Introduction

Wild blueberry (Vaccinium angustifolium Ait.) fields use agrochemicals to sustain high yielding crops (Yarborough 2004). These agrochemicals are applied uniformly using commercially available sprayers, hence ignoring the substantial variations in field and crop characteristics. The wild blueberry industry has set a goal to reduce agrochemical usage by 20–40 % (Zaman 2011). Wild blueberries are typically managed on a 2 year production cycle with the majority of herbicides being applied during the first year (vegetative growth stage) and insecticides during the second year (fruit production stage). Fungicides are applied during both years of production (Anonymous 2015). Propyzamide (Kerb™) herbicide is sometimes applied in late fall after harvest or in early spring of the vegetative year when the blueberry plants are dormant (Burgess et al. 2014). Propyzamide is used for advanced control of mainly fescue grass and broadleaf weeds including sheep sorrel (Burgess 2015). In central Nova Scotia, a tank mix of hexazinone (Velpar®) and terbicil (Sinbar®) herbicide is commonly used for pre-emergence weed and grass control (Burgess 2015). Metzger and Ismail (1977) reported that a newly developing field can have 20–40 % wild blueberry cover. Zaman et al. (2008) estimated 30–50 % wild blueberry cover in developing fields. Wild blueberries spread slowly by rhizomes underneath the soil surface and prefer grass or weed shaded ground as compared to barren soil for effective growth (Kender and Eggert 1965). Producers apply herbicides uniformly on both blueberry plants and bare soil areas of the field, which not only increase the production cost but also contaminate the environment via leaching, runoff and erosion (Zaman et al. 2010). Spot-application of pre-emergence herbicide to wild blueberry plants allows the crop to flourish weed-free. Not targeting bare soil can encourage native species of grasses or weeds to occupy the bare ground limiting soil erosion, keeping the organic matter and promote the spread of the wild blueberry rhizomes (Morrison et al. 2000). Mesotrione (Callisto®) and fluazifop-P-butyl (Venture®) selective contact herbicides are commonly tank mixed in June for post-emergence suppression of weed and grass species (Boyd and White 2010; Boyd et al. 2014). Spot-application of mesotrione and fluazifop-P-butyl herbicide solely in the target areas has potential to significantly reduce herbicide with effective weed and grass control. Tank mix fungicide applications of chlorothalonil (Bravo®) and prothioconazole (Proline®) in July of the vegetative year for control of Septoria (leaf spot) and Thekopsora minima (blueberry rust) can be reduced by applying spot-specifically only to the blueberry plants (Esau et al. 2014; Percival and Dawson 2009). In the crop year, propiconazole (Topas®) fungicide is applied in late May and/or early June for control of Monilinia vaccinia-corymbosi (Monilinia blight) (Percival and Dawson 2009). Chlorothalonil may also be applied a second time in June of the crop year depending on the disease potential within the field (Cote et al. 2015). Boscalid/pyraclostrobin (Pristine™) is typically a last fungicide applied in June of the crop year to control Septoria spp. (valdensinea leaf spot) and blueberry rust (Annis and Stubbs 2004). Blueberry maggot will contaminate blueberry fruit, so fly traps are set in July to estimate the infestation levels and spray acetamiprid (Assail®) insecticide if required (Renkema et al. 2014).

