The decision on which application method is best for herbicides boils down to two main factors: (a) target type and (b) mode of action. In general, it’s easier for sprays to stick to broadleaf plants on account of their comparatively larger leaf size and better wettability compared to grassy plants. There are exceptions, of course – at the cotyledon stage, broadleaf plants can be very small and a finer spray with tighter droplet spacing may be needed. Water sensitive paper is a very useful tool to make that assessment. Imagine if a tiny cotyledon could fit between deposits – that could be a miss!
Some weeds are also more difficult to wet, and those may also need a finer spray or a better surfactant for proper leaf contact. An easy test is to apply plain water to the leaf with a spray bottle. If the water beads off or the droplets remain perched on top in discrete spheres, the surface is considered hard to wet. Most grassy weeds are hard to wet, while most broadleaf weeds are easy to wet.
Grassy weeds are an especially difficult target because they have smaller, more vertically oriented leaves, and almost without exception are more difficult to wet than broadleaf species. All these factors call for finer sprays for effective targeting and spray retention.
Broadleaf weeds usually have more horizontally oriented leaves which also happen to be larger. As a result, they can intercept larger droplets quite efficiently.
There are about thirty mode of action (MOA) groups among the herbicides with about ten accounting for the majority in Canadian prairie agriculture. It’s probably an over-simplification to categorize them into just two groups – systemic and contact. But that grouping goes a long way to making an application decision.
Contact products (MOA Group 5, 6, 10, 14, 22, 27) must form a deposit that provides good coverage. Good coverage is an ambiguous term that basically means that droplets need to be closely spaced and cover a significant proportion of the surface area because their physiological effects occur under the droplet, and don’t spread far from there. One way to generate more droplets is to reduce droplet diameter, another is to add more water. A reasonable combination of both is ideal because simply making droplets smaller creates issues with evaporation and drift.
Systemic products (MOA Group 1, 2, 4, 9) will translocate within the plant to their site of action after uptake. As a result, coverage is less important as long as sufficient dose is presented to the plant. In practice, this means coarser sprays and/or less water may be acceptable.
When two factors are combined, either in a tank mix or a weed spectrum, the more limiting factor rules. Application of a tank mix or product that is active on both broadleaf and grass plants will be governed by the limitation placed on grass targets. A tank mix comprised of both systemic and contact products is governed by the limitations placed on contact products.
A factor we should also consider is soil activity and the presence of residue. Studies have shown that soil-active products are relatively insensitive to droplet size. But if they have to travel through a layer of trash to get to the soil surface, more application volume is the best tool.
Below are some recommended spray qualities and water volumes for use in Canada. The spray qualities listed in the table can be matched to a specific nozzle by referring to nozzle manufacturer catalogues, websites, or apps. Note that Wilger also offers traditional VMD measurements on their site, allowing users to be a bit more specific if necessary.
This article was co-written with Dr. Sean Westerveld, Ontario Ginseng and Herb Specialist.
An effective ginseng protection program begins with observing the Integrated Pest Management (IPM) process:
diagnose the problem,
monitor the problem,
control the problem, and
monitor the results.
When spraying is warranted, the operator should understand the basics of application technology. This not only includes the equipment, but the effects of changing spraying parameters (such as pressure or carrier volume), the impact of weather conditions (such as wind and relative humidity) and the product being applied (such as correct timing and safety requirements). The operator should also understand how to properly maintain, calibrate and orient the sprayer according to the nature of the target. Finally, monitoring the results requires the operator to respond to changes in the environment and target during application and to consider these factors when evaluating the outcome.
The ginseng garden
This is a four-year old garden, which represents one of the largest, densest ginseng canopies an applicator can spray. The six-foot wide beds in this particular garden are higher than most beds, making sprayer/tractor clearance an issue. It also means the distance-to-target from boom to canopy is less in the middle of the bed than it is nearer the alleys, making it difficult to ensure consistent coverage. Sprayer operators typically drive in the same direction over each bed, “training” the plants to bend in the same direction each time the tractor passes over the surface. This practice, combined with fenders on the tractor wheels, helps to minimize physical damage as the sprayer passes.
Ginseng gardens have high beds.Clearance is an issue in a four year old ginseng garden.
