Category: Drift

Articles about managing spray drift and inversions with horizontal boom sprayers

Three Features that Should be Standard on all Sprayers
One of my main activities in the winter is public speaking. Attending producer meetings gives me the privilege of meeting many farmers, learning about their operations, and sharing my research results.
I enjoy providing practical solutions to problems. But there are three issues that always come up to which I wish I had better answers. Here they are:
1. The Correct Spray. We’re stuck with compromises in this area. We need small droplets for coverage. We need large droplets for drift control. We need to keep application volumes moderate for productivity. We’ve basically asked the nozzle to shoulder the entire burden of our application needs, seeking a spray that hits all the right notes. Not too fine. Not too coarse. Able to work with fast and variable travel speeds and high, variable boom heights.
Based on our research in field crops such as wheat, canola, corn, lentils, etc., we can be confident that Coarse, even Very Coarse sprays, coupled with a reasonable water volume, are appropriate for most modes of actions and target situations. These sprays contain enough small droplets for good coverage, and their larger droplets work surprisingly well in most cases. Sure, a finer spray could save some water. And a coarser spray would reduce drift even more. But we need a compromise spray, combined with some lucky weather, to get the job done.
And yet we usually make spray quality recommendations with caveats, because droplet size alone isn’t enough. Drift is always a possibility, no matter how coarse we go. Coverage is not guaranteed, especially if the canopy is dense. Finer sprays will get deeper into a broadleaf canopy, but then we may have drift or evaporation to deal with. The nozzle size, volume, and travel speed relationship has to be just right so the spray pressure is in the correct range. And on it goes.
I’d like to give the overworked nozzle some help. We used to use shrouds to protect fine sprays from drift. Now it’s time to let air assist take over that task.
Air assist booms can accelerate (i.e., add kinetic energy to) small droplets so they’re less prone to off-target movement. Properly adjusted, air assist can carry these droplets deeper into the canopy and enhance their deposition.
A good air-assist system allows the user to select the strength and direction of the airblast to match canopy, boom height, and travel speed conditions.
Air assist is the workhorse of most fruit-tree and vineyard spraying. It has to be done right to provide all the benefits I mentioned, and certain approaches should be rejected. For example, there are some companies using air assist to promote very fine sprays with very low volumes. That’s the wrong use of the technology, and invites a backlash.
Instead, we need systems that work with existing spray practice to address some of its classic shortcomings, such as drift management, deposit uniformity, and canopy penetration.
Let’s see some products. It’s time to bring air-assist to the mainstream of agricultural spraying.
1. Boom Height, Level, Sway and Yaw Control. Boom height is so fundamental it’s almost boring. We’ve long said that it’s important to set the boom at the right height for proper nozzle overlap and drift control. It was easy with wheeled booms. But over the last 15 years, suspended booms coupled with fast speeds have caused booms to rise again (RISE OF THE BOOMS!).
Fact is that there are some tasks we’re asking of nozzles that they simply can’t achieve without level, low booms. Drift control is one such thing. Low booms are surprisingly effective at reducing drift, not only because winds are lower closer to the canopy, but also because droplet velocities are faster closer to the nozzle.
Angled sprays for fusarium headlight control are another thing that is more effective with low booms.
Spray droplets released from an angled spray soon slow down and get swept back by air resistance and begin to fall vertically, or move with wind currents, reducing their intended benefit. Low booms can prevent that.
Uniform and low booms also keep deposit variability more manageable. They can save energy needed for air-assist systems. The shorter the path to the target, the less air-velocity will be needed to get it there.
So how about it? Can we have boom linkages and suspension systems, coupled with sensors and hydraulics, that are stable and maintain 20” above canopy at 16 mph on uneven ground? Can we have systems that do this reliably enough that we’re prepared to invest in, say, expensive nozzle bodies? It’s possible.
1. Sprayer Cleanout. One of my favourite questions about cleanout is: “When do you know that you’re finished cleaning the sprayer tank and booms?” Inevitably, someone from the back yells: “In two weeks!” And we laugh, knowingly.
We have a terrible system of sprayer decontamination. It’s a process that is awkward, imperfect, and time consuming, often leading to poor practice. I’ll ask a group of producers what they do with their pesticide waste. The response is silence. I don’t blame them for not telling me that they dump the remainder on the ground somewhere, but I’d rather they didn’t. Sprayer designs don’t help.
What we need is a system that guarantees results. To start, a tank gauge that is reliably accurate to the nearest gallon would remove some of the filling guesswork and help minimize leftovers.
We need a remainder volume (volume left in the non-boom plumbing after the pump sucks air) that is known and small, because that remainder can’t be expelled and needs to be diluted. The smaller it is, the easier it is to dilute.
We need pumps that can run dry, so nobody has to fear spraying the tank out completely.
We need a wash system that requires little volume and works quickly, like continuous rinsing.
We need plumbing that is easy to understand and whose inside surfaces do not absorb pesticide, or hide it in corners and dead ends. Perhaps it’s a recirculating system. Perhaps it hasn’t been invented yet.
We need pesticide formulations that clean up easily. We need an easier way to inspect and clean filters. And we need a safe place to put any waste that can’t be sprayed out in a field.
I’d like to see a sprayer that can be decontaminated in 10 minutes without the operator leaving the cab, and without any spillage of spray mixture. Clean enough to spray conventional soybeans after a tank of dicamba. Clean enough to spray canola after a tank of tribenuron. I know it’s possible.
