Crop Science 40:444-448 (2000)
© 2000 Crop Science Society of America
CROP ECOLOGY, PRODUCTION & MANAGEMENT
Spray Chamber Evaluation of Air-Assisted Spraying on Broccoli
Bernard Pannetona,
Hubert Philionb,
Roger Thériaultb and
Mohamed Khelifib
a Horticultural Research & Development Centre, Agriculture & Agri-Food Canada, 430, Gouin Blvd, Saint-Jean-sur-Richelieu, Quebec, Canada, J3B 3E6
b Dep. of Soil Science and Agri-Food Engineering, Faculty of Agriculture and Food Sciences, Laval Univ., Quebec, QC, Canada, G1K 7P4
pannetonb{at}em.agr.ca
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ABSTRACT
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Conventional over-the-row sprayers achieve very little deposit on leaves near the ground and on the underside surfaces of leaves throughout the canopy. Air assistance has the potential to improve deposition of droplets on these leaf surfaces. To gain some insight on the effect of air assistance, the effects of airspeed, airflow rate, and air jet orientation were isolated. The study was carried out in a spray chamber with a standard spray boom over micro-plots of greenhouse grown broccoli (Brassica oleracea var. botrytis L.) plants. Air was delivered slightly behind the nozzles from a variable width slot producing a uniform two-dimensional air jet. The orientation of the air jet with respect to the vertical could be adjusted from -10 to 40°. The ranges of the independent variables were airspeed, 0 to 36 m s-1; airflow rate, 0 to 1.3 m3 s-1 m-1, and air jet angle, -10.2 to 40.2°. Two sets of flat fan nozzles (Volume Median Diameter = 230 and 400 µm, both delivering 250 L ha-1 at 6 km h-1) were used to carry out two full sets of experiments. Results showed that airspeed had the larger impact on leaf coverage. Higher airspeeds (>25 m s-1) and airflow coupled with finer sprays increased the coverage of the underside of the leaves at all levels within the canopy and of the top side of the leaves in the lower third of the canopy. However, lower airspeeds (<20 m s-1) are desirable for a better coverage of the upper side of the leaves in the higher two-thirds of the canopy. In all cases, angling the air jet forward at 20 to 25° is recommended.
Abbreviations: VMD, volume median diameter • CCD, central composite design • Y, number of bright pixels • Q, airflow rate • V, airspeed • 
, air jet angle
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INTRODUCTION
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BROCCOLI HAS BEEN IDENTIFIED as a vegetable useful in promoting good health and this has boosted the demand for this produce. Like many other field crops, broccoli is threatened by insects and fungal diseases. Cost of pesticides, environmental pollution, and avoidance of chemical residues on food are incentives to improve application methods to reduce pesticide usage.
Air-assisted spraying is a technique that has the potential to improve the leaf coverage through the plant canopy. Hadar (1991) reported that air-assisted spraying is an effective method of improving spray penetration and reducing spray drift. However, air volume and speed must be uniform along the boom. Manor et al. (1989) used an air-assisted sprayer in dense cotton (Gossypium hirsutum L.) with an airspeed set at 37 m s-1 pointing vertically downwards. Compared with a conventional sprayer, better penetration in the lower part of the cotton canopy and better coverage of the underside of leaves was obtained. Womac et al. (1992) used a Hardi Twin (Hardi International, Taastrup, Denmark) sprayer in cotton with three airspeeds (0, 8, and 16 m s-1) measured 10 cm below the slot and directed vertically down. At an application rate of 47 L ha-1, results showed an increase in leaf coverage for the top leaves with increasing airspeed, and a better coverage in the middle of the canopy at full airspeed. At 94 L ha-1, the top leaf coverage was not affected by increasing airspeed. However, middle leaf coverage of the canopy was the best at the highest airspeed.