The sprayer
This custom-built sprayer is a fairly standard design for most ginseng operations: Eight nozzles on each wing and nine on the centre boom. Spacing varies but this sprayer is on 11 inch centres, with the outermost nozzles on five inch centres and aimed outward towards the adjacent beds. Given the limited boom height, all nozzles are aimed back about 45 degrees to increase the distance to target and allow for overlap. The angle is critical to prevent gaps in the spray swath, but given the recommended practice of limited overlap for hollow cone nozzles, the 11 inch spacing may be a little shorter than required.
Custom-made ginseng sprayer. A standard design in Ontario.
Spray coverage
There is no hard and fast rule for spraying ginseng. The crop can receive 30 or more applications a year, most of which are fungicide applications. Tip: Monitoring the small plants inside the canopy is a good indicator of overall garden health.
The following lists products available for use in Ontario at the time this article was published. The application target varies for each product, depending on the pest or disease the applicator wishes to control. As such, the application volume should reflect the location of the intended target. For example, a foliar-and-stem application should achieve consistent coverage of all leaf surfaces without incurring run-off. An application intended to reach the crown through the straw will require some run-off down the plant stem and should require a higher volume than a foliar-and-stem application. Many products will become immobilized if they dry onto the straw. Applications are best done to wet straw, followed by irrigation or rainfall to wash the product into the root zone. Applications for diseases like Rhizoctonia generally take place early in the season before the canopy closes, and higher volumes may not be required to achieve root coverage. In order to know how much is required for optimal coverage, read on.
Table 1 – Spray target and relative volume by pest
Pest
Application Target – Specific Product
Garden Age
Relative Volume
Alternaria and/or Botrytis
Foliar and Stem – all products
Seedling – 2nd year
Low
3rd – 4th year
Moderate
Phytophthora Leaf Blight
Foliar and Stem – most products
Seedling
Low
2nd-4th year
Moderate
Foliar – Aiette and Phostrol
All
Low
Phytophthora Root Rot
Root – xylem-mobile root rot products
All
High
Foliar – Aiette and Phostrol
All
Low
Phytophthora Leaf and Root
Root – xylem-mobile root rot products
All
High
Foliar – Aiette and Phostrol
All
High
Cylindrocarpon
Root – all products
All
High
Rhizoctonia
Root – most products
All
High
Root – Quadris
Seedling
High
Pythium
Root – all products
All
High
Aphids
Foliar and Stem/Berries – all products
All
Moderate
Cutworms
Stem – all products
All
Low
Four-Lined Plant Bug
Foliar – all products
All
Moderate
Leafrollers
Foliar and Stem – all products
All
Moderate
Root Lesion Nematodes
Root – all products
All
High
History of the ginseng boom in Ontario
Historically, ginseng sprayer operators used brass hollow cone nozzles to spray ginseng. For reasons that are unclear, many then adopted the Casotti-style sprayer, which used higher volumes and an oscillating nozzle assembly to create a larger swath. This was determined to be overkill for ginseng, and it produced inconsistent coverage.
Many growers (sadly, not all) switched back to horizontal booms and began using the Arag microjet assembly. Drop nozzles (aka drop arms, drop booms, drop legs, etc.) were positioned with disc-core hollow cone nozzles behind the wheels to direct spray into the canopy from below.
Later, we demonstrated that the microjet mixing valve was difficult to set accurately, creating outputs +/- 50% the optimal rate. In response, a new variation on the Arag microjet was introduced, with a more reliable rate adjustment and a lower price tag (they are imported from Italy by a single North American distributor). The drop nozzles are absolutely critical for under canopy coverage, and growers have begun suspending them in each alley – not just behind the sprayer wheels. I predict the future boom arrangement will return to hollow cone nozzles, but in the form of molded poly nozzles with ceramic handling and drop nozzles with full cone disc-core assemblies. Air assist would be even better.
Sprayer settings
Most operators employ a ground speed of about 5 km/h (3.1 mph), operate at about 13.8 bar (200 psi) with nozzles spaced 25-30.5 cm (10-12”) spraying anywhere from 1,000 L/ha (107 gal./ac.) to 1,686 L/ha (180 gal./ac.). The application volume should reflect the stage of crop growth, the age of the garden and the target in question (see Table 1). Applicators should also consider droplet size (Table 2). This is difficult to control given that the majority use Arag microjets with the 1.5 mm orifice disc. In which case, pressure choice will affect median droplet size, with lower pressures increasing median droplet diameter and vice versa.