I also know what many of our European readers are thinking right now. Much of what I’ve discussed exists in the EU in some form or another. Why does the North American, and to a lesser extent the Australian market, not have these features?
Part of the reason is federal standards and regulations. Some European countries test and approve products for remaining tank volume, boom stability, and spray drift, for example. Others have sprayer performance criteria that must be met to be eligible for sale in that country. An increasing number have mandatory sprayer inspection.
These requirements serve to protect the producer and the environment. They’re an example of useful government actions. Despite, or perhaps because of, stricter rules, the entire EU marketplace is very competitive, with about 75 sprayer manufacturers. Bottom line: producers benefit.
We need leadership, preferably from a combination of government, industry, and producers, to achieve better sprayer designs. Our market has room for products that make it easier to prevent drift, protect water, and protect yields.
As they say, a rising tide lifts all boats. And it will certainly make my job easier.
March 21, 2017
Spray Drift – Why is it still happening?
Despite the abundance of information available on spray drift, we continue to see widespread incidents of damage to a variety of crops every year. Do applicators just not care or are they missing some vital information when making decisions to spray? I believe it is the latter.
What is the problem?
In my experience, the vast majority of spray drift cases (probably 90% or more) are the result of ‘inversion drift’. That means the drift has not come from an adjacent sprayed area, it has come from one or more sources that are some distance from the site of damage. The distance between the sprayed site and the location of the damage may vary dramatically, from a few kilometres to tens of kilometres.
Why is there so much inversion drift when labels specifically prohibit use of the products under surface temperature inversions? Many may argue that it is a blatant disregard of the label by a few applicators (translation = cowboy operators). I do not agree this is the main problem. While I can confirm the existence of ‘cowboy operators’, thankfully they are limited in number. I believe the problem is a lack of understanding about how to tell when there is an inversion and particularly not knowing how ‘day wind’ moves differently to ‘inversion wind’. I continue to see good farmers/applicators doing what they believe to be the right thing but it is not. These are people very concerned about minimizing spray drift; they honestly do not think they are doing anything wrong.
What is ‘day wind’?
After sunrise, the sun begins to heat the ground, the warm ground then heats the air close to the surface, and this air then rises. As that warm air rises, cold air from above sinks down to replace it. The ground then warms this cold air and it rises. This cycling of warm air rising and cold air sinking creates turbulence and then wind. This is a good thing; turbulent wind movement is much safer for spraying. ‘Day wind’ has a turbulent motion and is much more likely to pull any fine droplets to the ground within a reasonable distance. During the day, we can predict which direction and how far our fine droplets will travel.
What is ‘inversion wind’?
As the sun sets, the ground begins to cool quickly and this in turn cools the air next to the ground. As we all know, cold air does not rise and warm air does not sink. This means there is a layer of cold air trapped close to the surface and a layer of warm air above it. The result is no turbulent movement or mixing of the air. The air may become quite still and this is often observed around sunset when the daytime wind ceases or drops off. What happens next is where the real danger occurs for spraying.
As the night progresses and the ground cools more, the cool air close to the surface becomes colder and therefore more dense, particularly from midnight onwards. This cold dense air then begins to move across the landscape, often down slope and in very unpredictable directions. Remember this air is not turbulent, there is no mixing, it has layers of air, something like layers in plywood, and it flows parallel to the ground. Any fine droplets released into these layers of cold non-turbulent air will simply move sideways across the surface until the sun rises and heats the ground again. This is when the fine droplets are released from the layers and they come to ground, often in the lower parts of the catchment and a long way from the site of application. It is impossible to predict what direction this ‘inversion wind’ will go. For this reason, spraying in this type of wind is extremely high risk for spray drift.
Key indicators that and inversion is unlikely
- We should always expect that a surface temperature inversion has formed at sunset and will persist until sometime after sunrise unless we have one or more of the following: continuous overcast weather, with low and heavy cloud;
- continuous rain;
- wind speed remains consistently above 11km/h for the whole time between sunset and sunrise;
- and after a clear night, cumulus clouds begin to form.
Inversion wind movement – practical demonstration video
Wind is a key factor in any spray application. The problem is that not all wind is the same. To reduce the incidence of spray drift, it is critical that spray applicators understand how wind moves, particularly during a surface temperature inversion. This video uses smoke flares to visually demonstrate the air movement under inversion conditions.
Here’s what we’re looking for: moderate wind with consistent direction that disperses spray and drives it to ground.
Conclusion
Many factors affect the potential for spray to drift but the main ones are; the weather conditions at the time of application, nozzle selection, products/tank mix used, actual spray quality achieved, speed of rig, and boom height. The common denominator is that all of these things are within the control of the spray operator.
Spraying under inversion conditions is extremely high risk and prohibited on many product labels, that means it is illegal. If you are serious about preventing drift, then you must learn how to identify when an inversion is likely to be present and more importantly when it has broken.
All agricultural chemicals have the potential to drift; it is how we use them that increases or decreases that potential. Therefore, the problem is a human one, not a chemical one. There is a suite of information available but if you are still unsure or need any assistance, please seek advice from an expert. Maintaining long-term access to key products depends on us reducing spray drift.
March 8, 2017

