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Herbicide drift

Herbicide drift is the movement of pesticide away from the target area in the atmosphere. The three main forms of drift are droplet drift, vapour drift and particulate drift. Droplet drift is the main cause of off-target damage.

Spray emerging from a boom breaks up into droplets of varying size. Larger droplets fall onto the target area, while the smallest droplets may remain in the air.

Because droplet drift usually disperses as it moves away from the sprayed area, the type of crop damage it causes in adjoining areas is easily recognised. That part of the sensitive crop which is closest to the sprayed area is severely damaged but damage decreases away from the severe zone.

Vapour is produced by evaporation from the droplets when they leave the boom and from the target surface after spraying.

Like droplets, vapour disperses rapidly as it is carried away from the target area. The vapour will remain suspended in the air unless the contaminated air is forced back to ground level where it may damage growing plants.

Vapour can drift for long distances, and the characteristic feature of vapour drift damage is that no clear damage gradients can be seen. Damage, which is generally mild but widespread, is usually caused by a large body of contaminated air several square kilometres in size.

To understand damage caused by drift of herbicides there are four key aspects that require understanding:

Production of driftable sized particles

Most agricultural herbicides are applied as fine droplets produced by hydraulic nozzles on boom sprays, mostly set up to deliver droplets in the 150-300µm range. Droplets from 100-200µm in diameter usually stick to the first surface they encounter but droplets larger than 500µm are likely to bounce off leaves and end up on the soil or lower canopy and droplets smaller than 50µm are likely to float around the target plant and drift off. The smaller droplets provide better coverage which is important when the target plant is small, when the herbicide is poorly translocated or when low carrier volumes are used. The larger droplets result in greater interception or less drift, but the poorer coverage may need to be compensated for by using translocated herbicides or higher carrier volumes.

Most commercially available nozzles produce a range of droplet sizes so there is usually a proportion of very fine droplets that may drift. For emulsifiable concentrate sprays, drift reducing agents such as Nalcotrol® may reduce drift. For aqueous sprays the addition of a drift reducing agent may increase the production of droplets that are less than 100µm diameter and cause greater drift. Also, shear stresses in recirculating pumps (especially centrifugal pumps) can reduce the effect of polymer drift reducing agents.

Transport of herbicide away from the target area

Under the influence of gravity, all droplets fall at a speed called sedimentation rate. Large droplets fall faster than small droplets. Within a few centimetres of the nozzle the movement of most droplets is determined by gravity, their buoyancy and wind. The higher the droplet is released, the further it will move away from the target area because the wind speed is slower close to the ground and there is more time for the wind to move the droplet before it lands. Therefore, the amount of pesticide that drifts off target is closely related to the boom or flying height. Halving the height of the boom above the target will reduce drift by about 60%. In situations where drift can cause problems the boom should be operated at the lowest height possible for the nozzles and spacing.

Decreasing the nozzle spacing will allow the boom to be operated at a lower height. Large droplets contain more herbicide but they tend to land close by, whilst small droplets contain less herbicide but are moved over greater distances by the wind and are more likely to be affected by turbulence that may carry them upwards. (Note: About 2-10% of spray volume of aqueous formulations will be in drift-prone droplets - less than 100µm diameter - with normal flat fan or cone nozzles on aircraft and less on boom sprays. Emulsifiable formulations of herbicides will produce about twice as many droplets in this size range with the same equipment).

The size of the area sprayed also affects the amount of herbicide leaving the target area because successive runs contribute to drift. As small droplets drift away from the sprayed area they normally disperse to non toxic concentrations within 100-200 metres downwind. Under low level temperature inversion conditions the droplets may be trapped in a layer of cool air close to the ground and move greater distances at higher concentrations. These conditions are most likely to occur when there is little or no wind, clear skies, the weather is influenced by a high pressure system and the ground is cooler that the surrounding air in the evening and may last until the sun warms the ground on the following day or until the wind speed increases (most claims for drift damage are associated with inversions). This can be made worse by valleys that can channel drift laden air for a kilometre or more.

Interception or absorption of the transported herbicide by the off target species

As droplets leave the target area they usually decrease in size as the carrier evaporates. This affects the deposition on off-target plants and generally reduces the damage that would otherwise be predicted by drift. Damage is a function of the number of droplets impacting on the plant multiplied by the concentration of herbicide in the droplet. Small droplets that have been produced by the evaporation of large droplets carry more herbicide than small droplets produced at the nozzle.

Dose response curve of the herbicide for the affected species

The dose response curve for many plants to herbicides is a logistic curve as shown below. At very low doses there is no significant effect on the plant. As the dose of herbicide increases the amount of damage increases until at high doses the plant dies and increasing the dose will have no further effect.

Dose response curve of the herbicide for the affected species
Table 1 Dose response curve of the herbicide for the affected species

The graph also shows that wheat is damaged more than canola by low doses of glyphosate and the no-effect-level (NOEL) is around 10 grams of active ingredient per hectare (g.a.i/ha) for wheat and 25g.a.i./ha for canola. For canola, the dose required to kill it is similar for glyphosate and 2,4-D. However, because the slope of the dose response curve for 2,4-D is less that for glyphosate, the NOEL for 2,4-D on canola is much less that the NOEL for glyphosate. This means that canola crops are more tolerant to glyphosate drift than they are to 2,4-D drift.

Drift can be reduced by increasing the droplet size, followed by reducing boom height, followed by application at reduced wind speeds.

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