Herbicide application

Page last updated: Thursday, 10 June 2021 - 8:05am

Please note: This content may be out of date and is currently under review.

Nozzles for spraying herbicides

Changing the nozzle type, size, height and pressure will change the amount and effectiveness of herbicide reaching the target area.

The main types of applicators available are:

  • hydraulic nozzles
  • twin fluid nozzles
  • controlled droplet applicators
  • air induction nozzles
  • air assisted nozzles.

The information below pertains to hydraulic nozzles.

Hydraulic nozzles

Hydraulic nozzles are the main type used on boom sprayers designed to treat large areas.

Hydraulic nozzles form droplets when the spray liquid is forced, under pressure, through a small opening. They are classified according to the spray pattern produced. The four main patterns are flat or tapered fan, even fan, hollow cone and solid cone.

Hydraulic nozzles produce a large range of droplet sizes, from very small to large. Droplet size is quoted as the categories 'very fine' to 'extremely course' from the ASAE Standard S-572 and takes account of the droplet size distribution. Measurements are taken at 10, 50 and 90% of the spray volume to give a more accurate classification.

Nozzle size

Before the right nozzle size can be selected, you need to know:

  • spray application rate, in L/ha
  • travel speed when spraying, in km/h
  • nozzle spacing in metres

Calculate the required output in litres per minute per nozzle, using the formula below.

Nozzle output (L/min/nozzle) = Spray application rate (L/ha) x travel speed (km/h) x nozzle spacing (m) ÷ 600


Calculating the required output in litres per minute per nozzle

If you want to apply 40L/ha at a travel speed of 20km/h and the nozzle spacing on the boom is 0.5m, then nozzle output = (40 x 20 x 0.5 ÷ 600) = 0.67L/min/nozzle

A nozzle can then be chosen from the range available that will give this flow rate at a pressure within the recommended operating range.

Important: nozzle filters are essential to prevent blockages, especially with low volume nozzles. In general, 100 mesh screens are used for nozzles with a flow rate below 0.7L/min and 50 mesh screen for those with flow rates between 0.7 and 4L/min.

Non-drip check valves stop the herbicide in the boom from continuing to drip from the nozzles once the spraying has stopped. Retaining the liquid in the spray line also means that spraying restarts almost immediately once operations resume. Several brands of diaphragm and ball-spring check valves are available. All washers and diaphragms for non-drip check valves must be made from herbicide resistant materials.

Nozzle bodies come in two main types, suitable for either dry or wet booms. Dry booms have the nozzles fixed to the boom and connected by a small length of flexible hose. Wet booms have the nozzle bodies clamped around tubing, typically PVC, polythene or stainless steel.

Nozzle bodies may also contain more than one nozzle. If nozzles are to be changed frequently, the body can be rotated to the selected nozzle.

Using a boom sprayer

After selecting the nozzle type, correct operation is necessary to get the best result from the spraying. Operating factors that can influence the effectiveness of spraying are the height, orientation and pressure of the nozzles, as well as travel speed and environmental conditions. Flat-fan hydraulic nozzles are the main type used on broadscale sprayers; this section covers their use.

Nozzle height

Changing nozzle height will affect:

  • the uniformity of deposition across the boom
  • the amount of spray deposited on the target
  • the amount of drift from the sprayer.

The height at which the boom should be operated decreases as the fan angle increases. Most agricultural nozzles have spray fans in the 80-110° range. Manufacturers normally specify a suitable operating height that is determined by the angle of the spray fan produced. At the best height, the spray fans of each nozzle overlap correctly to produce an even coverage of droplets.

Nozzle pressure

Changing nozzle pressure affects both the flow rate from the nozzle and the droplet size. As the pressure increases, the flow rate increases and the droplet size decreases. At very low pressures the nozzles usually fail to atomise correctly and the fan angle decreases. The height of the boom also needs to be corrected.

Nozzles should never be used at a pressure outside the range recommended by the manufacturer.

Read the label

Prior to doing any spray operation, and particularly when using a herbicide for the first time it is advisable to reasd the label.

Spray records are required by law on the label of some products.

Some herbicide labels now specify the spray quality to be used.

Useful resources on nozzles and spraying