Disease Control in Berry Crops

In the spring of 2016, the Ontario Berry Growers Association (OBGA) conducted a survey of its membership to poll how fungicides were being applied. The results were very interesting.

Fungicide basics

Generally, fungicides registered for berry crops are contact products, so coverage and timing are very important. The fungicide has to be distributed evenly on the target before disease has a chance to infect the crop. That means the sprayer operator must be aware of the susceptibility of the crop to the level of disease pressure to ensure timing is appropriate. While kickback and post-application distribution of pesticide residue is sometimes possible, sprayer operators should not rely on it. The following table outlines application recommendations for a fungicide commonly used in Ontario. It combines labelled information and provincial recommendations and is representative of most fungicides.

	Summer-fruiting and Fall-bearing Raspberry / Blackberry	Highbush Blueberry	Day-neutral and June-bearing Strawberry
Labelled rate	2.5 kg/ha	2.25 kg in 1,000 L/ha	2.75-4.25 kg in 1,000 L/ha
Diseases (Labelled and Ontario provincial recommendations)	Anthracnose fruit rot, Spur blight, Leaf spot, Botrytis grey mould	Anthracnose fruit rot, Shoot blight (Mummy berry), Botrytis twig and/or blossom blight	Common leaf spot, Botrytis grey mold
Crop staging	Bloom, Pre-harvest, Harvest	First bloom, Fruit ripening	Flower bud, First bloom, 7-10 days after bloom, Pre-harvest, Through to fall

As of 2016

The spray target

The applicator reading the recommendations should be considering the best way to get the fungicide to the target. But, what is the target, and what is the best way to apply it? It seems the recommendations raise as many questions as they answer:

With the possible exception of blueberry, this fungicide can be applied through much of the growing season (especially when it’s been a wet season). That means the crop staging is highly variable.
The primary target is blossoms, but depending on the disease, leaves and stems are also important.
The label states a volume of carrier (i.e. 1,000 L/ha) for strawberry and blueberry, but not the cane fruit. It does not specify highbush blueberry versus the sessile, ground cover variety.

So, this means is the sprayer operator has to spray crops with highly variable physiology (e.g. bush, cane or sessile row crops), onto very different targets (e.g. leaves, canes, stems, flowers) throughout much of the season as the crop canopies grow and fill. This is a very challenging spray application. It would be wrong to suggest a single spray quality, water volume or sprayer set-up to efficiently accomplish all these goals (more on that later). The first consideration is the application equipment itself.

The application equipment

Berry growers employ a variety of sprayers to protect berries. Without considering models or optional features, there are three fundamentally different styles: Airblast, backpack and boom. According to the survey, the following table shows which sprayers are used in which berry crop in Ontario. Approximately 60 growers responded, and many grow more than one variety of berry and use more than one style of sprayer.

	Airblast Sprayer	Backpack or Wand Sprayer	Vert. or Hor. Boom Sprayer	Total
Highbush blueberry	8	1	0	9
Day-neutral Strawberry	3	0	21	24
June-bearing Strawberry	5	0	32	37
Raspberries & Blackberries	21	1	7	29
Total	37	2	60

So, generally, cane and bush berries are sprayed using airblast sprayers and strawberries using horizontal booms. The survey didn’t specify features such as air-assist on booms, or whether or not those booms are trailed or self-propelled. The type of, and features on, any given sprayer dictate the limits of what an operator can adjust to improve coverage.

