There has been rain and that means it’s weeds and spraying time. This time of year also lends itself to extremely risky spray conditions. Beautiful autumn weather is when surface temperature inversions are most common. For more information on inversions read my previous blogs here and here.

Strong inversion conditions March 2016. Image: AGRONOMORemember that all pesticides drift, it is just that some, such as Group I herbicides like 2,4-D, have a recognisable odour and produce unique symptoms on sensitive vegetation.

So what is the problem with off-target movement of pesticide? The most obvious agricultural issue is damage to sensitive crops. For example in the 2007-08 cotton growing season it was estimated that 10 per cent of the Australian cotton crop has some level of herbicide damage costing $5 million.

The second agricultural issue is pesticide residues. Organic farmers certainly don’t want anyone else’s pesticides. Drift is a particular problem when a crop is getting close to harvest. Signing a vendor declaration that you haven’t used certain pesticides gets complicated when the purchaser tests the product and it is ‘contaminated’.

Some of our major trading partners also have zero tolerance for certain pesticides. Ship loads of grain have been turned around and sent back for such breeches. The wine grape industry is particularly aware of the potential effects of unwanted residues on their markets.

Contamination of the natural and human environments is also a major concern.

Sensitive areas

Other than showing a duty of care, reading and following the label, using buffer zones and using best practice application, we need to be aware of sensitive areas in the vicinity of the farm when spraying. Talk to your neighbours and find out what crops they will have in, particularly in paddocks along the boundary. Be aware of state restrictions on pesticide use and spraying.

Some Australian states have restrictions on spraying in or near ‘sensitive’ areas. Western Australia has listed most Group I herbicides as ‘Scheduled’ meaning they cannot be used within certain distances of sensitive crops and high volatile 2,4-D ester is banned within 5 km of a commercially sensitive crop (vineyards or tomatoes), within 19 km of the Geraldton post office and within the Swan Valley, or within 10 km of the Kununurra Post Office. High volatile ester is not registered any other state or territory.

Victoria has nine designated areas where the type of pesticide and its application is regulated.

Agricultural Chemical Control areas in Victoria. Source: Agriculture VictoriaThe cotton industry has led the way with on online mapping with 95% of cotton crops mapped.  There is no excuse to say you didn’t know there is cotton nearby.

Cotton field awareness map for the western Riverina. Source: Cotton Australia & Cotton CRDCWestern Australia also has a voluntary system of registering your pesticide sensitive crop with the Department of Agriculture & Food WA.

DAFWA sensitive areas map 2015The map highlights organic farms, vineyards, tree crops, vegetable, bee hives and aquaculture sites. Being voluntary not all WA sensitive crops are mapped and a quick comparison with Google Earth will show additional vineyards and orchards. Another problem is that most broadacre farmers I have spoken with are unaware that this map of sensitive crops exits.  This is ironic because much of the potential drift affecting these sensitive crops could come from cropping country.

When it is spray time, do some planning, look at the weather forecast and for any potential risks, talk to your neighbours, use the right gear and get that pesticide where it is meant to be.

Most broadacre growers want the capacity to spray their whole farm within about a ten day period or less if possible. Generally the first way 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 meteorological conditions may not necessarily differ much between night and day.

There is a lot of preparation for successful, low risk night spraying. Image:AGRONOMOThis first article on night spraying discusses the issues encountered if there isn’t a surface temperature inversion. The second part will discuss surface temperature inversions and why 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 droplet spectrum with matching 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 amout of spray into the air compared with spraying diring daylight hours.

To demonstrate what can happen I will discuss research conducted by Bill Gordon Consulting in northern NSW comparing drift from night versus day spraying.

A 55 ha paddock was sprayed at 2:30 am and again at 7:30 am using a 36 m boom travelling at 22 kph, with Teejet AIXR 11002 nozzles, 50 L/ha at 4 bar producing a coarse spray quality.

Herbicide drift was measured 80 m downwind of the paddock using a 20 m 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 there was virtually one hectare’s worth of spray heading downwind from the sprayed paddock at night 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 and forced 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 be spraying at night launching small amounts of spray into the air which can accumulate in downwind areas of the landscape and becomes a much bigger problem to sensitive vegetation in those areas.

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 and the need to go 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 m into the air. Higher speeds usually means lifting the boom. Lifting boom height from 50 to 70 cm above the target increases drift potential 4 times while going from 50 to 100 cm increases it 10 times.
• Only spray paddocks with good catch surfaces, such as stubble or dense vegetation.
• Make sure you know if there are sensitive areas downwind.

Spraying paddocks with poor droplet catch surfaces like this will increase the likelihood of spray drift, especially with night spraying. Image: AGRONOMO

The next blog will explain surface temperature inversions and why most labels state that you are not to spray if an inversion has formed.