Different techniques are available for the application of crop protection products. A crop can be treated by spraying or by treating the air by means of fogging or low volume mist (LVM). What are the differences between these techniques? And what are the advantages and disadvantages?
Spraying is a widely used technique for the application of plant protection products. This requires a high pressure pump and tank for the liquid, which can be stationary or mobile such as a spray truck. With such a technique it is possible to reach the entire crop. The disadvantage is that compared with an airborne treatment it is accompanied by a large amount of water so the crop remains wet for longer.
A grower can also use a spray gun. This is often used, for example, for applying a product to pot plants. Growers sometimes use backpack sprayer with a spray arm for treating a few plants in a high upright growing vegetable crop. The advantage of this manual approach is the possibility to localise the treatment. The disadvantage is uneven spraying. It is also very labour intensive.
Automatic
Automatic application using a robot makes the distribution of the crop protection substance somewhat easier and is less labour intensive. The results of these treatments are also better than when using a spray gun. The major advantages of a robot are the regulated speed of administration and the direction of the spray.
The automatic approach can be implemented horizontally with a spray boom or vertically with a spray mast. For optimum coverage of the plant protection product it is important that for low and densely growing crops such as roses and gerberas, sufficient spray nozzles are present at the bottom of the spray mast. For high upright growing vegetable crops long masts with nozzles are needed in order to cover everything. A pressure of approximately 9 bar gives the best distribution.
Increased use of dosing pump
Dosing pumps are being increasing attached to spray robots, a development that is expected to rise. This pump works without power and uses water pressure as the driving force. The device sucks the concentrated substance from a tank and doses the required percentage, after which it is mixed with water in the mixing chamber. The water pressure ensures that the solution flows through. After an initial setup the dosing pump requires no further control or management.
The dosing of the concentration remains constant, even in the event of any pressure differences, and proportional to the amount of water flow. This extreme dosing precision completely prevents any overdosing. The advantage of adding the plant protection product at the end is that no substance goes through the pipes and the problem of residual fluids is reduced to almost zero.
With and without air support
Spraying without any air assistance is the traditional way. However, application with air assistance is becoming the standard procedure these days. In the traditional way a droplet floats through the air and collides into a leaf. A droplet is not able to penetrate a very densely growing crop. With air assistance the droplet is given extra power in order to go deeper into the crop. In this way more of the leaf canopy is covered, up to 95%, while with traditional spraying this is just 60-70%.
The need for air assistance depends on the density of the crop, but also the disease or pest that needs to be controlled. For light fungal infections and diseases such as aphids and trips in flower buds – which sit on the surface – additional air assistance is not needed. For persistent fungi and, for example, spider mites, which are present under the leaf, it is necessary.
Because the number of available substances is constantly decreasing and new substances work less effectively, it is important to use air assistance so that the crop protection product is distributed everywhere in the crop.
High-tech with air
Sprayers that have air assistance are more expensive to purchase. However, this can be achieved simply with a fan that further blows the liquid that is atomized by a nozzle. But there are more advanced techniques. Micothon has patented air assistance technology on all its spraying robots for optimal leaf coverage and less use of substances. The result is better due to a combination of fan position, volume of air, the pressure by which the air leaves the unit and size of the droplet.
Holland Green Machine works with air mixing instead of air assistance. The S55 Airmixer is a robot with a combined air-liquid spray arm. The air and the fluid are mixed under pressure in special mixing chambers on the spray arm. This creates a very powerful mist that penetrates deep into the crop. The Pieton spraying system works in the same way.
Horticultural supplier Horticoop sees a development in the application of air-injection nozzles on the spray mast. This nozzle operates without compressed air and instead sucks in air itself (venturi action). The larger droplet with air in it has more mass and provides a better penetration.
Setting the nozzles
The nozzles on the Empass equipment are set at an angle of 15 degrees. In this way it spays upwards onto the leaf for better penetration. By changing the position of the nozzles a grower can work with five different angles. As a result targeted action is possible to optimally hit the pest from above and below.
For a good result it is very important to set the nozzles correctly. This is simple to check by adding a fluorescent powder to the fluid and spraying it. By using a UV-lamp it’s possible to see where the fluid landed on the crop and if any changes are required in order to achieve a better result.
Mandatory inspection
In addition to the correct adjustment of the spray equipment, annual maintenance and the timely replacement of nozzles (at least once per year) are the basis for a good result. Also, since 1 January 2016 periodic inspection, every three years, of all types of equipment used to for apply crop protection products is mandatory in the Netherlands under the Crop Protection and Biocide law.
The tests are carried out by approved inspection companies. These companies have the correct testing equipment as well as trained personnel who perform the inspections correctly. An independent body periodically monitors these companies.
Air treatment
Treating the air with a plant protection product can be done with fogging or Low Volume Misting (LVM) equipment. The difference between these two applications is the technique and way in which the product is spread but the result is the same: mist. Fogging is thermal spraying using jet motors that run on gasoline. The droplet size is about 30 micron. LVM is cold spraying of tiny droplets (5 to 10 microns) by means of compressed air or vacuum or electrically powered air pumps. The versions with a pump are LVM-units without a compressor.
