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Spraying at Night - Minimising the Risk

Night Spraying AGRONOMOMost broad-acre growers want the capacity to spray their whole farm within about a ten day period or less if possible.

Generally the first method considered to get more spraying done in a limited time period is to spray at night. The widespread adoption of auto-steer and tram-tracking has meant that spraying at night is easier to do, thus giving more hours spraying per day.

Night spraying has also been promoted as giving better weed control because it is cooler with higher humidity so target weeds will be less stressed. This however is not always the case and anyone looking at meteorological data over a 24 hour period in summer will see that critical meteorological conditions may not differ much between night and day.

Avoiding Surface Temperature Inversion Conditions

At sunset the ground loses heat and under low wind conditions, air close to the ground cools, while air above it is warmer. The air doesn’t mix and is described as being stable. Air temperature increases with height compared with during the day when air temperature most often decreases with height. When this occurs close to the ground, it is called a surface temperature inversion.

This article on night spraying discusses the issues encountered if there isn’t a surface temperature inversion. We will be releasing another important article on surface temperature inversions in January. Surface temperature inversions are more common than most farmers realise. They are bad news for spraying. 

No inversion, so you think you are covering the bases?

Despite trying to do all the right things such as using a coarser spray quality (droplet spectrum) with matching increased application volume and keeping within the ground speed limits of your sprayer, significant drift can occur with night spraying. Night spraying can deposit 5 times the amount of spray into the air compared with spraying during daylight hours.

Research conducted by Bill Gordon Consulting, in northern New South Wales, compared drift from night versus day spraying, which showed what can happen, even though you meet label conditions!

In the trial, a 55-hectare paddock was sprayed at 2:30 am and again at 7:30 am using a 36-metre self-propelled boom travelling at 22 kilometres per hour. It used Teejet AIXR 11002 nozzles, 50 litres/hectare at 4 bars producing a coarse spray quality.

Herbicide drift was measured 80 metres downwind of the paddock using a 20-metre-high collection tower.

The experiment showed that less than 0.5% of total spray applied to the paddock was collected at the tower during the daylight application, compared with 1.5% from the night application. This means that at night there was virtually one hectare’s worth of spray heading downwind from the sprayed paddock despite the “acceptable” spray conditions. Looking at the meteorological data below there was little difference between the two application times.

Table 1: Meteorological conditions at time of spraying

  Wind Speed (km/hr)  Wind Direction (°)   Temperature (°C)  Relative Humidity (%) 
Night
(2:30 am)
 
12  19 26 64
Day
(7:30 am)  
18 4 29 61


Despite there being no surface temperature inversion, there was still 3 times the spray in the air moving downwind at night. Higher daytime wind speed would have created more mixing of the air, forcing droplets down to the target compared with night spraying.

The other thing to consider is it isn’t just what you do, but the cumulative effects of other sprayers in the local area. Often neighbours will also be spraying at night launching small amounts of spray into the air that accumulates in downwind areas of the landscape. This then becomes a much bigger problem to sensitive vegetation in those areas.

Night Spraying AGRONOMO

How to reduce the problems with night spraying?

  • DO NOT spray during a surface temperature inversion.
  • Keep measuring meteorological conditions during spraying and stop spraying if conditions deteriorate. Have a pre-determined cut-off for when you will stop.
  • Use extremely coarse (XC) spray quality – in this experiment XC spray quality would have reduced drift to 0.5% for the same conditions. Keep in mind the suitability of these coarse droplets on the target and the product to be used. Remember to increase application volume as spray quality becomes coarser.
  • Slow down and keep boom height to a minimum – higher speeds increase the volume of finer droplets being lifted behind the machine, creating a plume of spray lifting 15 to 20 metres into the air. Higher speeds usually mean lifting the boom. Lifting boom height from 50 to 70 centimetres above the target increases drift potential 4 times, while going from 50 to 100 centimetres increases it 10 times. Newer boom height controllers are very effective at controlling boom height.
  • Only spray paddocks with good catch surfaces, such as stubble or dense vegetation, NOT bare fallow.
  • Make sure you know if there are sensitive areas downwind.

Story and photos courtesy of Andrew Storrie - AGRONOMO consulting