Table 2 – The Impact of Droplet Size
Droplet Size
Drops per area
Retention
Canopy Penetration
Drift Potential
Fine
High
High
Low
High
Medium
Moderate
Moderate
Moderate
Moderate
Coarse
Low
Low
High
Low
Two versions of the ARAG Microjet.
The older style Arag microjets with 1.5 mm diameter discs have highly variable outputs. We developed tables listing their rates with the mixing valve handle set in two positions. They can be found here. We have also developed tables for the newer Arag nozzles for the 1.0, 1.2 and 1.5 mm discs based on 28 cm (11”) spacing. They are listed in Metric and U.S. Imperial.
Park the clean sprayer and get the pressure up to the desired level. Using a calibration vessel, perform a timed output test to determine each nozzle rate. I prefer the SpotOn SC-4 and a length of 1” braided line to direct the spray into the vessel. You will get wet, so ensure the water is clean and/or wear appropriate PPE.
Timed output test. Prepare to get very wet. Unless sprayer is sparkling clean, like this one, PPE is a must.
At 200 psi, we took readings from each microjet and found that while they were more consistent than the older model, there was still a lot of variation from tip to tip. This required us to turn the valve on the nozzle to get a more consistent output, then take another reading, and repeat until we liked what we saw. It became tricky to adjust the rate without reducing the hollow cone pattern to a solid stream because only a slight turn of the nozzle was required. Once we had it, we tightened the lock nut and moved to the next nozzle. Table 3 is a record of the procedure.
While calibrating, we noticed some of the nozzles would suddenly appear plugged, or dense lines could be seen in the spray cone indicating something was wrong. We cleaned them to discover bits of plastic from the poly tank. I asked about strainers, but they are not available for the microjets. I asked about in-line filters, but they aren’t rated for 200 psi. Filling the tank with clean water is very important, but even more so with these nozzles.
Table 3 – Calibrating the new Arag microjets
Nozzle Position
Rates in gpm (bold represents final rate)
Nozzle Position
Rates in gpm (bold represents final rate)
1
0.97, 0.96, 0.93
14
0.77, 0.92
2
1.07, 1.07, 1.26, 0.9
15
0.76, 0.8, 0.95
3
1.1, 1.1, 1.1, 0.93
16
0.97, 0.95
4
0.73, 0.92
17
0.73, 1.0, 1.07, 1.0, 0.98
5
0.92, 0.92
18
0.83, 0.94
6
0.94
19
0.77, 1.0, 0.99, 1.1, 1.24, 10.8, 0.93
7
0.88
20
0.77, 0.88
8
0.92
21
0.71, 0.95
9
0.95
22
0.77, 1.07, 1.04, 1.1, 1.27, 1.0
10
0.90
23
1.06, 0.97
11
0.86
24
0.77, 0.97
12
0.76, 0.83, 1.0, 1.0, 1.2, 0.92
25
0.68, 0.95
13
0.77, 0.92
Average output: 0.93 gpm, standard deviation of 0.03 gpm.
Ground speed
Once the nozzles were adjusted, we filled the tank ½ full and measured out 25 m in the bed. We would normally do 50 m, but the row was too short. The sprayer operator drove the course and we measured the time it took to travel the 25 m distance. Pass one took 18.5 seconds and pass two took 18.3 seconds. That’s an average of 18.4, which we then double so it works in the formula = 36.8 s.
( 50 × 3.6 ) ÷ 36.8 s = 4.9 km/h
Adjusting the drop leg nozzles
This sprayer had drops behind the wheels and two more to hang in the adjacent alleys. This is excellent because research has shown considerably improved coverage with directed spray from drop arms. In my mind, these are not optional – they are mandatory!
Drop nozzles in the alleys.
We swapped out the hollow cones we found in those positions for full cone disc and core (D5-DC35). Full cones increase the number of droplets that will clear the raised bed and enter the canopy. When adjusting them, be sure to minimize the portion intercepting the bed, while minimizing the spray escaping up through the canopy. It’s a fine line.
