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 is intended as a basic overview of what pesticide spray drift is and how to avoid it. If you want a more in-depth study of the physics of drift, head over here.
Defining Drift
Pesticide spray drift is the aerial movement, and unintentional deposit, of pesticide outside the target area. Aside from being illegal, there are a lot of compelling reasons for avoiding it. Drift can be measured in financial loss associated with wasted pesticide, wasted time and reduced crop quality/quantity. Plus, if an application is unsuccessful, the operator may have to re-apply, incurring further cost. Pesticide drift increases any risk of damage to human health, susceptible plants (e.g. adjacent crops), non-target organisms (e.g. wild and domestic animals, pollinating insects, etc.), the environment, and property.
We’ll limit our definitions to two forms of pesticide spray drift: Particle Drift and Vapour Drift.
Physical Drift is the initial off-target movement of pesticide droplets. This occurs at the time of application, and it is generally on a scale of tens-of-metres. There is a secondary component to physical drift wherein particularly small droplets (or the evaporated remains of droplets) stay aloft for longer periods of time, during which they can move laterally with wind or vertically with thermals and turbulence.
Vapour Drift is the off-target movement of pesticide vapours. This is a function of product chemistry (vapour pressure) and surface temperature. Rainfall (rewetting) can also affect vapour loss. If vapour gets caught up in a light breeze, moves downhill during a thermal inversion, or is redistributed in precipitation, movement is can be on a scale of kilometres.
Managing Drift
Drift cannot be entirely eliminated, but sprayer operators can greatly reduce the degree and impact. Much of what follows relates predominantly to particle drift from horizontal boom sprayers, but it’s never wrong to follow these best practices. Research and modeling have shown that the three biggest factors under the operator’s control are:
Apparent wind speed (i.e. the sum of wind speed and travel speed)
Boom height (i.e. release height)
Droplet size (i.e. nozzle spray quality)
Therefore, the degree and impact of drift can be greatly reduced by following these guidelines:
Reduce the distance between nozzle and target. For a herbicide application, that means lowering the boom to the lowest practicable height. There are exceptions, but a good rule of thumb is that the boom height should be approximately the same as the nozzle spacing.
Use the coarsest effective droplet size, generally achieved through the use of drift reducing nozzles such as air induction.
Work with the weather. Labels will specify appropriate weather conditions for spraying. Change sprayer settings to account for hot, dry and windy conditions or halt the job until conditions improve. Generally, avoid spraying when the weather is against you.
Identify any vulnerable nearby crop, landscape or environmental area. Choose a spray day when winds are blowing away from these sites. Explore voluntary watchdog sites like DriftWatch to see if there are registered sensitive crops nearby. Planting windbreaks or utilizing riparian areas can also help manage wind and provide localized downwind protection.
Observe labelled buffer zones and recommended sprayer settings. In Canada, using optimal sprayer settings in the right environmental conditions may reward the sprayer operator with buffer-zone reductions.
Work with your neighbours. Let them know your intentions. For example, greenhouse growers need to be notified to close vents during morning spray times to avoid any possibility of drift.
Understand the potential damage off-target herbicides can cause and make this part of your planning when selecting a herbicide. Where possible, choose herbicides with a low risk of volatility. Avoid products like dicamba near susceptible crops (grapes, tomatoes, peppers, sweet potato, tobacco, IP soybeans, etc.) or greenhouses. While not necessarily volatile, other synthetic auxins such as 2,4-D are extremely damaging to horticultural crops at very, very low doses.
Buffer zones or No-Spray zones physically separate the end of the spray swath for the nearest downwind sensitive area.Consider planting windbreaks between your operation and sensitive downwind areas. Be aware that the windbreak should slow and filter pesticide-laden air, not block it completely (~50 % porosity). Also be aware that there are potential impacts to nearby crop rows, such as creating shade as well as cool, still air conditions. Contact your local Nature Conservancy to discuss the right plants and management plan for you.
Running an Airblast Sprayer?
