Herbicide application methods
A boom sprayer is the most common type of apparatus for applying herbicides in broadscale farming. A sprayer has many components, the most important being the nozzles, which split the herbicide into many small droplets that are projected through the air to the target. The nozzle is the only component of the sprayer that directly determines the effectiveness of spraying. All other components are necessary to position the nozzles and provide them with a continuous supply of herbicide at the correct pressure. Correct nozzle selection and operation are critical for successful spraying.
Misters are a useful but imprecise way of applying herbicides to large areas quickly. They rely on wind to drift the herbicide. If the wind is too light or the spraying speed too high, the swath width will decrease, possibly causing overdosing and wasted chemical. If the wind is too strong or gusty, it increases the swath width, which will reduce the chemical application rate and increase the risk of damage from spray drift.
Blanket wipers are made of a vertical strip of material attached to a horizontal frame. The vertical strip, or blanket, acts as the wiping surface making direct contact with the target weed. This equipment has been developed as an alternative to rope wick applicators. A non-selective herbicide is generally used with successful weed control dependant on the height differential between crop and weed. Wipers are used in broadacre application to control radish or mustard in lupins or chickpeas or to 'top' grasses in pasture. Units have been designed to fit all terrain bikes and hand held equipment has been developed for back yard and environmental use to treat weeds such as cape tulip, Paterson’s curse, Guildford grass, arum lily, fressia and bracken fern. Herbicide can be selectively applied to these plants without damaging pasture legumes or native seedlings in revegetation areas. The best time to wipe weeds in crops is September to early October when the weeds are flowering and are 20–30cm taller than crop or pasture plants.
Rope wick applicators
Rope wick applicators consist of a series of ropes impregnated with a non-selective herbicide, usually glyphosate. They are not widely used, but they can be useful for the control of tall weeds in a crop or pasture. Normal spraying with a non-selective herbicide would not be possible in this situation, however a rope wick applicator can be moved above the crop or pasture and wipe the herbicide only onto the taller weeds, hence selective control is obtained. This technique has been partially successful for controlling cape tulip, docks, rushes, thistles and bracken in pasture. Because they can only operate at slow speeds and the ropes are very expensive, rope wick applicators have not gained wide acceptance.
Detection technology (for example, Weedseeker and Weedit) uses infrared and near infrared light to detect green weeds and sprays only green plants in paddocks. In action, light-emitting diodes (LEDs) point two different light sources, infrared and near infrared, towards the ground. Green weeds have a different reflective signature to stubble or soil. The system can operate at speeds up to 20 kilometres per hour (km/h), requiring a stable boom to aid operational efficiency.
Spot spraying, chipping, hand roguing and wiper technologies
Where new weed infestations occur in low numbers, eradication may be possible. In such situations more intensive tactics to remove weeds can be used in addition to 'ongoing' management tactics which aim to minimise weed impact.
Vigilance and attention to detail can be the difference between eradication and a prolonged and costly problem. Make sure you correctly identify the weed, understand the biology (when does it grow, when does it reproduce etcetera) and identify what control tactics are best suited to the weed at each growth stage.
To ensure the eradication program is successful:
- instigate accurate future monitoring by marking isolated infestations
- isolate the area of infestation to reduce the risk of further spread.
Techniques for localised eradication
Roguing refers to hand pulling or chipping of weeds prior to flowering or seed-set. It is also used in seed crops to reduce the chance of spreading weeds in the seed and when other options of controlling the weed are limited. If roguing is carried out after seed is physiologically mature, both the plants and their seeds should be contained and carefully disposed of. Roguing is an effective method of eradicating a new infestation in annual crops, despite being labour intensive and expensive.
This method is a quicker alternative to hand roguing and can be used to sterilise weed seed. Spot spraying usually involves the application of a non-selective herbicide to individual weeds using a sprayer in a backpack or mounted on an all terrain vehicle (ATV). The sprayer should have a single nozzle on a wand attached to a flexible hose. A boom sprayer fitted with weed detector units may also be used for applying non-selective herbicides to low-density infestations in fallows.
Wick wiping performed with a hand-held rope-wick wiper is an alternative to spot spraying when there is the possibility of herbicide drift onto sensitive adjacent plants. It is particularly useful if the weed is taller than the crop canopy. There are multiple 'wiper technologies' available, including wick wipers, rope wipers, carpet wipers and weed wipers.
Calibration of spray equipment
The importance of having your spray equipment accurately set up and calibrated cannot be overstressed. Huge losses can be incurred through incorrect herbicide application. These can range from total failure of the herbicide to kill the weeds, to the extreme where an overdose of herbicides kills both weeds and crop, and may even leave residues in the soil. Even if overdosing does not cause crop damage, the extra cost of the herbicide applied unnecessarily can be significant.