Aiming drop arms in a ginseng garden.
Calculating sprayer output
25 microjets at 200 psi = average of 0.93 gpm = 23.25 gpm 8 × D5-DC35 at 200 psi = 1.4 gpm × 8 = 11.2 gpm That’s ~34.5 gpm for the boom. Ground speed was 4.9 k/hr or ~ 3mph.
GPA = (GPM × 5,940) ÷ (mph ÷ nozzle spacing in inches) GPA = (34.5 gpm x 5,940) ÷ (3.0 mph × 11 inches) GPA = 204,940 ÷ 33 62.1 GPA or about 580 L/ha.
Diagnosing coverage
Water sensitive paper, which turns from yellow to blue when contacted by moisture, was placed in the ginseng canopy. Two sets of papers were set out, with four papers in each set. The canopy was still wet with rain, which made placement difficult as the papers would accidentally contact water on the leaves and change colour prematurely.
Water-sensitive paper wrapped around tubes for panoramic coverage.
Position#1
Clipped face-down on the underside of leaves at the top of the canopy.
Position#2
Clipped face-up on the upper side of leaves in the middle of the canopy.
Position#3
Clipped face-down on the underside of leaves in the middle of the canopy.
Position#4
Wrapped around a plastic tube and threaded over a wire flag, located at the foot of the plant to give panoramic coverage at the root.
The sprayer passed over the canopy spraying water, and papers were carefully retrieved, allowed to dry and scanned.
Panoramic papers in situ.Flags mark the locations of papers.
Generally, there were no “misses” whatsoever. Position 1 showed excellent coverage, with no indication of run-off and a high droplet count with even distribution. This is ideal for foliar applications, and under-leaf coverage is notoriously difficult to achieve. Positions 2 through 4 showed excessive coverage, with the exception of one of the position 3 papers, which was still adequate.
Example of coverage and paper locations in canopy.
Next steps
Ideally, the operator would drop the pressure by 20 psi increments, reducing output until coverage failed. It is important to note that the operating pressure must never approach the lower end of the nozzle’s recommended pressure range, or the spray quality will be compromised and so will coverage.
Once the coverage is considered a failure, the operator would return to the lowest output that did a good job, and the sprayer is calibrated for that crop (at that stage of growth).
Note that the calibration must be performed for each significantly different crop. With the exception of an early-season drench intended to contact the entire root, an emerging one year old garden would need a very different prescription than a four year old garden with a fully-developed canopy. Plus, the weather conditions will affect coverage, so do not calibrate in conditions you would not normally spray in. Hot and dry and windy conditions produce very different coverage compared to cool, humid and still conditions.
Once the operator knows what each garden requires, they will be able to mix their tanks using the same concentration of carrier to formulated product as they normally use, but likely go further on the tank. It will take some practice before the operator knows how much spray mix is required to finish the job.
A version of this article was originally written by @nozzle_guy as a guest blog for Farm At Hand, and is reproduced with permission.
One of the smartest decisions a grower could make is to consider a late-season harvest-aid application. Particularly in years with thinner stands, weeds can maintain a foothold. Late season moisture can give new life to late emerging plants or branches. When the crop is ready to cut, this could mean all sorts of cutterbar, pickup reel, feederchain, and sieve headaches.
A desiccant or pre-harvest herbicide application can help avoid those problems. The challenge is to get the spray into, or through, a mature crop canopy. Here are some pointers to do it right.
Evaluate where within the canopy the spray needs to go to do its job. If you’re considering a pre-harvest herbicide, are you looking to control dandelions or buckwheat near the bottom of the canopy, or are you trying to get thistles or quackgrass, whose leaves are near the top? If you’re mostly trying to accelerate drydown with a contact product, where in the canopy are the green stems and leaves that you need to contact?
Take a bird’s eye view of your canopy. That’s how the spray sees it. If you can clearly see your target, the spray application is pretty straightforward because most droplets will make their way there easily. But if the target is obscured by a lot of foliage, or if it’s vertical, the job is much more challenging and will require some combination of more water, slower speeds, angled tips or finer sprays.