For airblast sprayer operators, the environmental factors that affect drift are the same, but the rules for optimizing sprayer settings are slightly different. Droplet size is less of an issue, and in some cases droplet size cannot be controlled. Air settings are the primary tool for reducing drift potential.
Adjust fan settings to produce the minimal effective air speed throughout the season.
Use deflectors to channel air into, not over or under, the target.
If possible, increase droplet size by using air induction nozzles or disc & core (or disc & whirl) nozzles that produce a coarser droplet size. Depending on canopy size, you could use them in every nozzle position, or only in highest nozzle positions.
Any sprayer design the brings nozzles closer to the crop (e.g. tower or wrap-around designs) will reduce drift.
Canopy sensors that turn boom sections on and off to match the size and shape of the canopy will reduce drift.
It’s not only field sprayers that drift. Photo Credit – G. Amos and D. Zamora, Washington State.Monitoring airblast drift with ribbons and a tall pole with water-sensitive papers stapled along the length. This trial was run using only water so as not to expose the person holding the pole. Photo Credit – M. Waring, British Columbia.
If You Suspect Drift
If you suspect your crops or property have been damaged by pesticide drift, follow these steps (The contact info is specific to Ontario, so substitute your local authorities). The following information is based on this article in ONFruit which focuses on herbicide drift. Drift onto an organic operation would not necessarily cause visual injury, but steps are similar.
1. Diagnose the problem
Is there evidence of a spray application (agricultural or vegetative management such as roadside spraying)? Look for wheel tracks, weed symptoms, boom patterns and overlap on the headlands. Look for spray evidence in neighbouring fields, lawns, ditches, etc.
Familiarize yourself with the symptoms of drift injury on your crops.
Eliminate other possible causes. Disease, insects, nutrient deficiency, herbicide carryover, improper sprayer cleanout, and environmental stress can resemble drift injury.
Are there damage patterns? In the case of physical drift, damage is more pronounced on the upwind side of the damaged area, tapering away with distance from the source. In the case of vapour drift, damage can be uniform throughout damaged area and not necessarily downwind from the source. Pesticides can also move in cold air drainage and in surface run-off from rain events. If damage is patchy, it may be something else, such as soil pH or carryover (look where sprayer starts and stops).
2. Contact the appropriate people
Talk to your neighbour or the sprayer operator. Ask what was sprayed, when it was applied and who performed the application.
Contact the Ministry of the Environment, Conservation and Parks District Office or Spills Action Center (SAC): 1-866-663-8477. The SAC is available 24/7 and they will then contact the appropriate Environmental Officer and pesticide specialist in your region. Local MECP offices can be found here.
It is extremely important to report as soon as possible because the concentration of herbicide drops quickly within the plant. Do NOT wait until there are symptoms. Do NOT hesitate to call, even if you are unsure if it’s pesticide drift.
MECP officers can do a site visit, take samples of tissue and soil, and have them analyzed for suspect pesticides. Where appropriate, the offending applicator may face charges under Ontario’s Pesticides Act. Charges will be pursued only if off label use is identified from the information gathered.
Because of the wording of some of the labels and the difficulty of tracking down all the information needed, this has always been a very difficult thing to pursue in grower-to-grower drift incidents.
The results from the MECP lab are available for the grower and, if enough information is collected, the grower is encouraged to pursue civil court if insurance and/or cooperation with the applicator does not work. According to the label of most pest control products, the applicator is liable for any damage caused by the misapplication of a pesticide.
Contact your (crop) insurance adjustor and advise the applicator to contact theirs. However, do not rely on your crop insurance; Insurance companies may not provide coverage for drift incidents. It is prudent to determine if you are covered before you need to file a claim.
Report the incident to the manufacturer of the pesticide product. See the label for the toll-free number. Labels can be found on the PMRA label search.
3. Document all details of the problem and consider lab analysis
Collect spray records. This includes yours (to ensure it was not your application), and the potential offending applicators’.
Collect weather records (temperatures, possible temperature inversions, wind speed, wind direction, rainfall) for the date of application).
Take timestamped, geolocated photos (most smartphones include this information automatically, but check your settings). Repeat photos several times through the season.