The big advantage of a fogging unit is the spray capacity and mobility. The largest unit can spray approximately 60 litres per hour, which treats approximately two to three hectares per hour. Also, a fogging unit is not bound by an electricity cable or a compressed air line and therefore the user can control most of the distribution with his own hands. By moving along, trellis by trellis, the entire greenhouse area is filled with mist to give a good distribution of the crop protection product.
Different capacities
In general, a LVM-unit is fixed in one place. The fans ensure reasonable distribution of the mist but for large areas additional fans are required to send the droplets further into the greenhouse. The dispersal distance of a singly operating unit is about 50 m and for a double unit 50 m to the other side too. If the central path is longer, extra air assistance is required.
The capacity of LVM’s vary widely. It is very important that the crop and greenhouse are taken into account when choosing a misting unit. The conventional air pressure models with a compressor have a relatively low capacity of 4 to 8 litre/hour. However if a grower has a lot of compressed air capacity available, units are available with a large misting capacity of 40 to 80 litres/hour.
LVM with an air pump
A characteristic of the compressed air-LVM is that they produce the smallest droplet size. Fine mist is ideal for penetrating dense crops. For larger surface areas multiple units are required.
LVM’s without a compressor, but with a low pressure air pump, often spray more litres per hour (20 to 30 litres/hour) of larger droplets. Then the droplets fall faster so the distribution over the entire crop is less good.
The Fontan, which is supplied for example by, Horticoop and Vos Capelle in the Netherlands, and Powerfogger (agricultural hygiene company, Frans Veugen’s own brand), work with an air pump. The air under pressure creates a venturi action, which sucks the liquid from the nozzle with it. As a result the substance doesn’t cake on to the nozzle.
Compared with a compressor an air pump makes hardly any sound, gives a more constant pressure and uses no oil that could possibly leak. An LVM with an air pump is more expensive than one with a compressor.
Sufficient misting fluid
Regardless of the technique used, it is crucial to use enough misting fluid when carrying out a treatment. Of course, the dosage is not dependent on the technique, but on the surface area. The rule of thumb is to use 2 litres of water per 1000 m2 and then add the amount of plant protection product necessary for 1000 m2.
In practice, it appears that adding even more litres of water for better distribution – in which the amount of product per 1000 m2 per remains the same – has a positive effect on the result of the treatment. It is necessary to take into account the fogging unit being used because if the treatment lasts longer than 2 hours, the uniformity of the distribution of the mist decreases. To achieve optimum results when applying air-borne crop protection products the following parameters need to be taken into account: Greenhouse surface area; path length; duration of distribution (maximum two hours) and the amount of fluid used. When choosing a particular technique, fogging or LVM, growers need to be well informed by their suppliers of crop protection products and/or misting equipment.
Closed windows
Treatment by means of fogging or LVM is easy, saves time and compared with spraying uses very little water so the crop dries quickly. The disadvantage is that not all products can be used in this way so not all pests and diseases can be controlled with such an air treatment.
Treatment via air is only possible with all the windows closed and no wind. In practice a wind speed of 3 m/s is the ultimate limit. The greenhouse must stay closed for several hours during and after treatment. This is a particular disadvantage during the summer. And the temperature in the greenhouse should not rise any further, because damage can occur due to condensation.
Hearing protection required
During fogging no ventilation is required. However, the user needs to remain by the equipment because it needs to be moved. With an LVM the grower doesn’t need to be present and the equipment can be set beforehand. Fogging equipment has a nozzle, is made completely of stainless steel and has no moving parts but it is very noisy so hearing protection is required. The disadvantages of fogging are the combustion gases from the gasoline, the fire hazard, possible oil leakage, warm air and the warming or burning of the crop protection product.
In practice the preference is for low-noise, user-friendly LVM-equipment. In principle this equipment can be operated with the push of a button from the office. After filling the substance tank it then runs automatically.
GPS-controlled spray devices
Some new developments are expected in the future. Frans Veugen Bedrijfshygiëne, Dutch importer of Pulsfog equipment, is currently developing an electrothermal spray device that has the benefits of a fogging unit, but the convenience of the LVM. The heat ensures a reasonably small droplet size, which significantly improves the spread and spray penetration.
The huge greenhouse areas also remain a challenge for spreading the mist but the company expects that in the future localised area could be sprayed. Only selected sectors in the greenhouse will then be treated with smaller GPS-controlled spray device. This can happen automatically after working hours, so minimum disruption to the day’s work occurs.
Summary
Spraying is a widely used technique for the application of plant protection products. This can be carried out manually with a spray gun or automatically using a robot. Treatment without air support is the traditional way, but application with air assistance is now becoming the standard technique. Air treatment with a plant protection product can be via fogging or Low Volume Misting. When using LVM for large surface areas, fans are necessary to provide extra assistance. When fogging no extra ventilation is needed but someone is required to move the equipment.
Text: Harry Stijger
