a good sense of the variability in pressure across your boom. Your measurements might reveal an obstruction or a hose kink somewhere along the line.
Note that the pulse-width modulated systems offered by Capstan, Case, and Raven use a solenoid at each valve. This solenoid adds a known, and significant, pressure drop to the spray system as can be seen here.
Pulse-Width-Modulation (PWM) solenoids typically have internal flow restrictions that can contribute to pressure drop.
Here’s a fun video filmed by the Ontario Pest Education Program during a break at Ontario’s Southwest Crop Diagnostic Days:
Submitted while Dustin was the Commercial Horticulture Specialist with Alberta Agriculture and Forestry.
Horticultural chemicals and pesticides often have the dubious distinction of being more expensive than their field crop cousins. In order to reduce costs, growers may sometimes buy and use chemicals which have the same active ingredient, but are not registered for the crop they’re being used on. This practice of “off-label” spraying is not only illegal and can result in severe fines, but can also be incredibly dangerous for your clients, your livelihood, and the environment.
Health Canada’s Pest Management Regulatory Agency (PMRA) is the sector of the federal government that is responsible for overseeing the registration and regulation of pesticides in Canada. This includes products that producers are more familiar with such as herbicides and fungicides, as well as less thought about products such as animal repellents, rodenticides, and disinfectants. These products are all rigorously tested to compile data on residue, efficacy and long term effects, all of which is reviewed by Health Canada prior to registration. Furthermore, the PMRA is the body in charge of monitoring and enforcing appropriate use of these chemicals to ensure public safety.
When reviewed by Health Canada, all chemicals have specified rates, target pests, and the crops on which they can be applied. ‘Off-label’ spraying can include spraying above the appropriate rate, or spraying the chemical for a pest that it is not registered for or on a crop not on the chemical’s label.
Throughout the year, the PMRA randomly selects growers for pesticide use inspections in order to ensure compliance. These inspections could be random ones, arising from increased incorrect spraying because of a label change, the need to update information or a neighbour’s complaint. The purpose of inspections is twofold in that they serve as a deterrent to off-label spraying but also as an educational tool to encourage growers to follow labels.
In an inspection, samples of vegetative material or soil are collected and sent away for testing. Should these come back showing incorrect use of chemicals, growers may be subject to increased monitoring, financial penalty or even prosecution under the Pest Control Products Act. If somebody producing food is found to be non-compliant, it could even result in the Canadian Food Inspection Agency (CFIA) becoming involved and product being recalled or destroyed.
No grower wants to purposefully put themselves, their clients or the environment at risk, so how to avoid the temptation of spraying off-label? When possible, avoid the need to altogether! Good scouting and appropriate record keeping allows growers to track outbreaks over time and better plan for them in their operating cycle. Furthermore, a good chemical inventory will give producers a better idea of how much they have of needed chemicals and whether they have enough to deal with problems when they come up. Adopting biological controls in their operation may also allow growers to nip some problems in the bud before they become full blown issues.
As with all chemicals, proper storage, labelling and disposal are all part and parcel in running a safe, effective operation. By being aware of the process and how best to handle these chemicals, growers can ensure they grow a safe healthy crop without running afoul of the law.
