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.
In 2014 one of our OMAFRA summer students designed a short-and-gritty demonstration using a backpack sprayer, a variable-speed fan and some water-sensitive paper positioned downwind at 1.5 metre intervals. The intent was to illustrate how sprayer operators could reduce the potential for off-target drift by recognizing and accounting for three factors:
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)
Apparent Wind Speed
Spray operators know they should not spray when the air is calm or when the wind is too high, but they often forget that the nozzles experience “apparent wind speed” which means driving 10 km/h into a 10 km/h headwind is essentially spraying in a 20 km/h wind.
The result of spraying with a Medium spray quality in 10 km/h and 15 km/h wind: water-sensitive papers indicated that there is more downwind drift in higher winds.
Boom Height
Spray operators raise their booms to ensure their nozzles clear the crops, but this contributes to off target drift and greatly reduces coverage – particularly when using twin-fan style tips. Dr. Tom Wolf explains how to set your boom height here, or you could watch one of our Exploding Sprayer Myths videos on the subject.
The result of spraying with a Medium spray quality in a 10 km/h wind at 50 cm and 100 cm from the ground: water-sensitive papers indicated that downwind drift increases as the boom gets higher.
Droplet Size
The coarser the spray quality, the less likely the spray will drift off target. Remember, for a given volume, shifting to larger droplets means fewer droplets. Application volumes may have to increase to compensate for potentially reduced coverage.
The result of spraying with a Medium spray quality versus spraying with an Extremely Coarse spray quality: water-sensitive papers indicated that there is more downwind drift from smaller droplets.
Take-Home
This demo used percent coverage as a metric, which is convenient but greatly underestimates drift. So even when the spray window is small and the spray has to go on, take a moment to drop the boom, use a coarser droplet size and if it’s too windy, just don’t spray.
WUR Drift Calculator
There are many drift calculators available for home use. Some require more expertise than others to get a reliable result. This free downloadable calculator from Wageningen University & Research was made available in 2021. It can quantify spray drift deposits onto surface waters and non-target terrestrial areas near a sprayed field or orchard
The calculator uses statistically obtained regression curves to calculate spray deposition next to the sprayed field. The spray drift curves are based on the latest experimental data for field crops, fruit orchards and avenue tree nurseries.
Authors: T M Wolf, B C Caldwell and J L Pederson. Originally published in Aspects of Applied Biology 71, 2004, in expanded form.
Abstract
Spray drift deposition into water bodies may pose environmental and health hazards, and buffer zones have been suggested as a means of mitigating water contamination. Field trials were conducted to determine the effect of nozzle type and riparian vegetation on spray drift deposition into wetlands. Three riparian vegetative types, minimal vegetation (grass), low vegetation (willow shrubs), and high vegetation (aspen trees) were compared with open field conditions. Spray was released upwind of wetlands with these riparian characteristics with conventional and air-induced low-drift nozzles. Low-drift nozzles reduced drift deposits by about 75% in the absence of any vegetation, and by 88 to 99% when vegetation was present. Dense willow shrubs resulted in anomalous downwind deposits, possibly because of air turbulence caused by low porosity characteristics. By considering vegetation effects, a 15-m buffer zone could be reduced to 5 to 7 m for conventional, and 1 to 4 m for low-drift nozzles without increasing deposits at the edge of the sensitive habitat. Both variables should be considered by regulatory bodies in their risk assessment procedures.
Introduction
Airborne transport is an important vector for movement of pesticides from agricultural land to receiving waters. In an effort to maintain low pesticide levels in water bodies in accordance with risk assessment protocols, the Pest Management Regulatory Agency (PMRA) is mandating minimum setback distances (buffer zones) from water bodies during a spray operation. Several additional variables can complement buffer zones in preventing spray drift, including low-drift sprays and riparian vegetation. Germany and the United Kingdom already account for these characteristics in their buffer zone regulations (Kappel and Taylor, 2002).
Vegetation has been shown to be effective at mitigating droplet spray drift in several recent studies and reviews (Richardson et al., 2002, Hewitt, 2001, Ucar and Hall, 2001) by reducing wind velocities and intercepting spray. The documented magnitude of the spray drift deposit reduction in these studies ranges from 50 to >95%, dependent on variables that include vegetation height, porosity and orientation relative to wind direction, and wind speed.
