The potential for farm chemicals to move off-target and cause damage to sensitive areas is determined by two main factors: the amount of product that is able to be deposited, and the sensitivity of the area or species that the product lands on.

For most spraying activities the amount of product that remains in the air after each pass of the sprayer will be relatively small, depending on the spray quality selected. When using a coarse spray quality the amount of product that remains airborne after each pass of the sprayer may be as low as one or two per cent of the applied rate.

While one or two per cent may seem like a small amount, if we consider the capacity of modern self-propelled sprayers where it is possible to spray more than 60 hectares per hour, the total quantity of product remaining in the air can be large.

If just one per cent of the applied product remains airborne when spraying at a rate of 60 hectares per hour it is possible to leave more than 0.6 of a hectare-worth of product in the air for each hour of spraying that is completed.

If the weather conditions are wrong for spraying and the airborne product moves away from the target site to deposit onto a sensitive area, the damage can be substantial.

Where finer spray quality is used, the volume of airborne spray increases significantly. Consider the sensitivity of some crop species. Many broadleaf crops, such as cotton, tomatoes and grapes may produce visual symptoms of damage to products containing 2,4-D at rates about one gram of the active ingredient per hectare (1g a.i./ha).

At rates above 1g a.i./ha, yield losses may start to occur in some crops, particularly if the product deposits onto a crop when it is in a sensitive growth phase (for example, establishing reproductive structures).

As little as 3g a.i./ha of 2,4-D landing on a cotton crop at the wrong growth stage can reduce the yield of that crop by up to 50 per cent. At a value of $500 per bale and yield of 15 bales/ha, a 50 per cent loss of yield (equal to 7.5 bales/ha) could easily be worth $3750/ha in lost income.

Even using a coarse spray quality, just one hour of spraying under surface temperature inversion conditions with 500 millilitres/ha of a 470g a.i. 2,4-D in the tank mix could result in a yield loss to a cotton crop in excess of $176000.

This calculation is based on spraying 60ha/hour, using 3g a.i. 2,4-D, affecting 47ha, a 50 per cent yield loss, expected yield of 15 bales/ha and price of $500 per bale. (Note: for the reasons outlined, 2,4-D products must now be applied using a minimum of Very Coarse (VC) spray quality).

While the above example is based on a broad-spectrum product and an extremely sensitive crop, similar estimates can be made for other situations, crops or products.

As little as 20 to 30g a.i./ha of a glyphosate product has been reported to cause damage to cereals at the two-to-three leaf stage.

The pre-emergent herbicide Overwatch(R) (bixlozone) is another example where off-target ‘bleaching’ on sensitive species can be extremely visual, should spray drift move off the treated paddock. Species such as bifora, sow thistle, lupins, serradella, tillage radish, chicory and biserulla are very sensitive to this herbicide and will typically show the extent of spray drift that has occurred.

Spray drift is not just a ‘2,4-D in summer’ issue. The visual bleaching of Overwatch herbicide on sensitive species such as sowthistle is an indicator of the extent of spray drift occurring in autumn applications in some regions. Other products in the tank mix, or other products applied under similar conditions and sprayer setup, are also likely to be moving off-target in similar quantities. Although, due to their symptomatology, the drift from other products may be less obvious to the untrained eye.

Typically it requires the combination of both a very sensitive off-target species and a pesticide with very visual symptoms for spray drift to be easily identified. Many other products may be causing environmental contamination and potentially damage, but without visually observable symptoms.

Residue limits and the limit of detection

Often we will not see obvious visual symptoms when very small quantities of drifting products deposit onto an adjacent crop. However, if the product that has deposited onto the crop is not registered for use on that crop, or there has not been a residue limit established for this product in the destination market, this can create potential trade issues.

It is important to consider the destination market for an adjacent crop and whether there are animals grazing on neighbouring pasture.. When considering an application next to a crop or pasture, consider how close to harvest the crop may be or when grazing may take place on the pasture. This is particularly important if the crop or produce is being directly exported, for example container shipments of export grains or hay, or situations such as dairy cattle that are milked daily.

All applicators need to be aware that testing for pesticides within Australia that returns a result of ‘below the limit of detection’ for a product does not mean that produce is free of residue. If more sensitive laboratory equipment is used in a country receiving our exported products, a residue may still be detected.

The only safe time to spray adjacent to an export crop is when the wind direction is consistently away from the crop, and you have managed all the risks you can.

Spray drift onto houses, gardens, water courses and roof structures collecting rain water

There are many other sensitive areas adjacent to, or within the farm that also require specific protection from spray drift. On many newer product labels buffer zones to these areas will be indicated as required, but it is always good practice to ensure that wind is blowing away from these areas during application.

Causing herbicide damage to the household garden or having insecticide spray drift potentially deposit on the roof that feeds into the rain water tank is generally not a pleasant discussion around the dining table.

Loss of product efficacy

Under situations where excessive spray drift moves off the paddock, less pesticide will reach the target.

This can result in wasted cost of the pesticide and reduced efficacy, potentially leading to poor control and/or increased selection pressure for resistance from suboptimal application rates.