Water volume

Respondents also reported on how much carrier (i.e. water) they used to spray fungicide on their crops. Given Canada’s propensity to report volumes in many different forms, I have converted all values into the most common units: L/ha, US g/ac and the dreaded L/ac:

	n	L/ha ± std (max./min.)	US g/ac ± std (max./min.)	L/ac ± std (max./min.)
Highbush Blueberries	7	534.2 ± 340.1 (1,000/150)	57.1 ± 36.4 (106.9/16)	216.2 ± 138 (404.7/60.7)
Day-neutral Strawberries	22	418.5 ± 192.2 (1,000/224.5)	44.7 ± 20.6 (106.9/24)	169.4 ± 77.8 (404.7/90.8)
June-bearing Strawberries	33	403.1 ± 235.1 (1,000/50)	43.1 ± 25.1 (106.9/5.3)	163.1 ± 95.1 (404.7/20.2)
Raspberries & Blackberries	27	450.1 ± 279.4 (1,200/50)	48.1 ± 29.9 (128.3/5.3)	182.1 ± 113.1 (485.6/20.2)

There appears to be a lot of variability in the volumes applied, but on the whole, very few are using the 1,000 l/ha indicated in the fungicide recommendations. The ~430 l/ha overall average is no surprise; labelled volumes are quite often higher than what sprayer operators use. In some cases, high label volumes are warranted because the product requires a “drench” application to totally saturate the target, or to penetrate very dense canopies. Conversely, a high label volume might reflect outdated practices if that label hasn’t kept up with current cropping methods or application technology. Sometimes label volumes are suspiciously large, round numbers that suggest they are intended to encompass a worst-case scenario (e.g. a large, unmanaged crop with high disease pressure and a less-than-accurate spray application). In the particular case of crops sprayed with an airblast sprayer, it is very difficult for a label to accurately predict an appropriate volume due to the variability in crop size, density and plant spacing. This has led to methods to interpret labels, such as crop-adapted spraying.

The disparity between label language and grower practices is not entirely the fault of the label. Most sprayer operators don’t want to carry a lot of water because more refills prolong the spray day. In situations where the crop has reached a critical disease threshold, or bad weather has compressed the spray window, sprayer operators sometimes reduce the volumes in the belief that “getting something on” trumps “good coverage”. Perhaps that’s true, but insufficient volumes greatly reduce coverage. This can be further exacerbated when operators do not account for the increase in crop size and density over the season, or the impact of hot dry weather on droplet evaporation.

Improving coverage

So, is there an ideal sprayer set up and volume? As previously alluded, the variability in crop staging, crop morphology, target location and spray equipment make a single recommendation impossible. But that doesn’t mean there aren’t diagnostic tools and a few simple rules to help a sprayer operator determine a volume to suit their particular needs. Much can be accomplished with these three things:

Water-sensitive paper
A modest selection of nozzles and a nozzle catalogue
An open-minded sprayer operator willing to spend a little time and reconsider traditional practices

Rule-of-thumb fungicide coverage on water-sensitive paper.

Water-sensitive paper is placed in the canopy, oriented to represent the target (e.g. leaf, bloom, etc.). It is important to put multiple papers in at least three plants to ensure the coverage reflects a typical application. The paper changes colour when it’s sprayed and this provides valuable and immediate feedback. Did the spray go where it was supposed to go and did it distribute throughout the target? If so, then the operator now knows that they can safely focus on timing rather than targeting. If not, a little diagnosis is required:

1. Were targets completely drenched? If so, there is too much coverage. Operators can drive faster (if possible, and as long as it doesn’t create drift), reduce operating pressure (if possible, and as long as the nozzle is still operating in the middle of its registered range), or change nozzles to lower rates (as long as spray quality is constant).

2 .Were targets only partially covered, as if a leaf obstructed part of the target and created a shadow? This mutual-shading is the bane of spraying dense canopies. One possible solution lies in understanding droplet behaviour: Coarser sprays generally mean fewer droplets and they move in straight lines. Therefore, when they hit a target, they might splatter or run-off, but typically their journey is over. If the spray is too Coarse, a slightly Finer spray quality increases droplet counts and may help droplets navigate around obstacles and adhere to more surfaces. Sprays that are too Fine will not penetrate dense canopies without some form of air assist. They slow very quickly and tend to drift and evaporate before they get deep enough into a canopy to do any good. A Medium droplet size is a good compromise because it produces some Fines and some Coarser drops – the best of both worlds.