Document yield loss from the damaged area and an undamaged area. Choose a similar planting (same age, cultivar, rootstock, etc.). For perennial crops (e.g. vineyards, orchards, asparagus, berries) herbicides such as Group 4’s may necessitate documenting the effects for several years after the damage occurred.
Laboratory analyses of herbicide levels in plant tissue are often necessary to confirm the presence of herbicides, although symptoms may be helpful in diagnosing which herbicides caused the problem.
Research laboratories that will analyze crop samples for herbicide residues. Their requirements regarding sample size, labeling, storage, and shipping will vary, as will the list of pesticides they provide testing for and their minimal detection levels. Given the time-sensitive nature of pesticide detection, it would be prudent to know this information before need the service.
Applicator Liability
Anyone using pesticides is responsible for their safe application. For example, the Ontario Pesticides Act requires that licensed spray applicators carry a specialized liability insurance policy that provides appropriate coverage for their business. Operators who work on a “for hire” basis (e.g. a licensed spray applicator) or away from their own farm operation will need additional coverage. Where drift damages adjacent crops, insurance adjustors generally ask the following questions:
Was the damage to the applicator’s own crop? If so, it is unlikely that there will be coverage under any insurance policy.
Was the damage to a neighbour’s property? If so, the applicator’s liability policy may respond.
Was the product being applied according to label directions?
Other Resources
Managing spray drift is everyone’s responsibility. Extremely low, and often invisible, amounts of spray drift can be very damaging; even long after the application. For more information about drift mitigation, watch the following videos and download a copy of this Factsheet.
What is Pesticide Drift?- Ontario Ministry of Agriculture and Food and Ministry of Rural Affairs (2011)
Equipment and Methods to Reduce Pesticide Drift- Ontario Ministry of Agriculture and Food and Ministry of Rural Affairs (2011)
Preventing Pesticide Spray Drift- University of Missouri Extension (2013)
Three simple ways to reduce drift. Thanks to Real Agriculture for filming and editing! (2014)
Three simple ways to reduce drift. Thanks to Real Agriculture for filming and editing! (2014)
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.
It’s time to spray and what’s the first thing you do? Check the weather forecast, of course. More often than not, the suitability of the weather is the main factor in the decision to spray. Let’s have a closer look at what each weather component contributes to the decision.
Wind:
Everyone knows that small droplets can drift if it’s windy, and the windier, the worse it is. But that’s hardly the whole story. Here’s how can we improve our understanding of wind and its impact.
Look beyond the wind forecast. It’s standard practice to look a day or two ahead for wind forecasts. At any instant, the wind speed and direction may be acceptable for our planned spray job, but we know that it will change. Consider wind speed sites such as Windfinder, Ventusky, or Windy for added insight. These services show trends over time in a great visual interface, allowing users to anticipate changes in wind speed and direction for better planning. While they aren’t forecasts per se, visualizing wind patterns over a larger region allows a better understanding of what’s coming your way.
Figure 1: Sites such as Windy.com offer powerful visualizations of current and future wind conditions.
Use wind as an ally. We’re conditioned to think of wind as having a negative effect on spray drift. The less the better. Yes, droplet displacement increases with wind speed. But the “negative-only” perspective is being re-evaluated in light of dangers associated with wind-free conditions that often occur during temperature inversions (see “Temperature”, below). In fact, wind provides several advantages over calm conditions:
Directional certainty. We can assess the risk to downwind sensitive areas. This is not possible with calm conditions because inversion air flow may follow terrain, and as inversions dissipate, the first daily winds can be changeable and unpredictable in direction.
Turbulence. Wind creates mechanical turbulence which helps sprays deposit and disperse. Both of these effects have value. In a calm environment, such turbulent eddies don’t exist.
Low drift options. If it’s windy, we have options to respond. We can lower the boom or lower the spray pressure. We can mix the next tank in higher water volume, forcing either a larger nozzle (larger flow rates of the same model nozzle usually produce coarser sprays) or slower travel speeds. All these practices reduce drift when it’s windy. In comparison, nothing (except not spraying) can be done to reduce risk during inversion conditions. This is because even low-drift spray contain enough fine droplets to cause damage if they linger.