Sprayers used in the wild blueberry industry range from hand wand backpack models up to large 36.6 m wide self-propelled models. Studies have shown that the foam marker guidance systems can have error factors of up to 10 % of the sprayer boom width (Ehsani et al. 2004). Wild blueberry fields are naturally developed and typically situated on rough terrain with gentle to severe topography (Zaman et al. 2010). Keeping the sprayer nozzles close to the blueberry canopy without coming in contact with the uneven terrain and maneuvering over/around rocks, hills, trees or other obstacles in the field can be very challenging. A combination of the latest precision agriculture technologies and farmer’s knowledge of proper spraying procedures can improve the effectiveness of agrochemical applications (Doruchowski and Holownicki 2000). Smart sprayer machine vision technology and tractors equipped with auto-steer GPS guidance systems could be an ideal way for wild blueberry farmers to precisely travel the rough land without overlapping each spray boom pass. Early versions of auto-steer GPS systems have been difficult to use on wild blueberry ground, because travelling on a side slope, the GPS position was offset resulting in either over- or under-spraying. Many advanced GPS guided auto-steer systems now have a built in gyroscope to allow for proper GPS co-ordinate correction compensation based on the tilt angle of the tractor (Gomez-Gil et al. 2011). A gyroscope for tilt sensing may not be as important on flat agricultural ground but essential for the rough wild blueberry fields (Zaman et al. 2010). One of the most affordable types of GPS guidance is a light bar which involves a small display with arrows and embedded lights that visually guide the operator down each spray swath (Griffin et al. 2005). The light bar system is an affordable option, however, it does involve human error and requires more skill by the operator to drive effectively with accuracy as high as ±0.1 m (Griffin et al. 2005). Typically drivers are required to look behind, to ensure the boom is at proper height off the ground and to avoid collisions with obstacles, which can result in operator error. The human errors in a GPS auto-steering system can be reduced by automating the tractor steering system. Several tractor manufacturers now offer auto-steering as a factory option or farmers can choose from a variety of other companies that are now selling bolt-on kits. Auto-steering only offers command/control while the tractor is travelling along the sprayer swath paths, which is considered as one of the limitation to these systems. The operator is required to manually steer the tractor around corners and the auto-steer will start up and continue to steer along the next adjacent swath path. Research has been conducted to fully automate the tractor, however, further testing is required for spraying in rough wild blueberry fields (Pérez-Ruiz et al. 2015).

Advanced flow rate controllers are an important technology for precision spraying (Giles and Comino 1990). Original sprayers were designed to operate at a constant pressure given the number of nozzles and a set travel speed (Paice et al. 1995). Calibration consisted of determining the volume of spray being applied over a set period of time (Hoeg 2015). The operator is required to travel at a consistent speed when applying agrochemical to the field. Lower tractor speed due to maneuvering around corners and obstacles causes error which results in excess product application. Spray controllers have been designed to keep the application flow rate consistent with respect to ground speed (Giles and Comino 1992). The control systems are linked either to a wheel speed sensor or a GPS to compensate flow rate with the changing ground speed. Another advanced feature of newer flow rate controllers is the ability to automatically turn off boom sections to avoid overlap of application. Minimum overlap is achieved using an onboard GPS (Batte and Ehsani 2006). This technology has proved to be an effective option for famers to reduce production cost, minimize crop damage due to over-application, and mitigate environmental risks (Batte and Ehsani 2006; Burgess 2015).

In addition to advanced flow rate controllers and GPS guidance systems, some companies offer sensors that provide automatic green foliage detection and spray selectively (Michielsen et al. 2010). Gerhards and Oebel (2006) concluded that site-specific weed control offers farmers huge potential for herbicide reduction. Esau et al. (2014) tested a prototype smart sprayer for spot-application of agrochemical in wild blueberry fields. However, the economic analysis of this smart sprayer has never been performed. The objective of this study was to conduct an economic comparison of a basic sprayer versus a swath control and a smart sprayer.