On potatoe (Solanum tuberosum L.), Cooke et al. (1990) obtained a better coverage for the middle and lower parts of the canopy with high airspeed (36 m s-1). The coverage of the top leaves was the best at a medium airspeed (25 m s-1). Jeffrey and Taylor (1991) used a Hardi Twin air-assisted sprayer at three airspeeds (15, 22, and 28 m s-1). On the upper side of leaves, they found that the coverage in the upper section of the canopy was lower with increasing airspeed while at the middle and bottom sections of the canopy, a peak coverage was observed at 22 m s-1. On the underside of leaves, the top and middle sections of the canopy had a peak coverage at 22 m s-1 while the coverage at the bottom of the canopy increased with increasing airspeed. Quanquin et al. (1989) used a Hardi Twin air-assisted sprayer with airspeed set at 30 m s-1 to investigate the spray deposition on potato plant foliage. They reported that the coverage increased with air assistance and was even better with a spraying angle set at 30° backwards.
May (1991) used a Hardi Twin air-assisted sprayer equipped with flat fan nozzles on sugar beets (Beta vulgaris var. sacchariferr L.) with three airspeeds (0, 12, and 23 m s-1) and two air jet angles (30° forwards and 30° backwards). For both angles, targets between rows showed a better coverage at higher airspeeds, whereas targets under rows showed a better coverage at a forward angle with high airspeed. May (1991) also used a Degania sprayer (Degania Sprayers, Kibbutz Degania Bet, Israel) with hollow cone nozzles on sugar beets with three airspeeds (0, 23, and 56 m s-1). The higher airspeed gave the best coverage for targets between rows and under the crop. The angle of the air jet was not specified, however. With the Hard Twin sprayer used vertically, the best upper side leaf coverage was with either 0 or 12 m s-1 while the best coverage for underside leaves was with 23 m s-1. With the Doggone sprayer, the contrary was observed as the coverage increased with increasing speed for the upper side of leaves and decreased for the underside of leaves.
The literature review revealed that higher airspeeds promote overall a better coverage in the middle and bottom portions of the plant canopy while lower airspeeds are more appropriate for the top portion of the plant canopy. The main variables tested by the majority of researchers are airspeed and air jet angle. In all cases, the airflow was always linearly linked with airspeed.
Besides airspeed, airflow rate, and angle, other parameters such as wind, affect spray deposition on foliage. Conducting spray experiments in a spray chamber where boom spraying is simulated can control the variability of these operational parameters. This facilitates the optimization of controlled parameters for best coverage.
The objective of this study was to determine, for two droplet sizes, the combination of airspeed, airflow rate, and air jet angle which provides the best compromise in leaf coverage and canopy penetration for broccoli plants.
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Materials and methods
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Spray Chamber
The spray chamber (Agriculture and Agri-Food Canada Horticultural R&D Center, St-Jean-sur-Richelieu) is 12 m long and 3 m wide. It is fitted with a spray boom hung underneath a cart. The spray boom is a conventional boom to which an air diffuser was added. The air is supplied to the diffuser by a 610-mm diam. flexible pipe. The exit of the diffuser is a 2438-mm slot having an adjustable width from 17 to 67 mm (Fig. 1)
. This adjustment allows an independent setting of the airflow rate and the airspeed. The airflow angle is also adjustable (Fig. 2)
. Baffles were positioned inside the diffuser such that the exit velocity of the air is constant across the width of the boom. The air is supplied to the boom in a closed circuit. Before reaching the duct, the air goes through an 1830- by 1830-mm filter to prevent any contamination by the sprayed product. The filter is located at the starting end of the chamber and has a particle removal efficiency of 95% for 3-µm particles. Several components of the spray chamber are computer controlled. The cart is powered by a 0.746 kW electric motor having variable speed. The forward speed of the cart, set at 6 km h-1 (± 7%), is continuously monitored through a revolution counter connected to the computer. The cart is chain driven by the electric motor that makes slippage impossible. A Pitot tube installed in the air delivery duct upstream from the exit slot monitors the airflow. The Pitot tube, connected to a pressure transmitter, was calibrated beforehand against an inclined U-tube manometer. Calibration curves relating Pitot tube measurements and airspeed at the exit orifice were established for each airflow rate. Airspeeds given in this report are specified at the exit slot. The airspeed adjustment errors averaged 5%.