To hit plant parts that you can’t see, one of the main tools is finer sprays. The smaller droplets have an easier time changing direction to get around obstacles like leaves, and they are also much more likely to be intercepted by petioles and stems, and to stick to them. This can be both an advantage and disadvantage – for example, the awns in bearded cereals are notoriously effective at capturing the smallest droplets before they can do any good further down. If you don’t want to install a different nozzle to get a finer spray, simply increase the spray pressure of your low-drift nozzle to 80, 90, even 100 psi. This will create enough fine droplets. But don’t expect the higher pressure to push the spray into the canopy. Only air-assist can do that.
To get more spray deeper into the canopy, slow down, add water, and point nozzles backward. The backward orientation helps offset the forward travel speed, giving the droplets a slower net forward velocity that helps their downward movement.
If you’re using contact products like diquat, paraquat, saflufenacil or carfentrazone, use generous amounts of water, and slightly finer sprays. Make sure that spray drift control remains a priority and pay attention to water quality.
Test your water and make sure your water doesn’t have turbidity (suspended clay or other organic matter), for glyphosate and diquat or paraquat, and hardness, for glyphosate. Aluminum sulphate can help get rid of turbidity in a pond, but it takes time (treat turbid water at least 24 to 48 h before you need it). If treating a storage vessel, expect a layer of sediment. Ammonium sulphate (AMS) and other water conditioners can remove antagonizing hard water ions like magnesium and calcium. This is especially important as we increase water volumes with glyphosate to get better coverage. The higher water volumes give a concentration advantage to the hardness minerals.
Diquat and paraquat’s mode of action benefits from being applied in the evening. The absence of the sun allows it to be taken up and slightly moved (by diffusion, not true translocation) within the leaf before morning sunlight activates it. Once activated by the sun, these products exert their activity and movement stops. If you’re not careful, the tighter window of evening-only applications could get you behind. And of course, be aware of the signs of inversions and know when to quit.
Plan ahead and make sure you give yourself enough time, because to do the job right you’ll be using more water and driving a bit slower. Focus on productivity tools like a fast, efficient fill to make up the lost time.
A good job with a pre-harvest herbicide or a harvest-aid can save many harvesting headaches, and can help dry down during less than ideal conditions. It’s another reason why the sprayer may be the most important implement on the farm.
In Ontario, early variety tomato canopies get very dense in July. The inner canopy is relatively still, humid, cool and a perfect environment for diseases such as late blight. It is challenging to deliver fungicides to the inner canopy and this can lead to inadequate disease control. Matters are slightly improved as the fruit grows and pulls the canopy open, and staked tomatoes might allow for the use of directed sprays, such as drop arms in staked peppers. But, there’s no getting around it – from a droplet’s perspective, it’s tough to get through the outer canopy.
Imagine you are a spray droplet trying to get inside this canopy.
Study 1 – Qualitative Observations
In August, 2011 we worked in a market garden operation in Bolton comparing the spray coverage from four different nozzle configurations. We used the growers typical spray parameters: a travel speed of 4.5 km/h (2.8 mph), an operating pressure of about 4 bar (60 psi), a boom height of 45 cm (18 in) above the ground, and a sprayer output of 550 L/ha (~60 gpa). To monitor spray coverage, water sensitive paper was placed face-up in the middle of the tomato canopy. This diagnostic tool turns from yellow to blue when contacted by spray.
Water-sensitive paper at top of tomato canopy – easy to hit.
This particular sprayer was equipped with an air assist sleeve that blew a curtain of air into the canopy at about 100 km/h (65 mph) as indicated by an air speed monitor placed at the air outlet. When properly adjusted, air-assist booms have a number of benefits:
They part the outer canopy giving spray access to the inner canopy.
They rustle leaves to expose all surfaces to spray.
They permit the use of smaller droplets, which are more numerous and adhere to vertical surfaces, by entraining them and reducing drift.
They extend the spray window by permitting the applicator to operate in slightly higher ambient wind speeds.
Boom sprayer with air assist sleeve operating.