Materials and methods

Two electronically advanced precision sprayers were compared to a standard non-precision (basic) sprayer in this study (Table 1). Each three point hitch mount sprayer featured a 13.7 m wide self leveling x-fold hydraulic boom, with a 1135 l capacity polyethylene tank and standard pump and hose configuration mounted to a 75 kW farm tractor with enclosed cab. The basic sprayer used a factory installed foam marker drop kit for guidance, while the swath control and smart precision sprayers were equipped with an RTK GPS-controlled autosteer guidance system. The foam marker guidance system were assumed to have an overlap error of 9 % of the boom width with each pass across the field (Ehsani et al. 2004). The operators using an RTK GPS-controlled autosteer system were able to concentrate their efforts in keeping the boom at a safe distance off the ground as the tractor accurately manoeuvers across the field with negligible overlap error. The RTK GPS-controlled autosteer system installed on the precision sprayers was also assumed to have a tilt sensor correction to compensate for travel over steep sloped areas in the field. The basic sprayer used in this study operated at a constant line pressure with no nozzle or separate boom section control. The calibration of the basic sprayer required the operator to measure the output flow volume over a set distance and travel speed. After calibrations, the tractor operator was required to travel at a constant ground speed in the field to maintain a consistent output flow rate. Maintaining a constant ground speed is a difficult task in traditional wild blueberry fields because of the rough and unpredictable nature of the field layout resulting in over or under-application of agrochemical. A Midtech Legacy 6000™ (Midtech Technologies, Springfield, IL, USA) flow rate controller was used on both the swath control and smart sprayers which allowed the user to calibrate the flow rate more easily and without the requirement to travel at a constant velocity across the field (Esau et al. 2014).

Table 1 Parameter comparison between a basic sprayer, swath control sprayer and a smart sprayer
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Each sprayer consisted of 27 nozzles on a sprayer boom, spaced at a standard 0.5 m apart. The non-precision basic sprayer consisted of a manually controlled on/off boom switch that controls the entire boom as a whole. The swath control sprayer uses features built into the flow rate controller that is able to shut off three sections of the boom individually with an inline solenoid valve. Each section of the boom (4.57 m) controlled nine spray nozzles. Using the GPS attached to the flow rate controller, the swath manager has the ability to shut off each boom section individually once the boom is positioned in an already sprayed area in the field. They turn back on automatically once a non-sprayed zone is entered. This feature allows the operator to drive around obstacles with lower stress and avoids overlap/overspray into other spray tracks. Similarly the smart sprayer using an accurate GPS guidance system has control of boom sections. The solenoid valve positioned directly on each sprayer nozzle body allows for individual nozzle control with negligible overlap and drip. Another advantage of the smart sprayer is the ability to detect target areas within the field for spot-applications of agrochemicals. Wild blueberry fields are highly variable consisting of randomly distributed weed, grass and bare soil patches. The built in real-time machine vision system was programmed to detect the target areas and rapidly send triggering information to the corresponding solenoid valve, opening the nozzle in the precise location where a target has been detected. If the sprayer passes over a non-target area in the field, the corresponding nozzles will automatically shut off and agrochemical will not be wasted. For this study, the sprayer operating speed was 6 km h−1 with an application flow rate setpoint of 187.1 l ha−1 (Table 1). For adequate wild blueberry field management, each sprayer type required nine separate agrochemical applications to each field. Three of the applications were herbicide (propyzamide, hexazinone/terbicil tank mix, mesotrione/fluazifop-P-butyl tank mix), five were fungicide (chlorothalonil, chlorothalonil/prothioconazole, propiconazole, propiconazole, boscalid/pyraclostrobin) and one was insecticide (acetamiprid).

The initial purchase price of the basic sprayer (not including farm tractor) used in this comparison was $15 000 CAD, which included the price of the foam marker kit. Adding the additional RTK GPS auto steer control and swath manager is estimated to double the initial cost to $30 000. The addition of the machine vision system on the smart sprayer increased the price an additional $10 000 for an initial purchase price of $40 000 (Table 2). The basic sprayer using foam marker for guidance required an addition of a 23.8 l poly tank for holding the foam marker concentrate. Thirty minutes were required for refilling and mixing pesticide in the sprayer tank. The additional cost for operating the basic sprayer is approximately $150 for foam marker concentrate depending on the amount of land covered and the travel speed. Extra expenses each year for operating the swath control and smart sprayer include an RTK GPS correction subscription, solenoid valve and various other electrical component servicing. For the swath controlled sprayer, the additional cost was estimated to be approximately $1500 per year, whereas the smart sprayer with the added machine vision costs approximately $2000 per year due to the added solenoid valves and electronic components. The additional costs, not included in Table 2, could comprise of licensing software fee that may be required for the image processing. The diesel cost of $1.35 l−1 was used with an average consumption rate of 15 l h−1 for this study. The operator of the sprayer was assumed to have an hourly wage of $16.

