Aspects of Applied Biology 147, 2022
International Advances in Pesticide Application
Exploring variable air ow rate as a function of leaf area index for
optimal spray deposition in trellised vineyards
By M GRELLA1, F GIOELLI1, P MARUCCO1, E MOZZANINI1, A CAFFINI2,
D NUYTTENS3, I ZWERTVAEGHER3, S FOUNTAS4, L ATHANASAKOS4, N MYLONAS4
and P BALSARI1
1Department of Agricultural, Forest and Food Sciences (DiSAFA), University of Turin
(UNITO), Turin, Italy
2CAFFINI S.p.a., Palù, Italy
3Flanders Research Institute for Agriculture, Fisheries and Food (ILVO), Merelbeke, Belgium
4Agricultural University of Athens (AUA), Athens, Greece
Corresponding Author Email: firstname.lastname@example.org
In 3D crops, excessive fan airow speed may reduce deposition and increase spray losses
due to canopy compression. To solve this problem, variable fan airow rate as a function
of canopy characteristics is a key in the context of a sustainable crop protection technology.
The eects on spray canopy deposition of dierent air volume rates (low, medium, and high)
generated by a 700 mm diameter axial fan, combined with 4 and 8 km h-1 forward speed,
were assessed in a vineyard at an early and late growth stage. The objective was to identify
the fan airow setting (m3 h-1) which maximizes canopy spray deposition (% of applied)
with the nal aim to determine the relationship between leaf area index (LAI) and optimal
fan airow rate. The results conrmed that an excessive airow rate signicantly decreases
spray deposits. When the forward speed increased from 4 to 8 km h-1, the airow rate had to
be increased from low to medium to maximize canopy spray deposition. The results showed
the importance of total air volume applied per hectare (m3 ha-1) as a parameter to maximize
the spray deposits, irrespective of forward speed. Finally, the relationship between the LAI
(adim.), accounting for crop characteristics and canopy density, and the total air volume
applied (m3 ha-1) to maximize the canopy spray deposition was identied.
Key words: Precision agriculture, variable airow rate, electrically driven axial fan, spray
canopy deposit, airblast sprayer
The adjustment of the air jet (fan airow rate, velocity and, if possible, direction) in vineyard
sprayers in function of the canopy morphology (size, leaf density and row distance) can avoid
areas with under- or over-application of plant protection products (PPP) and can reduce losses
due to spray drift. Vines of the same variety can strongly vary in shape, size, and foliage density
within the same parcel and among plots. A precise and continuous air jet adjustment according to
the crop characteristics within the vineyard is thus key for an ecient and sustainable pesticide
application. In general, the fan airow aims to carry the PPP droplets onto the target and move the
foliage in order to improve the deposition on the internal part of the canopy and on the underside
of the leaves. Nevertheless, it is well known for 3D crops that an excessive air support may reduce
deposition, owing to canopy compression, and increase spray losses (Hislop, 1991; Balsari &
Marucco, 2004). For example, Pergher (2005) reported that a reduction of the air ow rate from
10.6 to 6.3 m3 s-1 increased mean foliar deposits by 25–30% in a vineyard. Therefore, the possibility
to vary fan airow as a function of canopy characteristics, especially density, is key in the context
of a sustainable crop protection technology.
Current technical solutions generally foresee the use of mechanically driven fans with dierent
revolution speed using a gearbox and/or lowering the Power Take O (PTO) of the tractor. Another
solution consists of reducing the fan air suction section by a diaphragm. In recent years, more
advanced sprayers with adjustable fan settings, which are able to vary continuously and in real
time the airow characteristics, have been developed. These sprayers are either equipped with
several axial fans mounted in dierent positions on the sprayer or can change the width of the fan
air outlet channel and/or the blades pitch, etc. (García-Ramos et al., 2012; Endalew et al., 2010;
Hołownicki et al., 2017; Salcedo et al., 2021). However, for all these solutions, the possibility to
vary airow settings continuously and automatically along the rows is limited.
While dierent methodologies have been established to determine the optimal spray application
rate depending on the canopy characteristics (Llorens et al., 2010; Garcerà et al., 2017), few data
exist concerning the relationship between airow characteristics and canopy morphology (size
and density). Instruction manuals (TOPPS, 2014), digital tools (Doruchowski et al., 2013), and
devices (Bahlol et al., 2020) that allow to roughly adjust the airow characteristics to the target
have been developed. However, such tools require a manual fan setting which cannot be done for
each specic canopy morphology and, in most cases, the airow adjustment is totally left to the
operator’s skills and experience.