We sprayed using the four different nozzle configurations, with and without air assist. Our goal was to make qualitative assessments (Good, Moderate, Poor), and here’s what we observed:
Nozzle Type / Sprayer Output
With Air Assist
Without Air Assist
80 degree flat fans /~550 L/ha (60 g/ac)
Good coverage in upper canopy
Poor / Moderate canopy penetration
Low drift
Good coverage in upper canopy
Poor canopy penetration
Moderate drift
80 degree air induction flat fans /~550 L/ha (60 g/ac)
The air induction nozzles performed poorly. Their Coarse/Very Coarse droplets impacted on the outer canopy, created run-off and resulted in very little canopy penetration. Medium droplets produced by twin fans and conventional flat fans were both inconsistent with inner-canopy coverage, but some advantage may have been observed with air assist. The TwinJets contributed to higher drift (likely because they were too high off the canopy) but otherwise produced coverage similar to the conventional flat fans. From these observations, the convention that spray shape (e.g. cone, fan, twin) has little or no impact on broadleaf canopy penetration holds true.
Acceptable spray coverage deep in canopy (harder to hit) using hollow cone nozzles and air assist.
After inspecting the papers deep in the canopy, we were surprised that air assist did not obviously improve canopy penetration. It did seem to help, but it wasn’t a slam-dunk. This may be because finer droplets (<50µm) are not easily seen on water sensitive paper. It might also be because we did not calibrate the air speed to the canopy: too little air and spray impacts on the outer canopy, while too much air forces leaves out of the way and spray is blown into the ground. It was obvious that drift was greatly reduced, so logically the spray had to have gone somewhere – we can only assume it entered the canopy.
The best results were achieved with hollow cones and air assist. Theoretically, smaller droplets should improve the potential for coverage by sheer number, but they slow quickly and are easily blown off course. Winds were only about 5 km/h (3 mph) during the trials. Had they been higher, the no-air-assist condition would have resulted in poorer canopy coverage. While we feel the air assist improved inner canopy coverage, we attribute much of the performance to the spray volume of 750 L/ha (80 gpa), which was significantly higher than we used with the other nozzles. When we attempted lower volumes using the hollow cones (not shown) the inner canopy coverage was greatly compromised. Higher volumes are a demonstrated means for improving canopy penetration, so this observation is consistent with what was expected.
The 2011 trial suggested that hollow cone tips used with high volume and air assist, improved canopy coverage and penetration. They are, however, very prone to drift and their use is not recommended without an air assist sleeve to counter the spray drift. Spray volumes over 500 L/ha are highly recommended.
Study 2 – Quantitative Observations
In July, 2016 we ran another study in Chatham-Kent. This operation was concerned about spray drift and recently changed from Hardi hollow cones on 25 cm (10″) centres to TeeJet Turbo TwinJets on 50 cm (20″) centres. They wanted to know if they had improved their coverage. We decided to test four nozzles at similar driving speeds and volumes.
Once again, we used water-sensitive paper. This time we placed two pieces back-to-back (face up and face down) about 1/3 down into the canopy. Then we placed two more in the same orientation about 2/3 down into the canopy. We did this for three plants for each pass. The next four images show the visual drift and weather conditions for each nozzle. Note that only one boom section was nozzled (indicated by a white line) in each condition.
It was very humid, making it difficult to place and retrieve the papers without smearing them. This made it tricky to discern differences in coverage, and the blurring prevented us from quantifying droplet density (i.e. number of drops per unit area). Nevertheless, papers were scanned and the percent coverage was calculated using the DepositScan software developed by the USDA’s Dr. Heping Zhu. The average percent-coverage (± S.E. n=3) is shown in the image below.
Coverage on the upward-facing papers in the upper portion of the canopy showed excessive coverage for all nozzles but the 3070. Little or no coverage was detected on the downward-facing cards, but without air-assist or a directed application (e.g. drop arms), this was expected. It’s the deeper canopy that’s of particular interest. The only significant difference may lie in the XR flat fan which showed more coverage on the upward facing papers and some (however little) on the downward facing papers.
This came as something of a surprise given that the XR produced a Fine spray quality and there was no air assist to guide spray into the canopy. I believe the high humidity and low winds played a role in this outcome by reducing evaporation and off-target drift. On a drier, windier day, we likely would not have seen this level of inner canopy coverage for either the XR or the hollow cone. By comparison, the Turbo TwinJet with its Coarse spray quality not only reduces off target drift, but would be more resilient in drier and windier weather and may very well have produced the best coverage by comparison.