Increasing volumes and reconsidering spray quality often helps, but there might be other options. If using air assist, there are tests that can confirm the air volume and direction are appropriate. Another solution might lie in canopy management (where pruning bushes and canes can help spray penetration immensely). Still another might lie in the use of adjuvants to improve droplet spread on the target.

3. Were targets missed entirely, or coverage is consistent but sparse? The operator is likely not using enough water, and/or the spray quality is too fine. It has been demonstrated time and again that higher volumes improve coverage, but operators can try any of the options listed previously for partially-obstructed coverage. All the reasoning is the same.

Conclusion

Spraying fungicides effectively requires an attentive sprayer operator. Timing and product choice are very important, but when it is time to spray the sprayer operator should diagnose coverage with water-sensitive paper, and be willing to make changes to the sprayer set-up to reflect changing conditions. Thanks to the OBGA for sharing the survey data.

June 27, 2016

Fungicide Application in Cereal, Pulse, and Oilseed Crops
Get ready for a busy fungicide season. If your growing conditions have been good, your crop is dense and vigorous, and soil moisture is adequate, you have yield potential to protect. A bit of moisture and warm temperatures at a critical time, and disease is likely to develop.
Before we delve into how to apply fungicides, let’s review the basics.
1. There is no substitute for an informed decision about whether to spray or not. Seek the advice of a professional to make sure you understand your crop’s genetic susceptibility to disease, the conditions conducive to its development, and the parts of the plant canopy that are affected and therefore need protection. How much yield or seed quality is actually at risk? What do the disease forecasts say for your area?
2. Identify the best fungicide product for your disease situation. Consider inherent efficacy, but also the longevity of the protection and the fungicide’s off-target toxicity (less toxic products can be sprayed in windier conditions without harming susceptible ecosystems). Remember that most fungicides are not curative and must be present on the plant foliage before infection takes place. Also remember that most fungicides are not easily translocated and are at best “locally systemic”. This means that fungicide deposit must cover the plant part that requires protection with an adequate droplet density. If the fungicide is systemic, these deposits must be absorbed through the plant cuticle and will only migrate a small distance within the plant tissue, usually in the transpiration stream, from the point of application.
3. Make proper timing the priority. Disease control is usually only effective if the fungicide is applied in a narrow time frame in which the crop or disease is at a certain developmental stage. A great application at the wrong time is less valuable than mediocre application at the right time. The use of low-drift nozzles should be considered an agronomic tool that permits the correct staging even under marginal wind conditions.
Let’s now review the major highlights of fungicide application in the major cereal and oilseed crops.
Wheat
In wheat, the early growth stagings for foliar fungicides are usually done to protect from leaf spot diseases such as tanspot, septoria nodorum blotch and septoria tritici blotch. Because these diseases are trash-borne, they tend to migrate up from the bottom to the top and good canopy penetration of the spray is important.
Better canopy penetration can be achieved the following ways:
- Higher water volumes. This is probably the most powerful tool in an applicator’s arsenal. More water usually delivers higher doses of active ingredient deeper into the canopy, and whatever dose does get deposited will be present in higher droplet densities. So in short, for any given spray quality (droplet size), more water provides better coverage. We all intuitively know this.
- Slower travel speeds. Moving slower imparts less of a forward velocity on the spray cloud, particularly in the larger droplets. As a result, these droplets move more vertically. In the case of a cereal canopy, more of the spray will reach the lower leaves. The finer droplets in the cloud tend to deposit with the wind direction regardless of travel speed.
- Backward pointed nozzles. If a droplet moves backwards at the same speed as the spray boom moves forwards, then it is basically standing still relative to the crop. It will have a greater chance of moving down towards the lower canopy than a droplet that’s moving forwards. The latter droplet will likely be intercepted by something vertical, like a wheat head or stem.
A single nozzle oriented back, applying a water volume that is at least 10 to 15 US gpa, will be sufficient to get good canopy coverage for leaf spot and rust protection.
Fusarium Headblight (FHB), caused by Fusarium graminearum, is a special case. It infects the wheat head at anthesis, and fungicide must be present on the head, at the glumes where the anthers emerge, at the time of infection. So we have a relatively large vertical target that is at the very top of the canopy. Initial work at North Dakota State University, followed up by work at AAFC in Saskatoon and the University of Guleph at Ridgetown, found the following:
- Angled sprays are essential. Field and lab studies showed that angled sprays were much more effective at depositing the fungicide on heads than vertical sprays. Backward pointed angled sprays provided additional help at targetting the other side of the wheat head. Twin nozzles are available from most manufacturers.