Know your wind speed. The international standard for wind speed measurement is 10 m above ground level. When 25 km/h wind speeds are reported, they are at 10 m, not the 1 m height where the boom is located. Within the surface boundary layer, the part of the atmosphere closest to the ground, wind speeds typically increase linearly with the natural log of the height above the canopy. The slope of that line depends on atmospheric stability and roughness length. Very close to the ground, the wind speed reaches zero, and that height is a function of the roughness of the surrounding terrain.
As a rule of thumb, over a short crop canopy, expect the wind speed at 1 m above ground to be about 0.67x of the speed at 10 m. So if the weather reports 25 km/h, the actual wind speed at boom height is closer to 17 km/h. Remember that weather stations can be far away, and local conditions will vary. Always measure your local wind speed and direction with your own weather station or handheld device, and keep a record.
Figure 2: Relationship of wind speed and height, for three roughness conditions (Source: Oke et al, 2017)Figure 3: Hand-held wind meters or weather stations are an essential part of a spray operation and record keeping.
Wind and Mode of Action. Coarser sprays are a common way to reduce drift in windy conditions. But some modes of action aren’t well suited to coarser sprays. We can schedule our spray jobs throughout the day to correspond to spray quality tolerance. Apply the products that require the finest sprays (contact products, grassy herbicides, insecticides) when conditions are best, and save the sprays that tolerate the coarser sprays (systemic products, broadleaf targets) for less certain conditions later in the day. Or treat the fields whose downwind edges border a sensitive crop during better conditions. Here’s a rough guide to spray quality and herbicide mode of action.
Temperature
Like wind, air temperature is more complex than it appears at first sight. Here are some other aspects to consider:
Understand temperature inversions. Temperature matters. But perhaps the most important aspect of temperature when it comes to spraying isn’t the temperature per se, but how it changes with height. The temperature change with height is used to identify dangerous temperature inversions.
Here’s how temperature profiles work (for a quick Sprayers101 overview, here, for the best in-depth explanation (NDSU), here): Due to atmospheric pressure, there is always a slight temperature decrease with height, about 1 ºC per 100 m (the dry adiabatic lapse rate). This temperature profile describes a “neutral” atmosphere, i.e., no thermal effects.
When it’s sunny, solar radiation heats the earth, which in turn warms the air near it. As a result, the rate of cooling with height is greater than the adiabatic lapse rate, and we have “unstable” conditions that are characterized by thermal turbulence (warm air rising, cold air falling) that actively mixes air parcels. Thermal turbulence is very good at dispersing anything in the air, including spray droplets.
When solar radiation is low or absent, the earth cools and this mostly affects the air near it. As a result, air temperature rises with height, and the daytime temperature / height profile is inverted. Air parcels no longer move up or down, in fact they return to their original location if displaced. This results in a “stable” atmosphere, also called an inversion.
Inversions are dangerous because they are associated with very low dispersion, and a spray cloud will remain concentrated and may linger over the ground for a long time, like ground fog.
Most weather services do not actively measure inversions. Instead, their presence has to be inferred by clues. For example, inversions: (a) occur primarily when solar radiation is low, from early evening, overnight, to early morning; (b) are more likely on clear nights, when soils cool more; (c) can be seen when ground fog is present, or when dust hangs, moving slowly; (d) are associated with low ground temperatures that also cause dew.
Recent findings about inversion in Missouri were summed up in this excellent webinar by Dr. Mandy Bish, Extension Weed Specialist at the University of Missouri. Her studies showed that inversions can begin hours before sunset, their presence and duration are dependent on local conditions such as topography and windbreaks, and recognition of telltale signs of inversions such as lack of windspeed are important for accurate local assessments.
Figure 4: Morning ground fog in Australia (picture provided to author).