Take Home
Drawing from both studies:
Properly calibrated air assist will reduce drift and has promise to improve canopy penetration/coverage.
Spray shape (e.g. twin, hollow cone, flat fan) does not seem to play a role in canopy penetration.
Spray quality larger than Coarse may negatively impact canopy penetration in tomato.
Coarse spray quality is perhaps the most versatile option when volume is sufficient (>500 L/ha).
Fine-Medium spray quality is only a viable option in high humidity and light winds. However, air assist is critical to counter drift, and high spray volumes (>500 L/ha) are still required despite the higher droplet count.
Underleaf coverage is exceedingly difficult to achieve, even with finer spray quality and air assist.
This occurred in Ontario (date and location withheld). The sprayer missed the outer edge of the tomato field during a late blight application. An unintentional field check, and amazing to see the results.
When T3 wheat rears its head, the first rainy day brings questions about spray angles. Let’s begin with a graphic that illustrates how angled sprays cover a vertical target like a wheat head. Assuming moderate wind and sufficiently large droplets, this is a simplified depiction of what we would expect to see.
But is this how the nozzles actually perform? Are dual angles really better than a single fan with an aggressive angle? We hoped to answer these questions when we demonstrated a selection of dual fan nozzles at Canada’s Outdoor Farm Show in 2013. But it was a very windy few days and what we saw was that regardless of the nozzle, most of the spray tended to deposit with the wind.
A 10 km/h wind will easily deflect Medium-and-smaller droplets and at 20 km/h all but the coarsest spray is deflected. This leads to non-uniform deposits and unacceptable levels of drift (yes, even through it’s a fungicide and you have lots of acreage.) To learn more, we turned to the literature to review studies performed in Ontario and Saskatchewan.
Wolf and Caldwell
In 2002, Dr. Tom Wolf and Brian Caldwell experimented with fan angles. They evaluated the impact of nozzle angle, travel speed, and droplet size on the “front” (facing the sprayer’s advance) and “back” (sprayer’s retreat) of vertical targets. They ran three laboratory experiments: spray configuration (single vs. double fan), travel speed (7.6 and 15.2 km/h) and spray quality (conventional versus air-induced droplets) using TeeJet XR’s and Billericay air bubbles at a rate of 175 L/ha. Here’s what they observed:
Larger, air-induced droplets produced higher average deposits than smaller, conventional droplets.
Twin fans improved overall average deposit compared to single fans.
Building on the first two points, twin air-induction fans improved overall average deposit versus conventional twin fans, and also improved deposit uniformity (i.e. coverage on the front versus the back of the vertical targets).
Higher travel speeds improved overall average deposit, but at the cost of reduced uniformity as the rear-facing target received reduced coverage (particularly in the case of conventional droplets).
Spray angle did not impact coverage from conventional tips, but increasing from 30 to 60 degrees improved coverage for AI tips.
While the coverage data was compelling, growers were not reporting improved efficacy with the improved coverage. The authors felt there were confounding variables like crop susceptibility, disease pressure and product effectiveness. Their conclusion was that applicators should strive for improved coverage, but only after integrated pest management (IPM) criteria such as product choice, crop staging and application timing are satisfied.
Hooker and Spieser
In 2004, Dr. David Hooker (University of Guelph) and Helmut Spieser (OMAFRA) started exploring nozzle configuration and sprayer set-ups to optimize Folicur applications in wheat. For several years they ran field trials exploring panoramic wheat head coverage. That is, not only the front and back of the wheat head, but the sides as well. Ten different nozzle configurations were used:
TurboTeeJets mounted in dual swivel bodies (backwards and forwards)
AirMix air induction nozzles mounted in dual swivel bodies
Air induced Turbo TeeJets mounted in dual swivel bodies
Single Turbo TeeJets angled forward or angled backwards
Single Turbo FloodJets angled forward or angled backwards
TwinJets
Single Hollow cones
Turbo TeeJet’s mounted in Twincaps
Turbo TeeJet Duos
Single Turbo FloodJets alternating forward and backwards
They explored boom height (0.5 m and 0.8 m above the crop), travel speed (10 km/h and 20 km/h) and application volume (93.5 L/ha and 187 L/ha). Here is a summary of their findings:
Travel speed did not appear to impact overall coverage.
