Dr Andrew Landers

Quarbarton, Sopworth, Nr Chippenham, Wilts. SN14 6PT

Summary

This paper describes the theory and components used in the design and

development of direct injection crop sprayers. The basic system requirements

are outlined along with suggestions for developing a successful system.

Introduction

Farmers and contractors are currently facing interesting challenges. The price of most agricultural commodities has plummeted, farmers are required to be more accountable to the buying public and to Government. In recent years we have seen the introduction of quality trails on a growing number of crops, including the disclosure of the number and frequency of pesticide application. The AEA have recently introduced, albeit voluntarily, an annual testing scheme for crop sprayers. The Government, quite correctly, continue their concern regarding environmental issues. Faced with these issues a number of farmers are seriously considering the benefits of owning a crop sprayer to apply pesticides. Precision farming has become topical in recent years, particularly the joint role of reducing inputs on the arable farm and, perhaps, reducing the chemical load in the environment. Direct injection crop sprayers have been developed to aid farmers reduce their pesticide costs, be more environmentally friendly and aid operator safety.

The basic principle

The principle of injecting a liquid is not new in agriculture, the diesel engine has been fitted to farm tractors for many years. The injection of a chemical into a liquid may be found in many water treatment plants and a number of road sweeping lorries use injection techniques to control kerbside weeds. Amsden (1970), first described various methods of pesticide injection, including the infamous spray train.

The basic principle of direct injection on a crop sprayer is that pesticide and water is kept in separate containers. When the sprayer is activated, a metered flow of pesticide is injected into the water stream, sometimes via a mixing chamber, at a point situated between the main water tank and the nozzles. Often a number of pesticide containers and pumps are fitted to allow the farmer to apply more than one product. A rinsing system may also be incorporated.

Landers (1988) stated the general requirements of an injection sprayer should be:

a) to help the operator carry out the spraying operation safely and efficiently

b) to allow thorough emptying and cleaning without contaminating the environment

c) to be robust, constructed of durable materials

d) to inject a wide range of pesticides with varying viscosities

e) to apply several pesticides/additives at the same time without any premixing

f) to mix the pesticide and water thoroughly

g) to function accurately at the range of dose rates found in practice

h) to change dose rate quickly and accurately due to changes in operating parameters

i) to be easy to use and understand

j) to be capable of being fitted to most existing sprayers

k) to be commercially viable

Subsequent recommendations to the above list are:

l) not to reduce the output of a conventional sprayer

m) to conform to current legislation e.g. CE approval

n) to link with commercial software for precision agriculture

o) to have, as an option, the ability to record pesticide/water/area for audit trails

The components

1. Containers

The traditional packaging of pesticides has been one of the barriers to success of direct injection sprayers. The use of small 5 to 10 litre containers require decanting into larger vessels and the parallel development of closed transfer systems has helped but is not the answer due to the sheer numbers involved, the speed of operation, rinsing out and the injection of rinsate and the disposal of containers. The use of refillable and returnable containers is one of the ways forward along with water dispersible granules and other novel techniques of packaging. Container weight and handling must be considered.

2. Injection techniques

The majority of developments have used pumps, either peristaltic tube pumps or piston pumps. Landers (1992) observed that a major concern is the ability to inject all types of liquids at varying viscosities, temperatures and containing all sorts of solvents. The pump chosen must behave accurately over a period of time and give repeatability.

Pumps, pipes, seals and connectors should be constructed from durable materials, particularly as they will work in a hostile environment pumping liquids which are very injurious to many materials.

The injection pump drive system can vary from mechanical, electrical or hydraulic. A mechanical drive (via the tractor p.t.o system) usually results in the injection pump(s) being situated at the front of the sprayer with long pipe runs and perhaps long injection delay periods. The use of electric or hydraulic drive does allow system designers to place the injection pumps wherever they may choose.

Landers (1997) describes the development of a pneumatic injection system which only requires an airline on the sprayer, the air supply being derived from a compressor mounted on the tractor engine.

Not all pesticides are formulated as liquids so an injection system which has the ability to handle powders/water soluble granules may be desirable. The system needs to be able to inject accurately over a period of time, with repeatability and to ensure adequate dispersal within the water during the short time it takes from the point of injection to the point of discharge at the nozzle.