Use Mesonets if you have them. Mesonets are networks of weather stations, and they can add valuable information. For example, North Dakota has an extensive network of about 130 weather stations that, among other things, measures and reports temperature inversions. NDAWN (ndawn.ndsu.nodak.edu) reports temperatures at 3 m and 1 m, and issues warnings of temperature inversions as they develop at a specific location. NDAWN information is available as an app. North Dakota isn’t the only place to have a public mesonet, check to see what’s available in your area. The added information is worth subscribing to.
Know the volatility of the product. Some pesticide active ingredients are volatile. This means they can evaporate from a wet or dry deposit during and after application (more here). Dicamba is a prominent example, but there are others, like trifluralin and ethalfluralin, 2,4-D and MCPA ester, and clomazone. Formulation can affect volatility, and the use of lower volatile esters of 2,4-D and better salts of dicamba have helped. Microencapsulation has been used to reduce the problem with clomazone. Volatility is strongly affected by surface temperature, and volatile products should not be sprayed on hot days or when the forecast calls for hot days following application. Volatile products have been found to evaporate from dry deposits for several days after application, and their vapours move under inversion conditions, causing widespread damage.
Sun
The sun plays a large role in spraying. Plants’ active growth improves herbicide translocation as well as activity in the photosystem, or in amino acid or fatty acid synthesis. The activity of herbicides has been shown to improve under sunny conditions for that reason.
Some herbicides, most notably diquat (Reglone), work too quickly when it’s sunny, and improved performance can be gained by spraying under cloudy or low-light conditions. The lack of photosynthesis allows for some passive translocation before the product causes tissue necrosis.
Sunny conditions also increase thermal turbulence we mentioned earlier, which is useful for burning off morning inversions. But what usually follows a sunny day is a strong inversion as the sun sets and the clear sky facilitates the earth’s rapid cooling. It would be possible to spray a bit later into the evening when it’s cloudy.
Humidity
Since about 99% of the spray volume is comprised of water, evaporation of this water can have strong effects on droplet behaviour. Droplets begin to evaporate as soon as they leave the nozzle, becoming smaller and more drift-prone while still in flight. Higher booms and finer sprays increase the flight-time of droplets, and this increases the sensitivity to evaporation.
The most common measure of water in air is relative humidity (RH). RH doesn’t tell the whole story, though, because the same RH at different temperatures results in two different rates of water evaporation. A better measure is wet bulb depression. Wet bulb depression is defines as the difference in temperature reported by a dry bulb vs. a wet bulb thermometer. Wet bulb depression has more recently been coined as “Delta T” in Australia. The Delta T value is directly related to water evaporation, and charts have been published showing acceptable values for spraying. A Delta T of >10 ºC is considered too high.
Figure 5: Delta T, also known as wet bulb depression, provides an indication of water evaporation rate.
After they deposit on a leaf, droplets can evaporate to dryness within seconds, and a dry atmosphere can result in rapid drying that reduces herbicide uptake. In one study, a Group 2 herbicide was applied to weeds in a normal sized spray, and also as a fine mist, both under very dry conditions. The normal spray showed the expected herbicide efficacy. The finely misted herbicide had no effect on the weeds, likely because the rapid drying prevented uptake. Interestingly, the product began to work again when the plants were later placed in a humid environment.
High humidity can also work against an application. Since humidity is often high during temperature inversions, droplets remain potent while they linger and drift over sensitive terrain. It would be better if they had evaporated and lost their effectiveness.
Some proponents of low water volumes and fine sprays have suggested oily formulations or adjuvants prevent evaporation. While this may slow evaporation, it also creates a dangerous condition in which many small droplets remain aloft and liquid for a long time, with high activity on any target they may encounter. The bottom line: Don’t spray low volumes with oily adjuvants.
The Perfect Day
We know that the ideal spray day is sunny, starts a few hours after sunrise once the dew has mostly burned off, and has consistent winds away from sensitive areas. Spraying should end well before before sunset, before calm conditions signal the onset of the inversion.