Table 2 Cost comparison of different components of basic, swath control and smart sprayer systems used in this study
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Two simulated fields each 20 ha in size were created in ArcGIS 10 (Esri, Redlands, CA, USA) for demonstrating the spraying pattern followed by the three different types of boom sprayers. Field A is a rectangle measuring 973.2 m long and 205.5 m wide (Fig. 1a). The operator would typically start spraying the field by completing the perimeter then proceeding to create vertical swath paths within the boundary of the outside initial pass. Field B is a triangle with a base dimension of 583.9 m and a height of 685.0 m (Fig. 1b). Similarly the operator would first spray the perimeter of the triangle then continue to spray straight line swathes in the vertical direction until finished. The variability in bare soil and weed patches is illustrated in Fig. 1. A typical sprayer applies a uniform blanket application across the entire field without taking into consideration this variability. The spot-applications using a smart sprayer based on accurate target detection can save significant amount of agrochemicals, which can lower the production cost and mitigate the environmental risks.

Fig. 1
figure 1

Field maps showing two different spray track configurations for a 20 ha area fields composed of 20 % bare soil and 30 % weed coverage

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Results and discussion

The 20 ha experimental fields used in this study were estimated to have 20 % bare soil and 30 % weed coverage (Table 3). It is estimated that over the 2 year production cycle of wild blueberries, it costs approximately $1735 ha−1 for agrochemicals. However, if the agrochemicals are applied spot-specifically on target areas, the cost would reduce significantly to $1093 ha−1. Additional costs associated with the agrochemical application includes labor, diesel and equipment.

Table 3 Agrochemical application costs over 2 year production cycle
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The cost comparison for the three types of sprayers used in this study is shown in Table 4. The swath overlap of the basic sprayer each time the operator made a pass across the field was estimated to be 1.23 m, resulting in two extra passes per application as compared to both the swath control and smart sprayer that featured the auto steer system. The operator travelled a total of 15 176 additional meters just in vertical spray distance (not including corners) when using the basic sprayer as compared to the swath or smart control sprayers. The increase in tractor fuel use and operator time were from the additional distance travelled (Table 4). Operation of the basic sprayer costs approximately $1994 ha−1 over the 2 year production cycle as compared to $1791 ha−1 with a swath control sprayer and $1137 ha−1 using the smart sprayer, when total agrochemical application cost (fuel, operator and agrochemical usage) was compared. The swath control sprayer had 10.2 % less input cost as compared to the basic sprayer. The smart sprayer had 43.0 % less input cost than the basic sprayer for agrochemical applications in field A.

Table 4 Agrochemical cost comparison between basic, swath control and smart sprayer for field A based on nine spray passes
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Additional agrochemical application cost savings can arise for farmers by replacing a basic foam marker single boom section sprayer with a smart sprayer, especially when their fields are irregular shaped such as the triangular field B (Fig. 1b). The operator first travels the perimeter of the field and over-spraying or wasting application on angles that are made when not spraying square edged parts of the field perfectly (Fig. 2a). Furthermore, because of the 1.23 m overlap for each pass using the basic sprayer, the operator is over-applying on one side of the boom width. The over-spraying implications were reduced significantly by adopting an RTK GPS-controlled auto-steer system. Results of map comparison showed negligible overlap between consecutive spray tracks (Fig. 2b). The reduction in agrochemical application was also witnessed with the automatic swath manager function, which helps to shut off boom sections once they reach already-sprayed areas of the field. A further reduction in agrochemical spraying can be achieved by increasing the number of boom sections of the automatic swath system.