We studied the integrated effect of buffer zones, vegetative barriers, and low-drift sprays to determine the overall impact of spray drift deposition onto downwind water bodies.
Materials and Methods
Overview and Site Description
The study was conducted in 2001 on a farm field near Aberdeen, SK. Sprays were applied upwind of a water body, and drift deposits were collected on petri-plates placed near ground level. Experimental sites were chosen to represent different vegetation heights and types around the water body in question: low (uncut grass), intermediate (willow shrubs), and tall (aspen trees). These were compared to nearby open-field conditions. Two sprayer nozzle types were used in the study: conventional flat fan nozzles and venturi-type low-drift nozzles.
The grass barrier was comprised of a mix of grasses dominated by bromegrass (Bromus spp.) growing to a height of 75 cm. Willows (Salix spp.) were approximately 3 m tall with a density of about 0.15 m-2 and presented a fully foliated barrier for their full height. Willows extended for a width of about 7 m toward the edge of the water body. Trembling aspen (Populus tremuloides Michx.) were approximately 8 m tall, with foliation beginning 1.5 m above ground. Trees were present at a density of about 0.25 m-2 and extended for 8 m toward the water edge.
Figure 1: Site and sampler layout
Spray Equipment and Application Method
A Melroe Spra-Coupe 220 was used to make the applications. This sprayer was equipped with conventional flat fan nozzles (XR8003) and air-induced low-drift nozzles (TD11003) at 275 kPa, producing ASABE Fine and Coarse sprays, respectively. The spray boom was 10 m wide and nozzles were 75 cm above ground. Sprayer travel speed was 12.9 km h-1, at which the application volume was 100 L ha-1.
The sprayer tank contained a mixture of 2,4-D amine4 (4 g L-1) and Rhodamine WT5 (2 mL L-1), a fluorescent tracer dye which would be used to quantify the deposits. 2,4‑D acted to photostabilize the dye, and also provided a spray formulation with physico-chemical properties representative of agricultural pesticides.
Top row: Fine and Coarse sprays used in study Middle row: Tall and medium vegetation Bottom row: Short vegetation and open field
Application was made in a direction approximately perpendicular to the prevailing wind, with the downwind edge of the spray boom at the edge of the wetland’s riparian vegetation. This was usually about 15 m upwind of the edge of the water body (due to severe drought conditions, the wetland did not contain any water at the time of the trials). Three consecutive passes were made along the same swath in a 10-min period to obtain average meteorological conditions for all three vegetation types. Wind speed and direction, temperature and relative humidity were monitored during application using a portable micrometeorological station.
Sampler Layout
Downwind of the spray swath there were 3 parallel lines of eleven 15-cm diameter glass petri-plate samplers starting underneath the sprayer boom and extending 46 m downwind from the edge of the spray swath (Figure 1). Samplers were separated by 5 m within the line, and lines were about 2 m apart.
The deposition profile was also assessed under open field conditions, using the same sampler layout but on crop land with no riparian vegetation. These are referred to as ‘bare soil, or ‘reference’ samplers in this report and served as a baseline to determine the impact of the riparian vegetation.
Sample Collection and Analysis
Sample collection began 5 minutes after spray application was complete (See Table 2 for trial times). Beginning with the furthest downwind locations, petri-plates were covered with a plastic lid, and placed into dark boxes. Spray deposits on the samplers were washed off in the laboratory using 95% ethanol in three 15-mL washes. Final samples were made up to 50 mL. and two 20-mL sub-samples were collected in borosilicate vials and stored in the dark.
Within 24 h, subsamples were analyzed using a fluorescence spectrophotometer with excitation and emission wavelengths of 545 and 570 nm, respectively (Shimadzu Model RF-1501 spectrofluorometer equipped with Model ASC-5 auto-sampler). Instrument readings were converted to µg L-1 using standard curves and expressed as a percent of the applied dosage under the field sprayer.
The fluorescence spectrophotometer data were averaged over the three replicate sampling lines, adjusted for photolysis, and expressed as a percentage of the amount applied on-swath. Relationships of spray drift deposits with downwind distance were first visualized by plotting all data points, and then mathematically related through appropriate regression techniques.
