Application and Herbicide drift

If pressure is used to apply herbicides, there will always be some herbicide drift through the air.  If there is no commercial crop in the vicinity, spray drift will have no effect, however, when weeds have to be controlled between rows of a commercial crop, any spray drift might cause considerable damage.

It is important to understand when spray drift is at its most dangerous and when is it likely to happed.  The grower should also understand which types of equipment are designed to help limit the amount of spray drift during applications.  There are also many adjuvants and thickeners which can be used to limit the size of droplets and so reduce the amount of drift.  Spray drift depends on the following:

  • The individual droplet size which is determined by the pressure at which it is released, the nozzle design and the surface tension of the herbicide solution.
  • The speed of the wind during application.  It is recommended that herbicides should be applied early in the morning when there is no wind or late afternoon.  The danger of wind is that some of the drift may be inhaled by the person applying the herbicide.
  • The distance from which the spray is applied to its target.  The further the nozzle is from the target the further the droplets must travel in order to reach its target.

The volatility of a herbicide refers to the ability of the chemical to  form fumes or vaporize.  The rate at which the fumes develop depends on the vapor pressure of the chemical.  Crops can be damaged by vapor drift, however, more dangerous is the effect it might have on the people mixing the chemical or applying it.  Herbicides lose their effectiveness when too much is vaporized.

Types of Equipment used to apply herbicides

Spray equipment

Spray equipment ranges from:

  • Small back packs which can hold between 15 to 20 litres.
  • Small motorized back packs with a capacity of 10 litres.
  • Larges spray equipment which is attached to a tractor and can hold between 200 and 1,000 litres of water
  • The most expensive method are the use of aeroplanes

The type of equipment that will be used will be determined by the grower, the conditions, geographic characteristic of the area, the type and amount of weed that needs to be controlled.


The most important component of the spray equipment is the nozzle.  The nozzles are responsible for converting the liquid into the required size droplets.  Each type of nozzle provides the correct size droplets within a pressure range.  Pressure ranges between 1.5 bar (20 psi) and 2.75 bar (40 psi).  The pressure with which the herbicide liquid is forced through the nozzle determines the uniformity and rate of application.  The size of each droplet and the uniformity of the size of all droplets is also determined by the pressure.

At ideal pressures:

  • Droplet formation is near the nozzle tip
  • Uniform small droplet across spray area

At too low pressures:

  • Droplets are large near the edges
  • A liquid film develops closer to the middle which stretches into large droplets the closer it falls to the soil

At too high pressures:

  • Very small droplets form very close to the nozzle tip
  • The small droplets are so small that they fall into the category of fog or mist size which drifts easily

The various types of nozzles that are commonly used for herbicide application are shown in Figure 12.  Note these are used to apply herbicides and may differ from those that growers used to apply pesticides and fungicides.  The pressures with which herbicides are applied differ from the pressures and equipment used to apply pesticides and fungicides

herbicide application nozzle design

Some common nozzle designed used to apply herbicides

Nozzle do have to be maintained every time they are used.  If nozzles are not properly maintained they will become blocked with dirt and dust and also some sediments.

The most common material used for nozzles are brass but stainless steel and nylon are also used.  The nylon types tend to break easily.

Applying granular herbicides

In some cases granular herbicides need to be applied.  It is obvious that equipment that is used for liquids cannot be used for granular types.  In most cases granular herbicides are applied with dry fertilizer spreaders or planters.  The dry fertilizer spreaders are not very accurate and care should be taken when calculating the rate at which they should be applied.  Quite often growers mix the herbicide with some sand in order to increase the volume of the mix which makes it more accurate when applied.

Soil conditions

Soil conditions are critical when applying herbicides that have to be applied into the soil.  These herbicides include EDB, methyl bromide and Fusillade. Ideally the soil should have as little clods as possible.  The soil should be irrigated a week before thoroughly, and only lightly three days before applying the herbicide.  If the soil is too moist, the herbicide will not mix readily into the soil and effectiveness will decrease. Too dry soil will increase evaporation.  Compaction layers also decrease the effectiveness of herbicides in the soil


Temperature plays a significant role in the amount of herbicides that effectively reaches the target area.  Since temperature has a significant effect on the volatility of the herbicide, a 10°C increase in temperature will increase the volatility three fold.  The opposite effect of temperature takes place on the soil.  The higher the temperature of the soil, the higher the increase in absorption of the herbicide to the clay particles.  However, if the application takes place either in the morning or late afternoon when temperatures are not in extremes,  a grower should obtained effective results.