Therefore, within the H2020-OPTIMA project (OPTimised Integrated Pest Management for
precise detection and control of plant diseases in perennial crops and open-eld vegetables,
www.optima-h2020.eu), a cost-eective integrated system, enabling to vary the fan revolution
speed automatically and continuously according to the canopy density, was designed (Grella et
al., 2022). Canopy density values are obtained from one ultrasonic sensor per sprayer side and
are processed through an algorithm in a controller. Based on the output from the algorithm, the
controller communicates the required fan revolution speed to the fan inverter. This algorithm is based
on the relationship between canopy characteristics and fan airow rate and has to be determined
The objective of this experimental work was i) to determine the optimal airow rate to be set
in a trellised vineyard to maximize canopy spray deposit for dierent canopy morphologies and
forward speeds, and ii) to investigate the feasibility of leaf area index (LAI) as a parameter for
dening the optimal fan setting.
Materials and Methods
Test location and vineyard characterization
Field trials were carried out at DiSAFA facilities (Grugliasco, Italy) in an experimental espalier-
trained vineyard (cv: Barbera) at two growth stages, namely early (BBCH 57, inorescences fully
developed; owers separating) and late (BBCH 89, berries ripe for harvest). The vine rows were
62 m long and oriented NW–SE (146° azimuth). Planting distances were 2.8 m between rows and
0.8 m within rows, resulting in a density of 4,464 vines ha-1. The inclined point quadrat technique
(PQT) was applied to accurately characterize the vineyard crop before the trials (Grella et al.,
2019). PQT measurements were taken in the vegetative strip of six vines coincident with those
used for canopy deposition sampling described in the following section (Experimental plot layout
and sampling system). The mean vegetative parameters, namely leaf layers and gaps, were thus
obtained, and leaf area index (LAI) was calculated from Eq. (1) according to Vitali et al. (2013):
LAI = Ly*(Hc/Rd) (1)
where LAI is leaf area index (adim.); Ly is the average number of leaf layers (n°); Hc is the canopy
height (m); Rd is the inter row distance (m).
An overview of the vegetative characteristics of the vines used for experimental sampling is
presented in Table 1. The average number of leaf layers was 0.77 and 2.31, the average of gaps was
60 and 14 %, and LAI was 0.33 and 1.33 (adim.), respectively for the early and late growth stage.
Table 1. Vegetative characteristics of the vines used for sampling at early and late growth stage
stage BBCH Depth (m) Height (m) N° of leaf
layers Gap (%) LAI
Early 57 0.39 1.06 0.82 61 0.35
Early 57 0.31 1.05 0.75 61 0.32
Early 57 0.38 0.89 0.81 58 0.29
Early 57 0.44 1.18 0.79 57 0.37
Early 57 0.38 1.07 0.72 60 0.31
Early 57 0.46 1.20 0.75 63 0.36
Late 89 0.42 1.65 2.13 19 1.25
Late 89 0.50 1.61 2.39 13 1.37
Late 89 0.44 1.55 2.11 18 1.17
Late 89 0.62 1.59 2.25 11 1.28
Late 89 0.42 1.62 2.31 17 1.34
Late 89 0.54 1.66 2.67 6 1.58
Sprayer characteristics and congurations tested in eld trials
A prototype vineyard sprayer, i.e. Smart Synthesis (Cani S.p.a., Palù, Verona, Italy), was
employed. It is a trailed sprayer with a 1,000 L polyethylene tank and an innovative electrically
driven axial fan (KEB automation KG, Barntrup, Germany) (Fig. 1a). The fan, 700 mm in diameter
and consisting of nine blades, sucks in the air from the front of a tower shaped air conveyor. The
latter is equipped with multiple adjustable deectors placed internally at the edge of the air-jet outlet,
thus allowing to direct the airow to precisely match the canopy height. An electric-control varies
the orientation of the whole air conveyor with respect to the central axis of the air-jet discharge
system backwards and forward, thus determining the incidence angle of both airow and spray jets
on the canopy. Furthermore, the sprayer was equipped with a DynaJet® Flex 7140 Pulse Width
Modulation (PWM) system (TeeJet, Spraying Systems Co., Wheaton, Illinois, USA) featured by
eight PWM solenoid valves coupled with a single nozzle holder per sprayer side (Fig. 1b). The
PWM valves can vary the duty cycle of the pulse signals continuously from 30 to 100% to change
the spray outputs but were only used at 100% DC in this study. Further details are provided in
Grella et al. (2021, 2022) and Zwertvaegher et al. (2022).