The nozzles used were Delavan 80-3R (Delavan Spray Technologies, Monroe, NC) at 385 kPa and Delavan 80-5R at 145 kPa. The 80-3R nozzles give relatively small spray droplets (Volume Median Diameter or VMD = 230 µm) compared with the 80-5R (VMD = 400 µm). Pressure was set to provide 250 L ha-1 for both nozzles at a travel speed of 6 km h-1. There was no more than 4% difference between nozzle outputs. The distance between nozzles was set at 50 cm and the spraying height at 45 cm above the plants. The computer controlled the starting sequence of the different components of the chamber. At the beginning of each treatment, the water pump was started first. After one second, the air blower started; four seconds later, the nozzles, and two seconds later, the cart started traveling over the plants. This starting sequence permitted the stabilization of each component so that target values of each variable were accurately obtained.
Micro-Plots Layout
To simulate field spraying, broccoli was laid out following the pattern used by Quebec growers. Holes were drilled in a false floor to place each pot of the greenhouse grown plants. Broccoli plants (cv. Emperor) were placed in double rows, 305 mm apart within and between rows (Fig. 3)
. The double rows were spaced at 1076 mm center to center. Because of the width of the spray chamber, one double row was centered on the floor and only two halves (i.e., single rows) were on the edge. The false floor holding the plants was positioned 254 mm above the chamber floor to allow the pots to fit in completely. The spraying of broccoli was done at the beginning of flowering when plants had between 10 and 15 leaves.
Experimental Design
A uniform precision rotatable central composite design (CCD) with three factors was used (Myers, 1971; Panneton et al., 1999). The three factors or independent variables were airflow rate, airspeed, and air jet angle. Five levels of each independent variable were needed to satisfy the requirements of the CCD, i.e., a coding of -1.682, -1, 0, 1, and 1.682 (Fig. 4)
. Target values of airflow rate were 0.40, 0.56, 0.80, 1.04, and 1.20 m3 s-1 per meter of boom. Those for airspeed were 12.00, 16.86, 24.00, 31.14, and 36.00 m s-1. Target values for air jet angle were -10.23, 0, 15, 30, and 40.23°. The 0 degree air jet angle is straight down. For the purpose of this experiment, the positive angles are towards the forward direction of the cart (Fig. 2). All treatment combinations are summarized in Table 1
. Treatments located at the center of the design have to be repeated six times in order to satisfy the requirements of uniform precision and rotatability of the CCD (Myers, 1971).
Data Collection
The spray coverage was measured on the upper side and underside of leaves at the bottom, middle, and top sections of the plants for a total of six sampling locations per plant. Kromekote card disks (Kruger Fine Paper, Ottawa, Canada), 10 mm in diameter, were glued with Tangle Trap insect trapping adhesive (The Tanglefoot Company, Grand Rapids, MI) at each location on the plants to collect spray droplets. Six sampling units (i.e., the Kromekote cards) were glued on each location for a total of 36 per plant. Each treatment was replicated three times on three different plants. All three replications of each treatment were carried out at once, since the only significant source of heterogeneity was the physical structure of the plants themselves. The location of the three plants used as replications for each treatment are shown as filled circles on Fig. 3. Trials were conducted separately for each of the two droplet sizes.
The spray solution was water with 0.1% (v/v) of Agral 90 (Novartis, Basel) and 3% (v/v) of the fluorescent dye Blankophor BA (Bayer AG, Leverkusen, Germany) liquid 80%. When illuminated under ultraviolet lamps at 254 nm, the fluorescent dye provided a high contrast between droplet stains and the Kromekote cards. Image analysis was used to measure the area covered by spray droplets. The camera resolution was 36 µm pixel-1. A 160 by 160 pixel square window was positioned over each sampling unit. Applying a single threshold as described in Panneton and Drummond (1991) yielded a good quality binary image from which the covered area was measured by counting bright pixels.
Data Analysis
The CCD data analysis was based on the following equation of a second-degree polynomial response surface:
 | (1) |
where the airflow rate Q, airspeed V, and air jet angle , are the independent variables. The response, Y, is expressed in terms of number of bright pixels. To obtain a percentage of leaf coverage, the response should be divided by 25600. A logarithmic transformation was required for all underside data because the variance was increasing with the mean. Each location within the canopy was separately analyzed and the CCD analysis was performed by the GLM procedure (SAS Institute, 1989). For each location and droplet size, the coefficients of Eq. [1] were generated. All linear terms are orthogonal to each other and so are the bilinear terms (Philion, 1995). However, the quadratic terms are not orthogonal to each other.