There are many methods for calibrating, usually involving calculations. A simplified method designed for boom sprayers with nozzles at 50cm spacings is summarised below.
- Measure the output of each nozzle for one minute. This should already have been done when choosing an even set of nozzles.
- Measure the combined output of all nozzles and divide by the number of nozzles. This gives average output per nozzle in millilitres per minute (mL/min).
- Decide on a speed of travel for spraying.
- Measure out a distance of 100m and record the time taken to cover the distance with the spray unit. It is important to calculate the speed on a surface similar to that being sprayed. You can calculate it by: speed (km/h) = 360 ÷ time (seconds) taken to travel 100m.
- Calculate the output using the following formula:
Output in L/ha =
Average output of a nozzle (mL/min) x 60
Nozzle spacing (cm) x speed of spraying (km/h)
To calculate the amount of herbicide needed for each tank of spray, the tank size (volume) must be known. Dividing the tank size by the output (L/ha) gives the number of hectares that can be sprayed with each tank. Multiply the rate of herbicide required per hectare by the number of hectares that can be sprayed per tank to get the amount of herbicide added to each tank.
Sprayer with a 1000L tank; output 50L/ha
Number of hectares sprayed/tank: 1000/50 = 20ha
If the rate of herbicide application is 2L/ha, the amount of herbicide added to each tank is 2 x 20 = 40L
To better judge the appropriate rate of herbicide and water requires continuous monitoring of conditions and results to establish the causes of failure or success. To achieve this you must keep records of every day's spray operations, and assessment of herbicide performance.
Herbicide performance is influenced by weather, soil moisture, growth stage and density of the weeds, herbicide rate, water rate, droplet size, and growth stage of the crop.
Record all of these for each paddock each time you use the sprayer. Weather conditions should be recorded at intervals during the spraying operation and for the preceding and following days.
A good marking system is essential to prevent overlap or missed areas when spraying. Overlap of the spray swath results in areas being sprayed twice, which waste both herbicides and time.
Missed areas result in uncontrolled weeds, which can reduce yield, provide seeds for future generations or contaminate crops.
Misters are a useful but imprecise way of applying herbicides to large areas quickly. They rely on wind to drift the herbicide. If the wind is too light or the spraying speed too high, the swath width will decrease, possibly causing overdosing and wasted chemical. If the wind is too strong or gusty it increases the swath width, which will reduce the chemical application rate and increase the risk of damage from spray drift.
Hydraulic nozzles for use on boom sprayers
The data provides the technical information needed to enable a good choice of nozzles for boomspraying in particular conditions.
|Spray pattern||Nozzle type||Common use||Operating pressure (kPa)||Droplet size D (v,0.5) †μm||Remarks|
|Flat fan||Standard||Herbicides||200-400||Fine–medium <144-340||Main nozzle used in broadscale spraying|
|Flat fan||Low pressure||Broad-leaved weed or pre-emergence herbicides||100-250||Medium-coarse 236-403||Large droplets result in less drift. Good control of broad-leaved weeds|
|Flat fan||Extended range||Herbicides||100-400||Fine-coarse <144-403||Designed to operate over a large pressure range. Droplet size between standard and low pressure nozzles|
|Flat fan||Air induction||Herbicides and fungicides||200-800||Coarse-extremely coarse 341-502||Large droplets result in less drift but have relatively good coverage. Good application on broad leaves|
|Flat fan||Air assist (twin fluid)||Herbicides and fungicides||200-600||Medium-coarse 236-403||Ability to change droplet size by changing air pressure|
|Flat fan||Twin||Post-emergence herbicides, fungicides||250-400||Fine <144-235||One fan is angled ~30° forwards and the second is angled ~30° back for better leaf coverage|
|Flat fan||Wide angle||Banding between rows||150-400||Fine <144-235||Two outlets project either side of the nozzle to produce one wide angle fan|
|Flat fan||Flooding||High volume pre-emergence herbicides||100-250||Coarse 341-403||Large droplets result in less drift. High volume application|
|Even fan||Standard||Banding between rows||150-250||Medium 236-340||Produce a uniform distribution across the fan pattern|
|Hollow cone||Standard||Low volume||400-800||Fine||Main nozzle used for insecticide|
|Hollow cone||Standard||Insecticides and fungicides||400-800||<144-235||Main nozzle used for insecticide and fungicide application|
|Hollow cone||Whirl||Herbicides||100-400||Fine-coarse <144-403||Nozzles available for the application of pre-emergence and post-emergence herbicides. Large passageway minimises clogging|
|Solid cone||Standard||High volume insecticides and fungicides||400-2500||Medium 236-340||High volume for good penetration of foliage|
|Solid cone||Full||Pre-emergence herbicides||100-300||Coarse 341-403||Large droplets result in less drift|
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 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.
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.
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.
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.