3. The point of injection

The majority of developments have injected pesticide into the water flow somewhere between the sprayer water pump and the nozzle. The pump must therefore be able to inject pesticide at a greater pressure than the sprayer operating pressure, resulting in the use of medium to high pressure pumps, usually piston pumps, to give a constant displacement at higher pressures. Other designers use the point of injection in the low pressure side of the sprayer, between the water tank and the water pump, this allows the injection system to operate at a low pressure with the major advantage of reduced pressure to pump concentrated pesticide.

The point of injection is one of the determinants of the delay time, the nearer the nozzle the less will be the delay. Whilst commercial reasons dictate that the market for injection systems is in the area of retrofitting to existing sprayers, the ultimate design would be a completely new sprayer layout with, perhaps, the point of injection at the boom sections or the nozzles.

4. Mixing

The degree of mixing of pesticide and water will depend upon the time available, the degree of turbulence and the design of the mixing chamber (if fitted). Designers have the choice of the use of a mixing chamber, relying on turbulence within the pipes, or momentary mixing at the nozzle. All the systems have their relative merits and some have major disadvantages. The other important role of an effective mixing system is to even out the pulses which may be caused by a mechanical pump.

Ease of use

The present injection systems in the market are regarded by some as expensive and many farmers cannot justify the added expense of up to £10,000 to the capital cost of a sprayer. Landers (1992b) indicated that some of the systems available may have short comings such as accuracy of application or time delay from the point of injection to the pesticide appearing at the nozzle. It is imperative that all injection systems are thoroughly tested in laboratory and field conditions to ensure accuracy and longevity. There must be a happy medium between the commercial pressures exerted by company accountants and the researchers quest for perfection.

The debate continues regarding the development of injection sprayers, should we develop a very simple system or a ‘sophisticated’ system? Detractors should be aware that modern crop sprayers bristle with electronics and as long as the control panel is easy to use and is reliable, then ‘sophistication’ may be quite acceptable. Often simple systems are unable to respond to changes in dose level such as when boom sections are switched on/off. The application of electronics to direct injection sprayers gives the operator the ability to change the pesticide dose rate while travelling across the field, so that varying soil characteristics or weed concentrations within the field can be allowed for. It is also possible to spot treat weeds within a field by switching the pesticide on and off, resulting in less pesticide being used (Marshall & Landers 1990). The use of precision farming techniques has seen an increase in interest in the use of injection systems, (Landers and Steel, 1994).

The farmer/agricultural contractor will readily identify with the major advantage of direct injection sprayers in reducing cleaning time and rinsate disposal, both operations often carried out at overtime rates of pay.

Conclusions

1. The advantages of direct injection systems in reducing environmental pollution and operator contamination plus simpler application are likely to outweigh the disadvantage of the extra capital cost.

2. The parallel development of returnable and refillable containers will overcome one of the major obstacles in the development of injection sprayers.

3. The concept of using compressed air in conjunction with returnable, refillable containers has definite advantages for the injection of pesticides.

4. The use of direct injection crop sprayers as part of precision farming will result in more appropriate use of pesticides with an overall reduction in application rates, thereby satisfying environmentalists, legislators and farmers.

Amsden, R.C. (1970). The metering and dispensing of granules and liquid concentrates.

pp. 124 - 129. British Crop Protection Council. Monograph No.2

Landers, A.J. (1988). Closed system spraying - the Dose 2000. In: Aspects of Applied Biology, 18, Weed control in cereals and the impact of legislation on pesticide application. pp361-369

Landers, A.J. (1992). The characteristics and performance of a direct injection crop sprayer. Ph.D thesis. University of Bath

Landers, A.J. (1992b). An evaluation of the Dose 2000 direct injection crop sprayer. Proc. of Ag Eng '92 International Conference on Agricultural Engineering, Uppsala. Swedish Institute of Agricultural Engineering, Uppsala, Sweden.

Landers, A.J. (1997). A compressed air direct injection crop sprayer. In:Aspects of Applied Biology,48, Optimising pesticide applications, pp 25 - 32

Landers, A.J. and Steel, D. (1994). Precision agriculture - a viable option for future arable farming in Europe. paper presented to the winter meeting of the American Society of Agricultural Engineers, paper no. 94/1604, St. Joseph, MI: ASAE

Marshall, I. and Landers A.J. (1990). When its better to close for business. Power Farming 79 (9) pp 24-27