The maximum coverage was obtained by computing the response over a three-dimensional Cartesian grid covering the space within a sphere having a radius of 1.682 units. This domain included the 2 by 2 by 2 cube and is referred to the experimental domain later in this paper (Fig. 4). The associated variance of the predicted response drastically increases outside this domain (Panneton et al., 1999). The independent variable increment used was 0.004 m3 s-1 m-1 for airflow, 0.12 m s-1 for airspeed, and 0.2523° for air jet angle.
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Results and discussion
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Experimental results were used to compute regression equations based on Eq. [1]. This resulted in 12 regression equations, one for each of the six sampling locations in two series, one for small and one for large droplets respectively. Most regression equations applying to the coverage of the upper side of the leaves are statistically not significant (Table 2)
. This is not the case for the underside coverage where most regression models are statistically significant with R2 ranging from 0.28 to 0.47. Therefore, underside leaf coverage can be significantly affected by changing the setting of the parameters associated to air assistance. For the upper side of the leaves, change in these parameters is not reflected into coverage values.
Regression coefficients associated with airspeed (V and V2) are significantly different from 0 in all cases where significant coefficients are identified (Table 3)
. Clearly, airspeed is the most important parameter to consider when adjusting air assistance within the range of airflow rate and air jet angles considered. Coefficients associated with airflow rate, air jet angle, or interactions (VQ, Q , V ) are significant for some of the sampling locations but no clear patterns could be identified.
The data in Table 4
can be grouped in two clusters according to airspeed values. The first one, labeled high speed cluster, groups all underside locations and the bottom upper side location in the case of small droplets. This cluster is associated with an airspeed above 25 m s-1. Other locations can be grouped in a low speed cluster associated with airspeed below 25 m s-1. Attempts to define clusters based on other criteria (airflow rate, angle, droplet size) were not successful. For small droplets, there is a very significant positive correlation (P = 0.008) between airspeed and airflow rate giving optimum coverage.
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Table 4 Combinations of airflow, airspeed, and air jet angle that provided the maximum coverage of broccoli leaves
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Numbers in bold in Table 4 designate points located on the edge of the experimental domain. In these cases, true optima are located outside the experimental range and could not be identified. For the high speed cluster, the airspeed is close to the maximum airspeed within the experimental range. This suggests that true optima are likely to be found at higher airspeeds.
The clustering described above was used as a basis for defining two sets of operational parameters, one for each cluster. For each cluster, average values of independent variables were calculated confounding data over droplet size (Philion 1995). For the high speed cluster, the resulting values are V = 29.2 m s-1, Q = 0.96 m3 s-1 m-1, and = 23.6°. For the low speed cluster, the values are V = 16.8 m s-1, Q = 0.65 m3 s-1 m-1, and = 20.0°. In both cases, the airflow rate is larger than the one delivered by commercially available air-assisted sprayers (estimated at 0.4 m3 s-1 m-1). From these new sets of independent variables, new coverage values were computed from the regression equations. Tables 5 and 6
present, for each location and droplet size, the optimum and the new coverage values for both clusters. In general, the high speed cluster is associated with improved underside leaf coverage and the low speed cluster with the upper side leaf coverage. Furthermore, coverage values for all tests with smaller drops are higher than coverage for the same tests performed with coarser sprays. Compared with non air-assisted spraying, air assistance improves coverage if the proper settings are used. For example, if it is required to improve the coverage of the upper side of leaves at mid-height within the canopy, settings associated with the low speed cluster are recommended. In this case, the high speed cluster settings does not improve coverage compared with conventional spraying. As another example, for both droplet sizes, air assistance did not improve the underside coverage of the top leaves.
