The pressure is on for better spraying

By Alec Nicol

"How many of you have a spray plan?" When consultant Graham Betts looks around the packed workshop he already knows how the majority will answer, and it still puzzles him why growers who would not think of starting a year"s work without a cropping plan or a fertiliser plan, let spraying fall through the net.

Mr Betts is the facilitator of a new campaign, "Spray Safe and Save", based on grower workshops in which conventional spraying wisdom is generally turned on its head.

He says the line is drawn between growers who set out in the morning to go spraying, and those who set out to actually kill pests and weeds. "It"s amazing... the results you can get when you put the chemical on to the target," he quips.

Getting it there means matching the chemical to the target and factoring in weather conditions and choosing the right nozzle, pressure and speed for the rig. That is the basis for the spray plan that every participant at the workshop completes during the day.

With a stated aim of saving money and dealing with environmental concerns, Mr Betts argues that "getting the nozzle selection right is the best way of changing the cost of spraying". He says that as much as 40 percent of the spray droplets produced by the average rig may not get to the target. "Conventional wisdom says that finer droplets give a broader coverage but the reverse is often true," he says.

Participants at this workshop, at Urana in NSW, are faced with a simple spray plan worksheet. It asks for the target or targets of the operation and the chemicals to be used. That decides droplet size. The task is to match the nozzle type and rig setting to those requirements. Each worksheet has two sections, one for ideal conditions and the other for less than ideal spraying conditions.

Few spraying operations have a single target and most rely on a compatible tank mix of chemicals: "But a compatible tank mix doesn"t necessarily mean a compatible target mix," says Mr Betts. "There"s likely to be variation in the ideal droplet size for each of the targets and you"re already forced to plan for less than ideal spraying conditions."

Target and chemicals decided, Mr Betts asks participants what speed they want to work at to get the job done.

While 12kph used be accepted as a standard speed, most participants opt for 18kph, with some going as high as 25kph. Cross-referencing the range of droplet sizes needed to hit the target, the volume of water and the pressure required to get there at the chosen speed, turns up a whole range of nozzle types and pressure options.

"There"s a theory that working at a higher pressure will wear the pump out quickly," says Mr Betts. "In fact, the modern pump is hardly working at the conventional 3 bar (300 kPa) mark. So don"t be afraid to operate at higher pressures."

The advice is backed by practical demonstration. A small stationary test rig plumbed to accommodate a range of nozzles operating at differing pressures is on show.

In a 6kph breeze, some nozzles operating at relatively low pressure produce a fog of drifting mist, while beside them others at twice the pressure get the chemical on to the ground and the target.

At the start of the day participants at the Urana workshop were asked to name the type of rig and nozzles they were using. The range was wide but most, if not all, stuck with a single type of nozzle. When they were asked what it would take to get them to change, it was obvious that cost was the factor.

A quick sum averaging the cost of a second set of nozzles against the annual chemical bill turned up a figure less than bank interest, yet nozzle selection proved to be a key element in getting the chemical to the target.

As one grower commented at the end of the day: "Clearly one set of nozzles is not enough - and nozzles now look cheap in comparison to the alternative."

The workshops, initiated by the GRDC"s northern panel, are part of a new campaign to tackle the problem of spray drift and chemical wastage.

For more information:
Graham Betts, 0427 622 214
Stuart Kearns, GRDC, 02 6272 5525,

GRDC Program 6