Spraying higher volumes improved coverage.
Lowering the boom improved coverage.
Coverage from conventional flat fans and TwinJets gave ~15-18% coverage and 22-26 mg of copper was deposited per m2, but alternating Turbo FloodJets gave ~29% coverage and deposited ~37 mg copper per m2.
The highest percent coverage was obtained using Turbo TeeJets or the AirMix tips mounted in dual swivels (~26% coverage), or single Turbo Floodjets alternating forward and backwards (34% coverage) as long as the spray was not obstructed by the boom structure itself.
Hooker and Schaafsma
A few years later, Dr. Hooker and Dr. Art Schaafsma worked with OMAFRA to explore efficacy. DON is a mycotoxin that may be produced in wheat infected by Fusarium Head Blight (FHB) or scab. There is an indirect relationship between wheat head coverage of fungicide and the reduction of FHB and DON: The higher and more uniform the coverage (with the right timing) the lower FHB and DON.
In two field experiments they performed in 2008, DON values in the untreated checks were around four parts per million. DON was reduced by an average of 22.5% using a single flat fan, 23.0% using a TwinJet and 41.5% using alternating Turbo FloodJets when averaged across two fields, two fungicides and four reps (n=16). They all reduced DON significantly. There was no statistical difference between singles and twins, but control from the alternating Turbo FloodJets was significantly better.
The Return of Wolf and Caldwell
Then, in 2012, Tom and Brian evaluated the new asymmetrical twin fan nozzles from TeeJet. The marketing claimed they could improve overall coverage at higher travel speeds because they decrease the contribution of the front-facing fan and increased the angle of the back. Tom and Brian’s lab-based experiments determined that:
Asymmetricals increased overall deposit amounts and uniformity versus single fan and symmetrical twin fans.
Nozzle orientation (alternating or not) seemed unimportant.
As suggested earlier, boom height was a big factor in coverage. Nozzle angle didn’t improve coverage when the boom was too high, but spray deposit increased significantly when the boom was lowered.
Coarser spray droplets have more momentum, so they can travel greater distances on their original vector. A coarser spray quality is the best choice for any angled fan.
Water volumes and FHB
Let’s address the notion that high water volumes might increase Fusarium Head Blight (FHB). This is a hypothesis that seems to have resonated with growers. Dr. David Hooker ran trials where he tried to favour FHB by spraying 40-50 gpa of water multiple times per day (even up to 100 gpa). There was no pathological impact (personal communication).
Consider that 1″ of rain is the equivalent of 2,715 gpa of water. Raising your carrier volume from 15 gpa to 20 gpa is the equivalent of 0.000184″ of rain. Admittedly, it’s all aimed at the wheat head, but it’s still a tremendously small volume. While studies have shown a diminishing return in coverage at 30 or 40 gpa, spraying with 20 gpa appears to be a safe way to improve coverage significantly.
Learn more about early morning spraying here, and a more in depth discussion of spraying when there is dew here.
Summary
So here’s what we can say based on all this research:
Higher volumes improve coverage (significantly up to ~200 L/ha or 20 gpa). Can you go to 30 gpa? Yes, and it will likely improve coverage, but it’s a diminishing return and at some point you will incur run-off.
When using angled sprays, coarser droplets improve vertical coverage. Compared to finer droplets, they move faster, survive longer (i.e. resist evaporation) and are less likely to be deflected by wind.
Maintaining the lowest operable boom height improves coverage from angled sprays. We want 100% overlap at target height, and with angled sprays that means getting pretty close. Aim for the highest wheat heads and not the tillers. If you’re 2′ away, you’re likely too high.
Symmetrical fans with shallow angles (e.g. 30°) improve coverage uniformity on vertical targets versus single fans, and a steeper backward-facing angle (e.g. 70°) improves coverage even more on the sprayer-retreat side.
Travel speed may or may not affect coverage, but slower speeds do facilitate lower booms, which do improve coverage.
Timing, weather and product choice are likely the most critical factors.
Angled sprays may offer some advantage in other situations, but they are primarily intended for panoramic coverage of vertical targets.