Results
Meteorological Conditions
Weather conditions were favourable during the trials. Wind speed and direction were appropriate for the sampler layout and the experimental objectives. Mean wind direction varied by up to 44º from the ideal (270º) in 6 out of 12 trials, and was within 30º for the remaining 6 trials (Table 1). Mean wind velocities were consistently between about 17 and 21 km h-1 in all but one trial. Air temperature and relative humidity fluctuated between 14 to 22º C and 31 to 80%, respectively, on the trial dates.
Deposition Profiles
A visual review of the raw data suggested that a linear regression of the log of deposit amount and log of downwind distance would be appropriate. It was noted that for willow, the deposit profile tailed upwards after the 26 m mark. Based on a survey of the site, it was concluded that this tail was probably caused by the length of the spray pass exceeding the length of protection offered by vegetation. In other words, beyond the 26 m sample, drift had not been attenuated by a vegetative barrier. It is also possible that the airflow was deflected up over the low, non-porous barrier and returned to ground level beyond the 26 m distance (Carter et al., 2001).
Figure 2: Spray deposit profiles from Fine (top) and Coarse (bottom) sprays. The deposition data for the willow were regressed from 6 to 26 m, all others were taken to 46 m (see text for explanation).
As a result of the questionable data for this vegetation type, it was decided that it would be misleading to include the furthest downwind data points. Implications of this observation will be discussed later in the manuscript. All regressions were statistically significant, explaining between 61 and 99% of the observed variation. In 5 of 8 trials, more than 90% of variation was explained.
Drift Mitigation by Riparian Vegetation and Application Method
The predicted drift deposit at 15 m was calculated for all trials based on the regression parameters (Table 3). For the conventional sprayer on bare soil, the deposit amount was 0.322% of the applied dose. The distance at which this specific deposit amount would be achieved was then calculated for all other trials. This value is the buffer zone distance at which equivalent protection to the reference system was offered. Buffer zones could therefore be reduced by 55% (grass), 99% (willow) and 69% (aspen) using the conventional nozzle and 56% (bare soil), 74% (grass), 98% (willow) and 92% (aspen) for the low-drift nozzle.
Table 1: Buffer zone distances based on observed drift, calculated from regression.
The calculated buffer zone reductions were not equivalent to the observed drift reductions due to the unique regression slopes of each deposition line. For example, expected drift deposits at 15 m downwind on bare soil were reduced by 77% when the air-induced low-drift nozzles were used (Table 4), whereas buffer zone distances could only be reduced by 56% (Table 3). Furthermore, the effectiveness of the grass vegetation diminished with distance, reducing drift by 64, 50, and 28% at distances of 15, 25, and 45 m, respectively. Therefore, a complete deposition profile will be required for each vegetation scenario to accurately adjust buffer zones.
Table 2: Drift deposits expressed as a percent of the reference deposition line for two application methods, four vegetation types, and three downwind distances. All numbers are the mean of three separate experiments on the same location.
Riparian vegetation was typically more effective than low-drift nozzles in protecting water bodies from drift deposition. While grass reduced deposition by 28 to 64% from the conventional nozzle (depending on the downwind distance), willow and aspen reduced deposition by between 95 and 99% (Table 4). The willow was not considered at further distances since the data used for the regression were truncated at 26 m. Low-drift sprays provided some additional protection in all cases except for trees at the 45 m distance, where deposits increased slightly relative to the conventional spray.
Discussion
The aerodynamics of vegetative barriers are a complex phenomenon. Wind, upon reaching a solid barrier, is diverted up and over giving strongly turbulent conditions on the leeward side and a rapid return to free wind speed. For a permeable barrier like a hedge, the return to free wind speed is more gradual since some air filters through, reducing the pressure differential and allowing for less turbulence (Davis et al., 1994). Wind speed reduction is most pronounced for a distance of 5 H upwind and 30 H downwind at the 1 H height, where H is the height of the barrier (Rider, 1951). Nonetheless, there may still be an upward diversion of air (and spray drift) which may simply delay, not eliminate, sedimentation (Hewitt, 2001, Ucar and Hall, 2001), particularly for dense hedges (Carter et al., 2001). Richardson et al. (2002) did not, however, notice such a deflection up to 10 m height.