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Table 5 Optimum and new coverage values of high and low speed clusters for broccoli sprayed with small droplets compared to coverage without air-assistance
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Table 6 Optimum and new coverage values of high and low speed clusters for broccoli sprayed with large droplets compared to coverage without air-assistance
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Conclusions and recommendations
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1. Results of the spray chamber trials showed that, compared with non air-assisted spraying, higher airspeeds (>25 m s-1), smaller droplets and a higher airflow promote the coverage of all underside locations and, at the bottom of the canopy, of the upper side of the leaves (small droplets). This result is obtained with no detrimental effect on the coverage of remaining locations. However, lower airspeeds (<20 m s-1) lead to a better coverage, in the middle and top part of the canopy, of the upper side of the leaves. In addition, air jet angles between 20 and 25° are recommended for a better coverage.
2. The optimum spraying conditions depend on the part of the canopy to protect. For example, if all leaf undersides are targeted, the best compromise would be an airflow of 0.96 m3 s-1 m-1, an airspeed of 29 m s-1, and an air jet angle of 24°.
3. The main interest of air-assisted spraying is to increase both the coverage of the hidden parts of the plants, and the spray penetration towards the bottom of the canopy. In this context, the use of an airspeed and an airflow rate higher than 25 m s-1 and 0.9 m3 s-1 m-1, respectively, coupled with a fine spray and a forward angle in the 20 to 25° range is the best general recommendation based on data presented here.
4. It should be stressed that the optimum values were found on the edge of the domain. Therefore, it is recommended to enlarge the domain of investigation and to carry out additional work to establish whether or not obtained values represent the true optima.
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ACKNOWLEDGMENTS
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This research was financially supported by Canada/Quebec Agreement for a sustainable environment in agriculture, and Grégoire et Fils Inc. The authors thank Sylvain Fortin, Jr., Eng. and Gilles St-Laurent for the technical support.
Received for publication March 17, 1999.
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REFERENCES
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- Cooke B.K., Hislop E.C., Herrington P.J., Western N.M., Humpherson-Jones F. Air-assisted spraying of arable crops, in relation to deposition, drift and pesticide performance. Crop Protection 1990;9(4):303-311.
- Hadar, E. 1991. Development criteria for an air-assisted ground crop sprayer. In A. Lavers, et al. (ed.) Air-assisted spraying in crop protection. BCPC Monogr. 46. Brit. Crop Protect. Council. Bury St. Edmunds, England.
- Jeffrey, W.A., and W.A. Taylor. 1991. Foliar distribution of air-assisted spray deposits in a potato canopy. p. 275. In A. Lavers, et al. (ed.) Air-assisted spraying in crop protection. BCPC Monogr. 46. Brit. Crop Protect. Council, Bury St. Edmunds, England.
- Manor G., Hofner A., Or R., Phishler G., Epstein Y., Nakash T., Jacobi M. Air stream facilitated application of cotton foliage treatments. Trans. ASAE 1989;32:37-40.
- May, M.J. 1991. Early studies on spray drift, deposit manipulation and weed control in sugar beet with two air-assisted boom sprayers. p. 89–96. In A. Lavers, et al. (ed.) Air-assisted spraying in crop protection. BCPC Monogr. 46. Brit. Crop Protect. Council, Bury St. Edmunds, England.
- Myers R.H. Response surface methodology. Boston: Allyn and Bacon, Inc, 1971.
- Panneton B., Drummond A.M. Digital image analysis of spray samples. Appl. Eng. Agric. 1991;7(2):273-278.
- Panneton B., Philion H., Dutilleul P., Thériault R., Khelifi M. Full factorial design versus central composite design: Statistical comparison and implications for spray droplet deposition experiments. Trans. ASAE 1999;42(4):877-883.
- Philion, H. 1995. Optimisation d'une rampe à jet d'air. Unpublished M. Sc. Thesis, Dep. of Soil Science and Agri-Food Engineering, Université Laval, Québec.
- Quanquin B.J., Anderson G.W., Taylor W.A., Andersen P.G. Spray drift reduction and on-target deposition using the Hardi Twin System air assisted boom sprayer. St. Joseph, MI: ASAE, 1989 ASAE Paper No. 89-1523..
- SAS Institute. 1989. SAS/STAT user's guide. Version 6, 4th ed. SAS Institute, Cary, NC.
- Womac A.R., Mulrooney J.E., Scott W.P. Characteristics of air-assisted and drop-nozzle sprays in cotton. Trans. ASAE 1992;35:1369-1376.
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ABSTRACT
INTRODUCTION