- Use Coarse sprays when angling. Angled and twin sprays have their challenges. The angle at which the spray is released dissipates quickly, particularly for smaller droplets. As a result, more aggressive angles and coarser droplets were found to be more effective. Larger droplets were able to maintain their initial trajectory for a longer distance, increasing the chance that the droplet hit the head from the side rather than passing it by vertically.
- Maintain low boom heights. Even coarse sprays are deflected by air resistance and will eventually stop moving in the direction they were first emitted. In fact, this happens within a short distance. Low booms, less than 25″ if possible, help.
- Watch wind speed and direction. Field observations show that even a moderate wind can over-ride the application practices described above, resulting in most of the spray deposited on the windward side of a target regardless of its initial release.
- Awns intercept small droplets. Many of our modern wheat cultivars are awned, and these fine structures are excellent collectors of small droplets. In early studies with durum, we found a large proportion of the spray volume on awns, where it served no useful purpose. The best way to minimize this awn interception is to ensure coarse sprays and sufficient water, no less than 10 gpa.
It’s important to maintain realistic expectations with FHB. Fungicide chemistry is improving but still offers only suppression. Crop staging is variable. Excellent application practices place the odds more in favour of disease control, but can’t change these facts.
Pulse Crops
Lentils and peas are increasingly important crops. They appear spindly in their early stages of development and are poor weed competitors. But under the right conditions, lentils soon form an impressive set of leaflets that creates one of the most impenetrable barriers in our cropping systems.
Here are some pointers for fungicide application in pulse crops:
- Understand the disease in your crop. Do you need to protect stems (anthracnose), leaves and stems (ascochyta complex, mycosphaerella), or senescing leaves or flowers (sclerotinia)? This is where the spray needs to go.
- Understand the time of disease development.
  - Trash-borne diseases like anthracnose and ascochyta will start at the bottom of a lentil canopy, and early treatment before canopy closure will be important to arrest or at least delay disease development as long as possible.
  - Late season diseases like sclerotinia and botrytis push the application timing towards a closing or closed canopy. Success of such sprays is more elusive because of the rapid development of new biomass.
- Take a bird’s eye view of the canopy.
  - If you can see the target you need to spray, the job is pretty straightforward and conventional water volumes and nozzles will work.
  - If the targets are hidden from view, it will take more water and slower travel speeds to get the required coverage. Consider the higher end of the recommended water volumes (15 gpa in most cases), slower travel speeds (10 mph).
- - When a canopy has many layers of cascading leaves, it is very difficult for a spray to get past these “umbrellas”. We’ve observed many times that a leaf is a very effective shield for anything below it. Large droplets have a hard time changing direction because of their mass and resulting momentum. But small droplets, especially those below 100 microns, can move with slight changes in air movement and get around these obstacles. Use higher pressures (to generate the finer sprays) or select finer nozzles to improve canopy penetration.
- Look at the size of the plant part you need to target. Large targets like leaves can capture almost any droplet size, but small targets like petioles or vertical targets such as stems may benefit from finer sprays, especially if they’re hidden in the canopy.
Generally speaking, dense pulse canopies will require higher water volumes and finer sprays than their cereal counterparts. Although twin fan nozzles have not been shown to provide an advantage in our studies on chickpeas, higher water volumes proved very effective at improving deposition and disease control.
Canola
Canola has two main diseases for which foliar sprays are used. A small number of producers choose herbicide timing for control of blackleg. Because the crop canopy is small and the spray targets are exposed, general herbicide application guidelines (Coarse sprays from a venturi nozzle, 7 – 10 US gpa) will provide good targeting and adequate coverage.
Sclerotinia control requires that the spray reaches buds and petals of canola that is between 20 and 50% flowering. Work at AAFC in Melfort compared conventional and low-drift sprays at two pressures, and showed that droplet size had no effect on disease control. In fact, the Fine spray produced by hollow cone nozzles at high pressure did not significantly improve sclerotinia control compared to a venturi nozzle at its recommended pressure of about 60 psi.
Subsequent lab work showed that the proportion of the applied spray that was retained by petals and buds was statistically identical for all tested sprays.
Water volumes may need to be increased for modern canola hybrids that have significant biomass at flowering. Such cultivars may grow over 1.5 m tall and present a large range of canopy positions in which buds and petals appear. As with the other crops, when a spray needs to cover more area, and especially when this area presents itself in layers, more water volume is appropriate.
Fine Sprays for Coverage