The reduction in drift deposition by riparian vegetation in this study is clearly significant, but is subject to some interpretation. These data were generated at a single site, and while this site was carefully selected to be representative and trials were repeated three times, it does not necessarily constitute an average result. There are clearly any number of possible arrangements of trees, shrubs and grass, plus any additional vegetative or landscape features which would influence drift deposition behaviour. However, due to the consistent nature of the data of this study, some confidence is attained in that the numbers are at least reliable for the given set of conditions. In this study, three spray passes were made along the same swath at the edge of the water body. Results could have been different had adjacent spray swaths been used, owing to the possible change in contribution of upwind swaths with the altered airflows under vegetated conditions.
Since the water body was dry, additional grass vegetation which had grown up could have made an effective collector of spray drift, possibly reducing deposit values beyond those that would have occurred in a water body. It is recommended that efforts be made to repeat these studies when water is present at normal values.
The mitigating effect of vegetation depends on the aerodynamic features of the vegetation, as well as the collection efficiency of their leaves, twigs, etc. This poses some difficulties because there are no absolute measures of these features. Permeability, for example, varies with wind speed owing to the movement of leaves, and winds speed itself varies with height (Davis et al., 1994). Collection efficiency of the vegetation varies similarly with target size, its movement, wind speed, and droplet size spectrum (Hewitt, 2001). However, there are opportunities for improved characterization with specialized equipment, such as that used by Richardson et al. (2002). Their LIDAR instrument was able to help calculate tree height and width, mean area index and mean area density. Work to further characterize vegetation will prove useful in future efforts to understand its mitigating potential.
Low vegetation such as grass has not received the recent attention of hedges and trees but has also been documented to reduce spray drift significantly. A study by Miller et al. (2000) documented significant reductions in airborne drift concentrations above uncut grass canopies, even at low plant densities. Bache (1980) documented similar reductions in spray drift when sprays were applied over a mature wheat crop compared to bare soil. Therefore the filtering effects of “low” canopies may be very significant and should be the subject of further study.
Riparian areas are regions of high biological activity and diversity, not only protecting adjacent water from outside influence, but also providing food and shelter for many species of wildlife. These areas must themselves be protected from harmful effects, which can include pesticides. Their efficient capture of sprays suggests some risk from pesticides capable of controlling perennial vegetation. Likewise, pesticide residues in this vegetation have the potential to be ingested by wildlife or be washed off with precipitation, resulting in movement into the water body. These effects must be considered when using vegetation to mitigate airborne drift.
Conclusions and Recommendations
Vegetative barriers reduced spray drift deposition from conventional or low-drift nozzles into water bodies by 24 to 99%.
Low-drift sprays reduced deposition by about 75%.
Of the vegetation types, shrubs and trees had similar effects, reducing deposition from open-field conditions by an average of more than 95%. Low-drift sprays improved on this reduction.
Calculated buffer zone reductions were less than drift deposit reductions. Accurate determination of buffer zone distances requires that the entire deposition profile be characterized.
It is suggested that both riparian vegetation and sprayer technologies are important components of water body protection. Both should be considered in BMP and regulation development whenever the impact of pesticide applications near water bodies is to be estimated or mitigated.
Acknowledgements
The technical assistance of Glenda Howarth, Jill Clark, Rachel Buhler, Murray Nelson, Trevor Linford, and Pam Reynolds is greatly appreciated. Financial assistance was provided by the Rural Quality Program of the Agri-Food Innovation Fund, administered by the PFRA. The authors wish to thank Darrell Corkal and Clint Hilliard of PFRA for their enthusiasm, support and guidance directed towards this project, and Raymond Malko for making his land available for the trials.
Citations
Bache, D. H. 1980. Transport and capture processes within plant canopies. Spraying Systems for the 1980’s. BCPC Monograph No. 24, 127-132.
Carter, M. H., R. B. Brown, K. A. Bennett, M. Leunissen, V. S. Kallidumbil, and G. R. Stephenson. 2000. Methods for reducing buffer zone requirements for pesticide spraying adjacent to wetland environments. Sainte-Anne-de-Bellevue, Quebec: Proc. 2000 National Meeting, Expert Committee on Weeds / Comité d’experts en malherbologie [on-line: http://www.cwss-scm.ca/pdf/ECW2000Proceedings.pdf].
Davis, B. N. K, M. J. Brown, A. J. Frost, T. J. Yates, and R. A. Plant. 1994. The effects of hedges on spray deposition and on the biological impact of pesticide spray drift. Ecotoxicology and Environmental Safety 27:281-293.