Using the electrically driven axial fan, dierent fan revolution speeds were set to test the eect
of fan airow rate (m3 h-1) on canopy deposition at the two growth stages. Three levels of airow
rate (low, medium, and high) were arbitrarily selected for tests. The fan revolution speeds at each
level varied according to the growth stage (as the canopy density increased) from 400 to 1,000
rev min-1 for the early growth stage, and from 700 to 1,750 rev min-1 for the late growth stage
(Table 2). The electric axial fan ensured the desired fan revolution speed, irrespective of the PTO
revolution speed, throughout the trials. In addition, two sprayer forward speeds (4 and 8 km h-1)
were combined with selected fan settings, at both growth stages, to test their eect on the canopy
deposition and possible interactions with airow rate (Table 2).
Fig. 1. Prototype vineyard sprayer equipped with electrically driven fan and PWM spray system during eld
trials at a) early (lateral sprayer view) and b) late (back sprayer view) growth stages.
Table 2. Overview of electrically driven axial fan settings tested in eld trials at early and late
growth stage in combination with two forward speeds for their eect on canopy deposition
Total air volume
applied (m3 ha-1)
Early 4.0 Low 400 4,728 4,225
Early 4.0 Medium 700 8,378 7,488
Early 4.0 High 1,000 11,876 10,614
Early 8.0 Low 400 4,728 2,113
Early 8.0 Medium 700 8,378 3,744
Early 8.0 High 1,000 11,876 5,307
Late 4.0 Low 700 8,378 7,488
Late 4.0 Medium 1,300 15,398 13,762
Late 4.0 High 1,750 20,663 18,468
Late 8.0 Low 700 8,378 3,744
Late 8.0 Medium 1,300 15,398 6,881
Late 8.0 High 1,750 20,663 9,234
The sprayer was equipped with eight standard at fan nozzles XR 80 02 VS (TeeJet, Spraying
Systems Co., Wheaton, Illinois USA) per sprayer side. All trials were carried out at a xed working
pressure of 0.40 MPa and 100% PWM duty cycle, providing an individual nozzle ow rate of 0.91
L min− 1. Based on a preliminary check of the vertical spray prole, only four and six nozzles per
sprayer side were activated at the early and late growth stage, respectively. Irrespective of growth
stage, the bottom nozzle was turned o on both sides while at the top three and one nozzles were
turned o at early and late growth stage, respectively. Resulting spray volumes were 390 and 195
L ha-1 at early growth stage and 585 and 293 L ha-1 at late growth stage for 4 and 8 km h-1 forward
In all cases the internal air deectors were adequately adjusted to match the canopy height and to
minimize spray losses over the canopy. The air conveyor was placed orthogonal to the rows (90°
relative to the central axis of the air-jet discharge system) in all trials.
Experimental plot layout and sampling system
The trials were performed by spraying the two outermost vineyard rows, with a total area of 347
m2 (62.0 m × 5.6 m) (Fig. 2a). Canopy spray deposition measurements were performed at three
locations along the sprayed rows, corresponding to three vine canopies per sprayed row. In total,
measurements were taken from six vines, as shown in Fig. 2a. At the early growth stage, spray
deposition was assessed at four sampling positions, i.e. at two heights (1 and 2) and two depths (A
and C) (Fig. 2b), while at the late growth stage nine sampling positions were assessed, i.e. three
heights (1, 2, and 3) and three depths (A, B, and C) (Fig. 2c). To assess the deposition, round lter
papers (120 mm diameter and 90 g m-2 extra rapid, Gruppo Cordenons S.p.A., Milan, Italy) were
clipped to vertical masts at each sampling position. Each collector represented a total exposed
surface area of 226 cm2. At the end of each spray application, samples were left to dry for 10 min,
placed into individual bags and sealed. To prevent tracer photo-degradation, the samples were
collected in closed dark boxes. Each test was repeated three times.
Fig. 2. Schematics of a) trial layout for the measurement of canopy deposition (aerial view) and related
sampling strategy at early b) and c) late growth stages; canopy depths A, B, and C, and canopy heights 1,
2, and 3.
