Spray application and herbicide efficacy in cereals and oilseed rape

Find out how to optimise herbicide application techniques to make every drop count in the fight against arable weeds. This page covers spray equipment (e.g. nozzles), rates and timing.

Application technique can significantly affect herbicide performance, particularly for small weeds early in the season. Consideration should be given to:

• Timing of the application
• High levels of deposit of active substance
• Optimum droplet size
• Controlling spray drift

Find out the pros and cons associated with the key herbicide spray timings in autumn-sown cereal crops.

The highest deposits on target weeds result from reducing both application volume and droplet size.

Using lower volumes (around 100–150 l/ha) is generally more effective than volumes of 200 l/ha and higher.

Reducing water volume from 200–100 l/ha, achieved by changing nozzle size, gives an estimated 40% increase in work rate for a 24 m boom on a 3,000-litre, self-propelled sprayer.

Increases in work rate can also be achieved by increasing forward speed and boom width. However, faster forward speeds and wider booms may require increased boom height. This increases the risk of drift, as does spraying reduced volumes through smaller-sized, conventional nozzles.

Some horizontal movement of droplets is necessary to ensure adequate deposition on small grass weeds. Angling nozzles is one way to create horizontal velocities, which increases active ingredient deposition on small vertical targets.

For many herbicides, finer-quality sprays are more reliable than coarser sprays. Often, air-induction nozzles giving the smallest droplet sizes perform as well as medium-fine conventional sprays.

Larger weeds are more suitable for treatment with air-induction nozzles, particularly those giving the smallest droplet sizes, which can still significantly reduce drift compared with conventional nozzles.

* Acceptable

**Preferred

Drift control = best for drift control

Where conditions limit available spray days, high work rates are necessary but must be balanced against the risk of spray drift. Increased drift is associated with:

• Fine sprays
• Applications to small canopies
• Increased boom heights
• Higher wind speeds
• Particular formulations*

*Water-soluble liquid formulations with a high level of surfactants  – such as glyphosate – may also increase spray drift, so additional precautions for controlling drift may be necessary.

It is essential to comply with product labels and the Code of Practice for using plant protection products.

It is important to keep pesticides out of watercourses.

Water quality legislation may affect or restrict use of several herbicides, particularly those used extensively, as well as those used at high rates and applied at times of year when drains may be running or there is potential for run-off to watercourses.

The result of new legislation could be restrictions on rate and/or timing, with product withdrawal as a last resort.

The Voluntary Initiative provides best-practice guidance for the main pesticides that post a particular concern to water.