Hewitt, A. J. 2001. Drift Filtration by natural and artificial collectors: a literature review. Special publication by Spray Drift Task Force, 12 pp. [on-line: http://www.agdrift.com]
Kappel, D. and W. A. Taylor. 2002. Buffer zones and “low drift” equipment. Hardi International Discussuion Paper, available from Hardi International A/S Helgeshøj Allé 38 DK-2630 Taastrup.
Miller, P. C. H, A. G. Lane, P. J. Walklate, and G. M. Richardson. 2000. The effect of plant structure on the drift of pesticides at field boundaries. Aspects of Applied Biology 57:75-82.
Richardson, G. M., P. J. Walklate, and D. E. Baker. 2002. Drift reduction characteristics of windbreaks. Aspects of Applied Biology 66:201-208.
Rider, N. E. 1951. The effect of a hedge on the flow of air. Quarterly Journal of the Royal Meteorological Society 78:97-101.
Ucar, T. and F. R. Hall. 2001. Windbreaks as a pesticide drift mitigation strategy: a review. Pest Management Science 57:663-675.
Wolf, T. M. 2000. Low-drift nozzle efficacy with respect to herbicide mode of action. Aspects of Applied Biology 57:29-34.
OK, fine. We confess to the shameful use of click-bait in our title. Nevertheless, it’s absolutely true: Drift can be good. The reason this statement is unsettling is because of the lack of context, which is really what this article is about.
The majority of sprayer-related information available to ag stakeholders relates to horizontal boom sprayers. Most of it is relevant to broadacre field crops and often pertains to herbicide applications. If you’re unsurprised, or still struggling to grasp our point, it’s likely because you’re part of that world. But consider everyone else.
Agricultural spraying is diverse and many usage patterns are grossly underrepresented. As a result, those operators struggle to find relevant information. And what information is most readily available? Yes – broadacre herbicide spraying. Even the experts (i.e. agronomists, salespeople and consultants) often make the error of responding to specialty crop questions with field crop answers. Or relatedly, they assume their entire audience is comprised of field croppers and fail to use a disclaimer before making sweeping statements. The problem (and it is a problem) is so pervasive that we often hear about specialty crop operators taking training courses intended for field crop applicators… because that’s all that’s available.
So how different can spraying be for a given crop? Surely a droplet is a droplet and the laws of physics don’t care what you grow? This is true, but droplet size, spray volume, distance-to-target, environmental conditions, sprayer type and product formulation combine in complicated ways. The result is that the best advice for one operation can be disastrously wrong for another.
Case in point: Drift is good. If we were persnickety (and we are) we would suggest that moderate drift is the lateral movement of spray with the prevailing wind, and that this helps ground and/or disperse spray in a predictable direction. It’s not a bad thing. But we know that drift quickly becomes bad in high winds and especially when there are sensitive downwind areas. We won’t even talk about dead calm.
However, in controlled environment applications (e.g. greenhouses) operators use very small droplets in high numbers and absolutely rely on air circulation to expose all crop surfaces to the spray. Without drift, most stationary foggers would only have a limited and localized effect. And in airblast operations, a wind that would never deter a field sprayer operator would derail an airblast operator. Wind is much faster with elevation and airblast sprayers use very small droplets that span considerable distances to the tops of tall targets. In their world, droplet size is not the primary means for drift mitigation – it’s air alignment.
The following table is a relative comparison of key factors in spraying. Note how different they are depending on the usage pattern. And if this isn’t diverse enough, recognize that we didn’t include a column for vegetative management (e.g. roadsides, industrial and forestry) or aerial application (which might even be split into piloted and remotely-piloted).
And so where does this leave us? We have two pieces of advice:
If you are looking for information on spraying, take the time to find out who your source is and understand the context of the information. More often than not it will have relevance, but in some cases it could be completely wrong for you.
If you are providing information on spraying, be clear who the information is intended for. While we don’t propose caveats amending every statement, context is always appreciated. A sentence in an article, or a brief interjection during a presentation, might help someone that doesn’t know what they don’t know.
This presentation was delivered virtually for the 2021 OMAFRA Controlled Environment Webinar Series. If you’d like to learn more about strategies for spraying in closed environments, settle in and give a watch. It’s 45 minutes plus questions.
So, a minor error in the presentation. The image of the ascospore was not quite right. This new version (below) is correct. Perspective can get tricky at the micron-scale of resolution.
