With Dr Tom Wolf, Agrimetrix Research & Training (Saskatchewan).

Efficiency is not all about speed. When it comes to spray application efficiency, the keys to success are firstly hitting the target with the right product at the right time, and secondly, maximising the time the sprayer is in the field rather than refilling.

Dr Tom Wolf, spray application specialist with Canadian research and extension company Agrimetrix, says growers must accept that covering more ground per hour is not a good measure of spray efficiency.

“We understand that growers are generally spraying more times in a season and that there is limited time available,” he says. “We also know that travelling at 30 km/hr with a boom height of 90 to 100cm above the ground is not going to result in an effective application of the product. For weeds, this can also have implications for herbicide resistance in circumstances that result in low-dose applications.”

It pays to consider what percentage of sprayer engine hours are spent in the field spraying, compared to the time spent travelling to refill. Mr Wolf says investing in filling systems can be a ‘gold mine of efficiency’ that can allow the operator to slow down their ground speed and apply the spray more effectively.

“When you have other time efficiencies in place you can afford to slow down and you’ll do a better job on the weeds when you use the correct nozzle, boom height and ground speed,” he says.

How can poor spray application cause herbicide resistance?

In brief: Under-dosing applies strong selection pressure to weed populations.

The details: Generally, grasses require medium to coarse nozzles and broadleaf weeds require very coarse nozzles for optimal spray application. When a tank mix is used to target both grasses and broadleaf weeds the compromise made with nozzle selection should usually favour the finer nozzle for grasses.

Failure to do this can result in under-dosing for grassy weeds, particularly for products that do not translocate efficiently in the plant. These modes of action rely on excellent coverage, which is not achieved if the wrong nozzle is used.

Turn compensation is also very important to avoid under-dosing. Without turn compensation, the outside wing is applying 20 to 50 per cent less product than the intended rate and can promote herbicide resistance in weeds.

Another under-dosing situation occurs when wide sprayer tyres are used at high ground speed, resulting in high air turbulence along the wheel tracks and up to 50 per cent less herbicide deposition behind the wheels.

How often should I calibrate the sprayer and replace nozzles?

In brief: Aim to replace nozzles after spraying 20,000ha. Check for blockages before every spray event.

The details: The rate controller on the sprayer does not tell the operator if there is a problem, so it is necessary to conduct a flow test on the nozzles at least once a season.

Flow testing a set of nozzles is time consuming, but it is the only way to determine if the nozzles are delivering the correct flow rate. Each nozzle should be within five to ten per cent of the correct flow rate. If a few nozzles are off specification, it is usually best to replace the set as they are likely to all be worn to a similar degree. To save time, many operators chose to replace sets of nozzles annually to be on the safe side.

How can I make the most of pulse width modulation?

In brief: Pulse width modulation controls nozzle flow along the boom and can help avoid under-dosing with herbicide.

The details: The duty cycle’s pulsing solenoid can shut off the flow to the nozzle intermittently, between 10 and 100 times per second. When selecting nozzles for use on a pulse width modulation sprayer, the aim is to have the duty cycle solenoid open about two-thirds of the time.

This allows for ideal turn compensation, more flexibility in ground speed, optimal spray quality without changing speed and higher flow rates in certain nozzles (e.g. behind the wheels).

WeedSmart is an industry-led initiative that aims to enhance on farm practices and promote the long term, sustainable use of herbicides in Australian agriculture: www.weedsmart.org.au

Ask your questions about spray application on the WeedSmart Innovations Facebook page WeedSmartAU or Twitter @WeedSmartAU