But what to do when that day never happens? All too often, high winds persist day after day, and night spraying is the only alternative. In that case, do what you can to minimize potential damage. Survey downwind areas. Choose cloudy skies that suppress inversions. Incoming weather systems are usually associated with consistent winds, and these may reduce inversion risk. If drift is a possibility, apply more water and use the coarser nozzles at your disposal to minimize it. Any investments made to boost productivity will pay dividends, allowing you to get a greater proportion of your work done when conditions are better.
In 2016, an asparagus grower in southern Ontario picked up a used De Cloet Hi-Boy originally used to spray tobacco. His vision was to create a three-row herbicide sprayer for asparagus and we were invited to participate. His concept was to design shrouds that would contain the herbicide, but not snag the asparagus or drag heavily on the ground. This article follows the development of the sprayer from concept to testing to final product.
The sprayer itself was a classic three-wheel, self-propelled affair. The asparagus was planted on four foot centres, leaving a three foot alley. While the goal was to hang three shrouds off the boom, we started with one to work out the bugs.
This operation uses 2,4-D to control weeds in the alleys and while a little can hit the asparagus stem up to 12 inches (where the branching starts), we wanted to avoid contact at all costs. That led us to the TeeJet AI 95° flat fan nozzle, which produces a Very Coarse to Extremely Coarse spray quality. A single nozzle could be suspended to span the 3 foot width of the alley.
The first version of the shroud was suspended off the boom from four anchorage points. A certain amount of of play was allowed so the shroud would find plumb (i.e. hang vertically), even when the sprayer boom yawed or pitched over uneven ground.
The shroud was constructed of sheet metal, angled to reduce the potential for contact with the asparagus branches, and terminated in stiff, nylon brush-style mud flaps commonly seen on trucks. These brushes were cut to a few inches in length to span the distance between the side of the shroud and the ground. This would create a “seal” to prevent spray from escaping, maintaining some degree of contact with uneven ground.
We tested the first version by placing water sensitive paper in two positions on the ground, just inside the reach of the brushes. We had to be careful not to run them over with the centre wheel of the sprayer. We also adhered two papers to the angled inner walls to see how much, if any, spray was hitting the inside of the shroud.
Our first pass on June 16th was at 9:00 am, 19.1 ºC (66.4 ºF) with a cross wind of 5 to 7 km/h (3.1 – 4.3 mph). relative humidity was high at 85% and travel speed was slow at 3.2 km/h (2 mph). We started with the .06 AI tip at 50 psi, but we drenched all the targets with excessive coverage because we were travelling so slow. We also found the stiff brushes were creating furrows in the soil, as shown below.
For our second pass, we tried the .04 tip and raised the shroud while dropping the tip to keep it suspended 15 inches over the ground. We were still drenching the targets and noticed the shroud was hitting the asparagus spears, causing physical damage. The damage is shown below – note the dark green on the bent spear.
This led to a decision to flare the side walls more aggressively, bringing them further into the centre of the alley and away from the spears (shown later in the article). This had the added benefit of angling the brushes as well to get a maximum span for weed control in the alley. For the final coverage pass we used the AI .03 tip, which gave more than 45% coverage on the ground, with even distribution, and there was no indication of spray on the papers adhered to the inside of the shroud. This coverage is more than is likely required, and the operator should be able to spray up to 6.5 km/h (4 mph) without compromising coverage.
Since the coverage tests, the grower added additional sheet metal fenders to the the existing fenders, encasing the wheels and creating a smooth transition for the shroud to gently deflect the asparagus. The fenders were needed because the grower found the asparagus was being pushed out by the wheel fender only to bounce back in front of the shroud, which snagged the fern and damaged it. The additional fenders keep the fern spread and prevent it getting caught in front of the shrouds.
The grower was very happy with the sprayer’s performance and planed to build another. Why be satisfied with the status quo when you can tap into your creative side and be innovative? If you don’t think you’re imaginative enough to try upgrading equipment on your farm, here’s a simple test to prove that it’s in you. It’s easy to see the bird in the image below, but with a little concentration you’ll be rewarded with a ski-jumping rabbit.
Thanks to TeeJet for donating the nozzles and water-sensitive paper and to Ray and Brad Vogel of Lingwood Farms for inviting me to participate.