Sprayed mixture and laboratory analysis
To measure the collector spray deposits, E-102 Tartrazine yellow dye tracer was added to the
sprayer tank at a target concentration of about 10 g L−1. Before and after each spray application, the
tank mixture was sampled directly from the nozzles to determine the precise tracer concentration
of each repetition.
The collectors were washed with deionized water to extract the tracer. The Tartrazine concentration
was determined by measuring the absorbance of the washing solution with a spectrophotometer
UV-1600PC (VWR, Radnor, PA, USA) set to 427 nm wavelength for peak absorption of the dye
and comparing it to the calibration curve obtained in the laboratory prior to the analysis. In all
cases, three absorbance measurements were taken from each sample.
The deposit on each collector (Di), expressed per unit area in μL cm−2, was calculated from Eq.
(2) as follows:
where Di is the spray deposit on a single collector (μL cm−2); psmpl is the absorbance value of the
sample (adim.); pblk is the absorbance of the blanks (adim.); Vdil is the volume of the deionized
water used to extract tracer deposit from the collector (μL); pspray is the absorbance value of the
spray mixture concentration applied during testing and sampled at the nozzle outlet (adim.); Acol
is the projected area of the collector exposed to the spray (cm2).
For a broad comparison of data, the spray deposit values (μL cm-2) were transformed to be expressed
as % of applied volume.
Di = ((psmpl - pblk) * Vdil) / (pspray * Acol) (2)
Data processing and statistical analysis
All statistical analyses were performed using IBM SPSS Statistics (Version 27) predictive
analytics software for Windows®. To investigate at what sprayer setting combinations the highest
spray depositions were obtained, a three-way ANOVA was performed with growth stage (early
vs late), airow rate (low vs medium vs high) and forward speed (4 vs 8 km h-1) as independent
variables and canopy deposition (% of applied volume) as dependent variable. The interactions
among independent variables were also investigated. To investigate the dierences among spray
deposition obtained at dierent airow settings, the means were compared using a Duncan post-
hoc test for multiple comparison (P < 0.05). The fan settings able to maximize the canopy spray
deposits at dierent forward speeds at late and early growth stage were then objectively identied.
A one-way ANOVA was carried out to investigate the eect of forward speed (4 vs 8 km h-1) on
spray canopy deposit obtained just from the fan settings able to maximize the canopy deposition.
Results and Discussion
The three-way ANOVA (Table 3) indicates that growth stage exerted a statistical inuence on
the canopy deposition (% of applied volume) irrespective of fan airow setting and forward speed
Table 3. Results of the three-way ANOVA for the canopy deposition (% of applied volume)
Sources DF P > (F) Signicance a
Growth stage (GS) 1 3.24E-22 ***
Fan airow setting (ARF) b21.39E-07 ***
Forward speed (FWS) 10.393 NS
GS × AFR 20.969 NS
GS × FWS 1 0.447 NS
AFR × FWS 2 1.01E-04 ***
GS × AFR × FWS 2 0.576 NS
a Statistical signicance level: NS P > 0.05; * P < 0.05; ** P < 0.01; *** P < 0.001,
b Fan airow rate corresponds to Low, Medium and High levels.
Average canopy depositions expressed as % of applied volume were 38.5% . 25.5% for early
and late growth stages, respectively. The huge increase in canopy density along the growing
season resulted in a decrease in average deposition, as the inner parts of the row are much more
dicult to reach. In addition, airow setting signicantly aected canopy deposition (Table 3), as
it varied the airow rate (m3 h-1) (Table 2). The higher the fan airow rate, the lower the canopy
deposition irrespective of growth stage (33.4% low, 32.5% medium and 25.9% high). A signicant
interaction was found between air ow setting and forward speed, meaning that eect of forward
speed depended on air ow setting (as can be seen in Fig 3). Fig. 3 displays the average canopy
spray deposition obtained for dierent air ow setting forward speeds and growth stages. At 4
km h-1, the adoption of a low fan speed (400 and 700 rev min-1 at early and late growth stages,
respectively) allowed to obtain a signicant increase in spray deposition compared to the other
fan speeds. At 8 km h-1, the highest spray depositions were obtained with the medium fan speed
(1,000 and 1,300 rev min-1 at early and late growth stages, respectively). The medium fan speed
resulted in a signicantly higher deposition than the low and high fan speed at the early growth
stage, but at the late growth stage only the dierence with the high fan speed was signicant. The
results are in line with those found by other authors reporting a decrease in canopy deposition due
to an excessive airow rate generated by axial fan sprayer (Pergher, 2005; Balsari & Marucco,
2004). However, at 8 km h-1 the low fan speed probably did not provide enough airow to open
and move the dense (late stage) canopy, resulting in a reduced spray penetration and lower spray
deposition value. When increasing forward speed, the airow rate must be increased in order to
maximize spray deposition at both growth stages.