Conventional wisdom says that fungicides require finer sprays for coverage and best effect. This is certainly true in some cases, particularly where the leaf area index is high and leaves are arranged in cascading layers. But it’s time to retire this notion as general advice and adhere to research results for guidance. For FHB, the recommended angled sprays benefit from being applied in coarser, not finer sprays. And in pulses and canola, research showed that there was no benefit from finer sprays. In fact, finer sprays can impair proper timing because of their propensity for drift and rapid evaporation under dry conditions.
Modern coarse sprays produced by air-induced nozzles are less susceptible to these environmental conditions and therefore offer an important advantage: they allow for better timing accuracy. For this reason, I view them not so much as drift control tools, but rather as agronomic tools.
There is a downside to the coarser sprays – they do require more water. Volumes should always be above 10 US gpa, and many recommendations go to 15 gpa if the canopy is dense. In some cases, 20 gpa may be beneficial. These higher volumes are a reasonable price to pay to protect a valuable crop, and we certainly have the equipment to make this price bearable.
Aerial Application
Aerial application is an important way to apply fungicides. An aircraft’s chief advantage is to cover large areas with no crop trampling, and can do so even in wet conditions. As a result, they offer the timing advantage we so often mentioned in this article.
A producer hiring an aircraft for spraying ought to have a conversation with the pilot and discuss water volume and droplet size. Aircraft, out of practical necessity, apply less water and distribute it in finer sprays to offer the required coverage. Although this has been shown to be effective, it creates drift and evaporation potential. It is worthwhile to ask for higher water volumes if it means that the spray can be applied somewhat coarser, creating less drift.
The rotary atomizers on many aircraft produce fairly uniform droplet sizes and do a good job of eliminating the larger droplets. This makes even more droplets available for coverage. However, even with this technology spray drift still matters and all steps to prevent it should be taken. This means using larger average droplet sizes and increasing water volumes accordingly to their label recommendations.
June 24, 2016
The Case for Low-Drift Sprays
This article was written by Tom Wolf for “PEI Potato News Magazine”, a publication of the Prince Edward Island Potato Board (http://peipotato.org/). It is reprinted with permission.
PEI Potato News Magazine
“Should I be using low-drift nozzles?” It seems like a simple question with an obvious answer. We all want to reduce spray drift, and this easy-to-use technology is the fastest way to get there.
And yet, the question is more complicated than it first appears. Yes, all applicators want to reduce drift, but many worry about the coarse sprays produced by low-drift nozzles. As a spray volume is divided into coarser (i.e. larger) droplets, there are fewer of them, and that can reduce coverage. It’s a legitimate concern.
Let’s start with our shared value first – the desire to reduce spray drift.
Given the economic, environmental and health impacts of spray drift, the importance is hard to over-state. That’s why spray drift management is a primary concern of our federal regulators whose job is to protect the public interest. It’s also a concern for the neighbours who have a right to keep unwanted products off their property, whether it’s residential or agricultural.
Conventional flat fan nozzles (XR8004) operating at 40 psi
Glyphosate drift with 20 km/h side wind, XR8004 40 psi
Low-drift nozzles (TD11004) operating at 60 psi
Glyphosate drift with 20 km/h side wind, TD11004 60 psi
For these reason, managing drift should be a foremost concern for applicators. The technology is vital to the crop production industry, and if we don’t take care of the issue, someone else will take care of it for us. That’s not the best path.
Much has been written about how to reduce drift. The key points are:
- choosing days with better weather,
- lowering booms and travel speeds,
- watching spray pressure,
- protecting the spray with shields,
- using coarser spray qualities on the whole.
Of these, the most economical and practical is using coarser sprays via low-drift nozzles. Engineered to emit fewer fine droplets, they are proven to reduce drift by anywhere from 50 to 95% compared to a standard flat fan of the same size. When it comes to reducing drift, they work.
When these tips first hit the mainstream as “pre-orifice” nozzles in the late 1980s, and later as “venturi” nozzles in the mid 1990s, we were impressed with their ability to reduce drift. And the obvious question was, what about product efficacy? Can fewer, larger droplets do the job? The answer, to our initial surprise, was yes.
In the late 1990s, the crop protection industry (including governments, universities, and the private sector), participated in studies throughout Europe, Australasia, and North America looking at low-drift spray performance. In Canada alone, we conducted over 100 studies and concluded that pesticide efficacy was not harmed when a properly adjusted low-drift nozzle was used. A surprising result showed that fungicides did not seem to need finer sprays, contrary to popular opinion, as long as water volumes were sufficient to provide adequate coverage.
As we did more and more studies, it became apparent which points were critical:
