Archive Article: 1997/11/08

8 November 1997

Youve chosen the best product for the job in hand and picked the right day to spray. But are your nozzles up to it, asks Sarah Henly?

TRUTH or myth? Venturi nozzles improve the performance of all agrochemicals? Fitting venturi nozzles is the same as using the Airtec twin fluid system? Airtec and Degania are directly comparable?

If you answered yes to any of those questions, youd better read on. To get the best from a herbicide, fungicide or insecticide, you need to know precisely which type of nozzle to use, as well as when and what to spray.

By selecting the right nozzles, you could find theres scope to improve the efficacy of sprays, increase the number of spray days, and possibly reduce application rates, says Nigel Western, applications specialist at IACR-Long Ashton near Bristol.

First consider your target. The type of nozzle you need for a pre-emergence residual herbicide application will be very different from what you would choose for a fungicide spray. You may need to keep a whole selection, he suggests.

"Soil-applied herbicides are typically applied using coarser droplets than foliar-applied sprays since target coverage is not as critical. Venturi nozzles for example would suit the first use, but because the coarse droplets are retained poorly on very upright leaves, not the second use.

"Conventional hydraulic nozzles would do for both provided suitable flow rates were selected, but they wouldnt offer the operator as many chances to spray because drift would be of greater concern," he explains.

Drift is one of the biggest stumbling blocks to successful spraying. Using a low drift nozzle or spraying system can bring more spraying opportunities, but is practical only if suited to the particular task in hand, warns Mr Western.

While twin fluid systems such as the Airtec can be effective against any target, venturi nozzles for example are less suited to early post-emergence herbicide applications where a fine spectra is desirable.

This months Brighton conference sees the launch of a new BCPC classification rating nozzles according to potential drift hazard, which should help in selection. That complements the current BCPC nozzle selection scheme which classifies nozzles and sprays into fine, medium or coarse.

As a general rule, most applications can be made using a medium quality spray in 150 to 250 litres/ha of water.

Some post-emergence contact herbicides, insecticides and fungicides work more effectively when droplet size is small, since retention on the leaves is usually greater.

Some guidance is given on product labels. But its worth studying the advantages of each of the different types of nozzles before buying.

As a guide, conventional flat fan nozzles are the simplest and cheapest to use, which is probably why they account for 70% of the usage figures. They come in a range of angles from 65-110í and produce an overlap, so the target is sure to receive the correct dose of spray.

Droplet size is determined by outlet size and pressure, so can be chosen specifically for the target in mind. The smaller orifices are prone to block when low application volumes are used, warns Mr Western.

However he would probably spend his money on flat fans, unless he had a large farm where low volumes were essential and drift potential was high.

"If I farmed a large area where high winds were often a problem, Id probably choose an Airtec system. It is the only thing that has shown consistent advantages over conventional nozzles. Twin fluid nozzles can be adjusted to suit a variety of targets," he says.

It is claimed that using air to atomise the liquid flow adds further energy to the spray, giving some direction and velocity to the smaller droplets.

Improved penetration and foliage retention is reportedly achieved because some of the air enters the larger droplets and slows down their movement.

The Airtecs flow rate is determined by the ratio of air to water within each nozzle, and droplet size can be changed on the move. That has numerous advantages – the operator can assess the drift risk and increase droplet size when approaching, for example, a stream or houses, explains Mr Western. The application rate or volume can be altered easily.

The downside is cost and complexity. Booms must be modified to take the air compressor and large nozzles. Retrofitting to existing sprayers could cost about £7,000 for a 24m boom plus labour charges.

Where growers want a low drift system without all the caboodle, there are two other main options. Venturi nozzles offer some air entrainment effect without the need for a separate air supply.

The Billericay Bubblejet, for example, is a pre-orifice low drift nozzle which is claimed to give better foliage retention than other coarse nozzles.

The disadvantage is that the droplet size is not ideal for very upright leaves. And because the nozzle draws air in through tiny holes in its sides, blockages could be a risk and spotting them would be difficult, believes Mr Western.

The second option – and the newest nozzle on the market – is the anvil-based Turbo TeeJet from Spraying Systems.

It is a modification of the original anvil type but produces a coarser droplet at lower application volumes and can overlap.

"The Turbo TeeJet rather cleverly allows spray operators to maintain coverage with low application volumes without significantly increasing drift. It is particularly useful for applying soil-acting agrochemicals," says Mr Western.

To get the best out of any agrochemical, growers must weigh up the pros and cons of each type and be prepared to change nozzles as often as they change targets, he concludes.

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