Fig. 3. Average canopy deposition (% of applied volume) for the dierent fan speed setting (low, medium,
high) at 4 and 8 km h-1 forward speed and at early and late growth stage. The bars show the mean ± standard
error of the mean. Dierent letters on the bars denote signicant dierences within forward speed and growth
stage (Duncan’s post hoc test, P < 0.05).
Focusing solely on the fan settings able to maximize the canopy deposition, the one-way ANOVA
underlines that there is no signicant eect of forward speed (4 vs 8 km h-1) on canopy deposition
[F(1, 1) = 0.697, P = 0.406]. This is because an adequate level of air volume per ground area (m3
ha-1) was maintained through changes in fan revolution speed setting. Indeed, at early growth stage,
increasing forward speed from 4 to 8 km h-1 did not signicantly aect spray deposition because
the fan speed was increased from low (400 rev min-1) to medium (700 rev min-1) resulting in similar
amounts of total air volume applied, i.e. 4,225 vs 3,744 m3 ha-1 (Table 2). Similar results were found
at the late growth stage with highest spray deposition values for total air volumes applied of 7,488
m3 ha-1 (4 km h-1 and low fan speed - 700 rev min-1) and 6,881 m3 ha-1 (8 km h-1 and medium fan
speed – 1,300 rev min-1) (Table 2). It indicates that within each growth stage, canopy spray deposition
can be maximized by applying an adequate total air volume per ground area (m3 ha-1) through the
correct combination of fan revolution speed and forward speed. Total air volume applied was thus
demonstrated to be the main factor to take into account to maximize the canopy spray deposition.
Based on this nding, a linear t model was obtained to describe the relationship between the LAI
(Table 1) and the corresponding optimal air volume applied (m3 ha-1) to maximize canopy spray
deposition (Fig. 4). The model thus provides values for the total air volume to be applied at the
dierent growth stages. A threshold LAI value of 2.5 (represented by the red line perpendicular on
the x axis in Fig. 5) was set because in modern commercial trellised vineyards leaf layers is kept
below 2.5 through adequate canopy management techniques, such as shoot trimming, positioning,
and tying (Intrieri & Poni, 1995). As an example, a LAI value of 2.45 can be achieved by a trellised
vineyard with “extreme” features, i.e. 2.2 m inter-row distance, vegetative strip of 1.8 m width,
canopy depth of 0.8 m and mean leaf layer equal 3.0. Based on the linear model, the corresponding
optimal value for the total air volume would be 10,735 m3 ha-1. At 4 km h-1 forward speed, this
corresponds with a fan speed setting of about 1,000 rev min-1, or slightly higher (Table 2). At 8 km
h-1 forward speed, fan settings higher than those tested in this experiment (1,750 rev min-1) would
Fig. 4. Linear t model describing the relationships between the Leaf Area Index (LAI, adim.) and the total
air volume applied (m3 ha-1) based on experimental data obtained in the tests at 4 and 8 km h-1 forward speeds.
The red lines represent a maximum threshold LAI value in modern vineyards that denes the maximum
total air volume to be applied (10,735 m3 h-1) in vineyard spray application.
The results obtained from this preliminary, experimental work are the basis for the development of
a smart sprayer equipped with an electrically driven axial fan able to provide an optimized variable
airow rate based on a real-time measurements of canopy characteristics. This study explores the
possibility to use the LAI as a parameter for setting the optimal fan speed (rev min-1) automatically
in order to deliver the desired total air volume applied (m3 ha-1). Further relationships among other
available canopy parameters (i.e. number of leaf layers, % of gaps, tree row volume, and leaf wall
area) and total air volume applied per ground area (m3 ha-1) are under investigation in order to
dene the most useful canopy parameter for selecting the optimal fan setting. Additional trials are
ongoing to evaluate the eect of a wider range of fan revolution and forward speed settings on
canopy spray deposition.
This project has been funded by the European Union’s Horizon 2020 research and innovation
program under grant agreement No 773718 (OPTIMA-project). Special thanks go to KEB ITALIA
S.r.l. for providing technical support, and to Marco Resecco for the help provided during eld
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