Fig. 2
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Field B close-up map showing travel direction and spray overlap using basic sprayer (a) and swath control sprayer (b)

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The distance travelled while using the basic sprayer to spray field B increased by 8588 m as compared to field A because of the irregular shape of field B, although both fields had the same area (Table 5). The distance travelled while using the swath control sprayer and smart sprayer for field B increased by 11 435 m as compared to field A. Results reported that the basic sprayer was required to travel across field B 414 times, while the swath and smart sprayers only required 369 passes over the nine applications required. The operator travelled a total of 12 329 additional meters in vertical spray distance (not including corners) when using the basic sprayer rather than the swath or smart control sprayers on field B. Each application resulted in an overspray volume of 304.8 l ha−1 using the basic sprayer and 13.5 l ha−1 using the swath control sprayer as compared to 0 l ha−1 overspray using the smart sprayer. The total application cost was increased over field A by $116 ha−1 when using the basic sprayer and $16 ha−1 using the swath control sprayer and $2 ha−1 using the smart sprayer. The total application cost was $2110 ha−1 when using the basic sprayer, $1807 ha−1 when using the swath control sprayer, and $1139 ha−1 when using the smart sprayer. The swath control sprayer had 14.4 % less input cost as compared to the basic sprayer. The smart sprayer required 46.0 % less input cost than the basic sprayer for agrochemical applications applied to field B. The training required to operate the swath control and smart sprayer may require reading the operator’s instructions manual (approx. 1–2 h) to understand how to use the flow rate control and the machine vision software interface. An additional 1–2 h of preliminary operational practice with the added control systems (touch screen interface for camera control, flow rate controller) may also be required for beginners. Additionally, a flow meter correction may be required for the first time which can take up to 1 h depending on the volume of water used for calibration.

Table 5 Agrochemical cost comparison between basic, swath control, and smart sprayer for field B based on nine spray passes
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Benefits of precision agriculture technologies to the wild blueberry industry

Real-time spot-application is the most effective way of applying agrochemicals in wild blueberries. Incorporation of the precision agriculture technologies to existing sprayers can enable the sprayer to operate spot-specifically, thereby eliminating the need to purchase a new sprayer. The payback period for the swath control sprayer used in this study ranged from 2 to 1000 years over a farm size of 10–60 ha. For the smart sprayer, the payback ranged from 0.92 to 8.8 years based on field sizes from 10 to 60 ha. Results suggested that the larger growers can have their technology upgrades paid back more rapidly. One of the contributing factors in the payback period is the additional $1500 years−1 fee for using the swath control technology and approximately $2000 years−1 for the smart sprayer. Results suggested that wild blueberry producers with field sizes of 20 ha or larger can benefit greatly with the added technology. Farmers with field variability in weeds and bare soil coverage within the field can benefit from upgrading to a smart sprayer with a payback period of only 2 year with a 30 ha field as compared to almost 5 year with using only a swath control sprayer (Table 6).

Table 6 Payback period for application with an irregular shaped wild blueberry field
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Conclusions

The three boom sprayers used in this study had agrochemical application costs ranging from $1137 to $2110 ha−1 over a 2 year production cycle of a typical wild blueberry field. The average savings by using a swath control sprayer was 12.3 % in the 20 ha fields in this study. The smart sprayer was able to save 44.5 % input cost as compared to the basic sprayer used in this study. The extra cost required to purchase the equipment can be offset by applying less inputs with a payback period of 2 y for a swath control sprayer and less than 1 y with a smart sprayer with a field size of 60 ha or more. Farmers with field sizes of 20 ha or greater should consider a swath control type sprayer while those with significant weed and bare soil variability within fields should consider smart sprayer for spot-application of agrochemicals. Extra time may be initially required for training on use of the technology but results showed the potential of time saving while applying agrochemicals and the reduced trips to refill sprayer tank. Further research could involve testing a sprayer with a lower cost GPS and light bar guidance to determine if there are benefits for growers that have smaller-sized fields. Spot-applications have great potential to reduce production cost with the aided advantage of reducing environmental risks.