The sprayer, an essential piece of equipment

The sprayer is necessary in all types of agriculture (including organic), supporting them accurately and adequately

By associating sprayers with only “chemical” treatments, there is the the risk of endorsing the perception that represents them as icons of the alleged “harmfulness” of conventional agriculture. Actually, the sprayer is a fundamental piece of equipment for the agricultural production process, necessary in all types of agriculture, including organic, which it manages to support accurately and adequately.

The sprayer, as the name suggests, is used to distribute liquid substances through nozzles that allow spraying, with droplets produced by the impact of a pressurized liquid inside a nozzle, a surface or a crop. The liquid is generally water in which are dissolved, emulsified or suspended the products that are intended to be distributed, attributable to the categories of pesticides, fertilizers or growth promoters.

Pesticides are substances with antifungal, antibacterial, antiparasitic, phytohormonal and herbicidal actions and in order to be used they must be registered in the EU pesticides database (Reg. (EC) No 1107/2009 and subsequent regulations) regardless of whether they have a synthetic, mineral or biological origin or whether they are permitted by organic specifications. In fact, only some of the 316 active ingredients permitted in agriculture are the product of chemical reactions carried out in laboratories; a large proportion are counted among the biopesticides, a term by which we tend to identify organisms used in biological control, such as Bacillus subtilis or Bacillus thuringiensis (in this group not all are required to be registered, essentially only those that can be used with a sprayer), others such as botanicals, which include plant extracts and phytocomplexes of plant origin. Each of these substances has its own approved report that regulates, among other things, against which pests and on which crops they can be used.

There is also great interest in the distribution of liquid fertilizers, especially those capable of stimulating certain aspects of plant physiology and which must be absorbed by the foliar apparatus. However, even conventional fertilization in liquid form on open field crops allows an improvement in the efficacy of the fertilizing principle because it is possible (at least in part) to exploit the foliar absorption capacity. Particularly with nitrogen and microelements, significant absorption efficiencies can be achieved. There is obviously a need to examine the economic convenience of liquid fertilization and to evaluate changes in operational sequences and work organization, but this technique has the undeniable advantage of using the sprayer for distribution in the field. The mechanization analysis that follows will therefore also take into account these possible uses .


Working capacity

The sprayer is an efficient and technologically very advanced piece of equipment. The high working width and the stability provided by the boom, which allows discrete working speeds to be maintained in the field, make it possible to achieve excellent effective working capacities.

The possibility provided by the actual distribution system to apply low or ultra-low volumes per hectare, together with the excellent tank capacities that distinguish towed (and self-propelled) equipment, guarantee excellent operational autonomy.

When the machine is equipped with good pumps and filtering systems, water withdrawal during refilling can also be carried out in the field, by exploiting the water resources of the area. This reduces tank refilling time and thus the sprayer’s utilization capacity.

With 24 m booms and 3,000 liter tanks, the low volume distribution allows real working capacities of over 10 hectares per hour.

From a technological point of view, a treatment carried out with volumes of 150-200 litres per hectare, in the case of weed control or treatments on crops with a reduced leaf volume, is technically suitable to achieve the result. However, there tends to be, especially among the customers of contractors, a “need” to see the spray coming out of the nozzles, and so contractors operate with spraying volumes of at least double, more often triple. The result is that the number of stops to fill the tank and prepare the new mixture increases and the real working capacity is reduced.

Dwell times for this operation can be optimized when the time for filling the tank is similar to the time the operator spends mixing the product. In towed sprayers, this is generally achieved with flow rates of 200 liters per minute or more. After this, it must be said that the work area assigned to this task must be ergonomically organized and allow operation with speed, precision and compliance with the rules for protecting the health of the operator and the environment.


Quality of work

Good distribution requires that every area of the field receives the prescribed dose. However, during distribution, conditions occur that lead to an alteration of the distribution uniformity. These can result from movements of the boom with respect to the machine, such as oscillations and yaws, which locally alter the distribution uniformity and, at least in the first case, can even leave small areas untreated.

An answer to the first group of problems is provided by the devices connecting the boom to the machine, which concern the ability to contain the oscillatory movements of the boom itself through proportionate suspension systems and very efficient cushioning systems. In this context, of course, the skill of the driver is also fundamental, who must have the sensitivity to evaluate the forward speed suited to the operating conditions and manoeuvre well, opting for low forward speeds and, when possible, wide radius of curvature.

Also very useful are the automatic horizontal adjustment systems, which generally use an ultrasonic sensor that reads the distance from the ground of the nozzle placed at the ends of the boom. A plc checks for any difference between the measured height and the set value and acts accordingly on the hydraulic jacks acting on the boom (or rather separately on the two arms).

A second problem arises between two contiguous passes: overlapping passes leads to the distribution of an overdose which can be, in addition to being wasteful, counterproductive for the outcome of the intervention or, vice versa, the lack of overlap can leave untreated strips. In fact, if it appears intuitive that a smaller distributed quantity can nullify the effect of the active ingredient, it is not so well known that this can also happen by distributing a higher dose. In fact, an excess of active ingredient can alter the behavior of the plant towards the product (what occurs with some herbicides such as glyphosate and in general with active ingredients characterized by phytotoxicity because they locally alter the physiology and morphology of the plant by interfering with their action).

The only effective solution is the adoption of automatic guidance systems. This device, very useful in all contexts, is absolutely advantageous in operations with sprayers and fertilizer spreaders. In fact, with operations that do not leave evident marks on the ground and are characterized by large working widths, the operator is unable to identify the correct point for entry into the field, unless tramlines (autumn-winter cereals) are used or entry points are marked on the field.


Working precision

The automatic shut-off of sections, combined with satellite guidance, is a typical tool of precision farming. In its most common sense, this system intervenes by blocking the spraying of a section of the boom (some equipment allows action on individual nozzles), when the navigation system recognizes that spraying at that point has already been carried out or if it is outside the field perimeter. This avoids double distributions (overspraying), treatments on sensitive areas such as ditches and irrigation canals. In addition to shutting off spraying, the systems are equipped with devices that close the boom in front of obstacles such as pylons or trees as long as they are marked on the map.

A further development in Precision is the possibility of changing the dose distributed during spraying. In conventional machines, if you want to change the dose above or below certain values, without changing the feed rate, you need to stop the operation and change the nozzle. This usage limit makes it difficult to adopt the variable rate, i.e. the modification of the dose according to information collected in real time or constituted in production maps. In fact, the dose to be distributed and the rate of advance determine the pressure and flow rate of the liquid to be delivered by the nozzle and, as is well known, for each pressure and flow rate range there is a nozzle whose colour determines the group to which it belongs. To overcome this limit, some manufacturers mount on their booms a device capable of activating between two available nozzles the one most suitable for the required delivery conditions. Another interesting technological proposal that aims to overcome the physical limits of the nozzle concerns the possibility of performing a pulsed distribution of the liquid: the distribution from continuous is fractioned into micro-deliveries, thus extending the range of use. Therefore, this device allows a single nozzle to work in a much wider range of forward speeds and doses, enabling the sprayer to perform variable rate effectively.

Finally, some manufacturers are beginning to propose systems capable of detecting the presence of a plant or even distinguishing weeds from the crop. In the first case, the system can be used on a bare field to condition the dispensing to the presence of the weed or on a cultivated field to limit the dispensing of the treatment where the plants are; in the second case, the herbicide is only dispensed when the weed is detected.

These systems use hyperspectral optical sensors placed on the boom supported by advanced software and hardware capable of providing a response in time compatible with the speed rate.

Lorenzo Benvenuti

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