Module 21: Assessing spray deposits

21.2 Assessing the results of spray jobs

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Published 24 January 2025 | Last updated 20 January 2025

The fastest and generally least expensive way to assess if adequate spray coverage has been achieved is to carefully assess the results of an actual spray job. The most obvious results can be seen when using a fast-acting contact herbicide, such as paraquat.

If there is an issue with the sprayer set-up, often there will be a pattern that is repeated at particular locations throughout the sprayed area. Before starting an assessment of the results of a spray job it is very useful to have a copy of the spray record of the actual job available. This way, information about the application parameters and the weather conditions (especially wind direction) can assist in determining the source of potential problems.

The following suggestions have been provided to assist applicators and advisers when evaluating the results of various spray jobs.

Always look at several locations across the paddock

Ensure that the areas that are chosen to assess the level of control are representative of the possible variability in soil type, drainage or other agronomic factors that may be present in a paddock. Where there are apparent differences in the level of control obtained, it is important to work out whether this is due to an agronomic factor or an issue caused by the spray application.

Examine multiple sets of wheel tracks across the paddock, including those going in both directions of travel

Compare the level of control obtained in the centre of the wheel tracks, adjacent to the wheel tracks, at the mid-point of the boom on each side of the sprayer, and at the boom ends to ensure sufficient overlap between spray passes.

For herbicide applications into standing stubble also compare the level of control in the inter-row and at the base of the standing stubble (to evaluate stubble penetration). This should also take into account the direction of travel and the wind direction at the time of spraying, noting if reduced control is consistent in particular locations that can identify the source of the problem.

Coarser droplets tend to be affected by travel speed, moving with the direction of travel. Smaller droplets, such as medium and fine spray qualities, tend to be more affected by the wind direction and wake effects in the centre of the sprayer.

For assessing the level of control onto larger weeds, also compare levels of damage caused by the herbicide on various sides of the weed in relation to the direction of travel and the wind direction.

The impact of spraying with a very coarse spray quality at 33km/h

The thistle in this image was sprayed with a phenoxy herbicide. The damage to only one side of the plant indicates the plant must have been very stressed when sprayed and the droplets landed mostly on that side of the plant (with the direction of travel of the sprayer). Source: Simon Rodgers. (509.8 KB JPG)

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Compare areas of low spraying speed with areas where the average spraying speed has been maintained

Carefully look at the ends of the spray runs, areas within the headland and areas just outside of the headland where the sprayer has commenced a new spray run.

For most sprayers that utilise a standard rate controller the pressure at the nozzle will decrease as the sprayer slows down. This will tend to increase the droplet size, causing the spray quality to become coarser. Where pressure becomes too low, this may also reduce the angle of the spray pattern, which may lessen the overlap of

the spray patterns. Both of these outcomes may lead to reduced levels of control in locations where the sprayer has travelled at lower speeds without a minimum setting in the controller to maintain pressure at the nozzle.

Where a minimum hold has been set in the rate controller, this can lead to overdosing in the areas of lower spraying speeds. This may be observed through increased damage to the crop with some selective herbicides, or as areas of poorer crop establishment when higher rates of residual herbicides have been applied.

Where wider headlands have been used, crop damage, poor establishment or poor control may be the result of incorrect timing of auto section control or nozzle shut- off. This is likely to be visible close to the edge of the headlands (where used) and in locations where the sprayer enters an area that has already been treated.

Where overdosing leading to crop damage has occurred (due to the minimum hold being engaged or set to remain at too high a speed or pressure), this may be visible across the entire headland.

In areas where poor weed control has occurred as a result of reduced coverage (due to no-minimum setting in the rate controller), this tends to be noticeable outside of the headland into the sprayed paddock, particularly where narrow headlands have been used, or from the edge of the paddock when no headland has been sprayed (e.g. some furrow irrigated blocks).

Where operators typically spray headlands at lower speeds than the rest of the paddock it may be worth considering applying the tank mix to the headland at a slightly higher water volume to maintain the pressure at the nozzle. This would require preparing a separate tank mix to be applied at the higher total application volume than the rest of the paddock, but this strategy may help to minimise overdosing or reduced coverage due to lower pressure at the nozzles.

Within the preset operating parameters, sprayers fitted with pulse width modulation systems are designed to overcome this challenge when slowing down on headlands by maintaining constant pressure at the nozzle and automatically reducing the duty cycle to moderate flow rate.  Note - duty cycles that drop too low when speed reduces can also create coverage issues.