- When using venturi nozzles, spray pressure had to be increased from the industry standard of 40 psi to about 70 psi. This is because of a venturi nozzle’s two-stage design. The high pressure compensated for an internal pressure drop inside the nozzle. Sprays remained low-drift, but patterns and overall efficacy were better at this higher pressure.
Spray pattern of conventional spray (XR8002, 40 psi)
Spray pattern of low-drift spray (ULD12002, 60 psi)
Spray deposit of conventional spray (XR8002, 40 psi. ~10 gpa)
Spray deposit of low-drift spray (ULD12002, 60 psi, ~10 gpa)
- Spray pattern overlap needed to be greater with low-drift sprays – a full 100%. In other words, the edge of one nozzle’s spray pattern should reach the middle of the adjacent nozzles’ patterns. The pattern width at target height was now twice the nozzle spacing and this ensured good distribution of not only the spray volume, but droplet numbers, along the boom.
- We needed to pay attention to the target plant architecture and leaf surface properties. Plants such as grasses (with vertical surfaces and difficult-to-wet leaves) often had less spray retention with coarser sprays. Low-drift nozzles worked, but we couldn’t go as coarse in these cases. Careful selection of low-drift nozzles as well as more attention paid to operating pressure solved these issues.
- Our minimum water volumes had to increase slightly to compensate for the fewer drops produced by low-drift sprays. This was especially true for contact modes of action where too few droplets-per-area reduced performance. Using an Extremely Coarse spray at a very low water volume was asking for trouble.
Much of my efforts in recent years have been to advise applicators just how coarse they can safely go without harming product performance. This involves things we’ve touched on in this article, like water volumes, modes of action in the tank mix, target plant or canopy architecture, growing conditions, and the like. We’ve arrived at a few rules of thumb, like those above, but as always, it’s dangerous to oversimplify and there are always new situations to grapple with.
While we were learning how to tweak low drift nozzles to get them to perform, we also learned there were significant advantages to using coarser spray qualities.
1. Foremost, there was an immediate reduction in drift. One applicator told me years ago that switching to a low-drift spray removed a huge burden of worry from him, and that alone was worth it.
2. Low-drift sprays made it easier to spray on-time, even if weather conditions were marginal for conventional sprays. The result: the timely removal of weeds, or the correct staging of fungicides and insecticides. This has paid large dividends in terms of protected yield.
3. Coarser sprays can protect product performance from some adverse conditions, such as days with high evaporation rates. On such days, fine sprays evaporate to dryness so quickly that uptake can be limited. Larger drops stay liquid longer, with more uptake the result.
4. Directed sprays, be they banded sprays or twin fan nozzles for fungicides, make more sense from coarser nozzles. The reason is that these coarser sprays go where they’re pointed, whereas fine sprays lose their path in wind or through travel-induced deflection, very quickly.
5. We also learned about the air-entrainment that coarser sprays can produce. Large droplets dragged air with them, and smaller droplets could hitch a ride in their wake. This provided a form of air-assistance that reduced drift and carried small droplets into the canopy. Finer sprays had a harder time producing this type of drag, and sustaining it in the canopy.
When we analyzed the droplet size spectrum of coarse and fine sprays, we confirmed that the total number of droplets produced by any given volume of water had been reduced. Not a surprise. But two things struck us.
First, even though the average size of droplets in coarse sprays were very large, they still contained a population of small droplets. In fact, if you counted every single droplet in the spray, the vast majority were small and they were still taking care of coverage.
Second, the critical amount of coverage (measured as the percent of the surface area covered by spray deposits) that was necessary for a given product to work was lower than what we’d been aiming for. In other words, we didn’t need as much coverage as we thought we did, and any excess didn’t actually add to product performance in most cases.
We later analyzed the relationship between spray coverage and herbicide performance and found that the uniformity of the deposits was actually more important than the amount of coverage per se. So, if we focussed on proper overlap and spray pressure there was greater benefit than increased coverage alone. Deposit uniformity has become our research focus of late.
So, should you be using low-drift nozzles? By adopting the changes in pressure, overlap, and water volume outlined above, and paying more attention to the plant architecture and pesticide mode of action, we’ve been very successful in implementing low-drift sprays in all field crops. In my view, we can safely retire Fine sprays for all field crop pesticides. This means conventional flat fan nozzles, hollow cone nozzles, and the like. Get rid of them. All they do is add drift potential.
It’s safe to adopt low-drift sprays. Research and experience from the field prove that they work. Low-drift sprays should be viewed as an agronomic tool that improves application timing and accuracy. And with less drift, we show that agricultural practice can be both efficient and environmentally responsible. That’s going to be a very important story to tell, now and in the future.
May 20, 2016