Limiting spray drift and improving coverage

Figure 7. Spray workshop held at Douglas Daly Research Farm

Callen Thompson, Senior extension Agronomist Katherine

Agriculture in the Northern Territory is famous for its vast cattle stations but we know that within this landscape horticulture and hay production is often concentrated in specific areas where there is good soil and water. There is also likely to be lifestyle blocks in the same areas. In these intensive areas there is a significant risk of spray drift damaging neighbouring crops, pastures or the environment if chemicals are not applied properly.

Using spraying equipment, whether it be for weed, pest or disease control, is part of most agricultural systems and the Australian Pesticides and Veterinary Medicines Authority (APVMA) ensures that chemicals are safe for use as long as the applicator follows the directions on the label.

High value horticultural crops like mangos, Asian vegetables and sandalwood are susceptible to herbicides such as 2-4,D, which are commonly used for broadleaf or woody weed control in pasture and hay paddocks. If these chemicals are applied in-correctly, in poor weather conditions or with inappropriate equipment, they can drift onto neighbouring crops. This may cause yield loss or even death to the susceptible “off target” crop, which can lead to litigation if significant damage is caused.

Even if the chemical does not land on an off target crop it may contaminate the environment. In addition, chemical that is not landing on the desired crop is wasted, effectively decreasing the rate of chemical applied, decreasing the effectiveness of the job.

By understanding weather conditions, your equipment and the product you are using, you can limit the risk of off target drift while still getting good coverage.


Coverage is important as chemical that does not land on the target is wasted. This is effectively reducing the rate of product you are using, which can lead to reduced efficacy and low rate herbicide resistance. Products where this is especially important are insecticides fungicides and contact herbicides, especially when applying as a protective spray.

Products like glyphosate are translocated through the plant so coverage is not as important as the chemical will travel from the point of contact to the rest of the plant. Some products, like gramoxone, are extremely poorly translocated and rely on the whole plant being covered to achieve effective control.

Pre-emergent products like S-Metolachlor need good coverage as they act as a film, stopping weeds emerging through the top soil. Timing, solubility of the product and ground cover can affect the degree of chemical coverage achieved.

Droplet size

Different nozzles can give you different droplet size ranging from very fine to extremely course. Fines droplets are small, slow moving and highly susceptible to drift while course droplets are large, fast moving and less susceptible to drift.

If landing on the desired target, fine and medium droplets will give a more even coverage compared to course droplets, unfortunately weather conditions and equipment setup can decrease fine droplet survival and increase movement off target. Good coverage can be achieved using course droplets if the water rate is increased. Figure 1 shows water sensitive paper comparing two spray rates, 100 and 50 L/ha using air induction nozzles. The higher water rate achieved far greater coverage.

100L/ha of water on the left vs 50L/ha of water on the right. Both very course droplets.

Figure 8. 100L/ha of water on the left vs 50L/ha of water on the right. Both very course droplets.

Weather conditions effecting application

Weather in the Northern Territory is generally marginal for spraying. It is important we understand how weather effects spray quality and how we can manage it.


Wind can blow droplets of target, this is called physical drift. It is important not to spray when wind is blowing towards susceptible crops. Risk of drift can be reduced by using course droplets because larger droplets have greater velocity and are less susceptible to being blown of target. Spray when wind speed is between 3-20km/h and is not gusting (some products like 2-4,D label requires less than 15km/h). Never spray when there is little to no wind (less than 3km/h) as this can allow fine droplets to float upwards into an inversion layer (see below). When you are measuring wind speeds always measure at boom height.


At high temperatures the weed may have shut down and not translocate the product so never spray when weeds are showing signs of stress. High temperatures can cause volatile chemicals to vaporise and lift of the target. This is called vapour drift and can happen hours after the chemical has been applied. Chemical companies recommend that you don’t spray at temperatures over 30oC.

High temperature combined with low humidity can effect droplet survival due to evaporation. Using courser/larger droplets will decrease the effect of evaporation. Because of this relationship, understanding Delta T is very important.

Humidity and Delta T

Increased humidity can reduce droplet evaporation rate. The relationship between temperature and humidity is expressed by delta T. High delta T (high temperature/low humidity) causes droplet evaporation and can limit the product hitting the target. Low delta T (low temperature/high humidity) can be a drift issue as fines droplets have greater survival as they are not evaporated. The graph in figure 2 is a great tool to determine the right Delta T conditions for spraying. Delta T between two and eight is ideal.

Surface temperature inversion

A surface temperature inversion is commonly known as an inversion layer, and has the potential to cause chemical drift up to 30km from the area sprayed.

Figure 9. Selecting the right Delta T conditions for spraying, Source: Jorg Kitt, Spraywise Broadacre Application Handbook, 2008

Under normal conditions temperature decreases further away from the ground. In an inversion event temperature increases with height. The cool air doesn’t mix with the warm air and layers are formed. Fine droplets or vapour can be trapped in these layers and float away.

This often happens when there is low wind, high daytime temps and low night temps. Inversion layers usually occur early evening, night time and early morning. Dust, smoke and fog can indicate an inversion layer. Fine droplets are most susceptible.

For information on using chemicals responsibly visit the Northern Territory Government website.

Last updated: 19 December 2017