Archive Article: 1997/11/08

8 November 1997

Fasten your seat belts. New research shows going fast is best, reports Debbie Beaton.

MANUFACTURERS have been very successful in producing sprayers that can go faster and faster. For trailed sprayers the Fastrac, among others, has shifted the operation up a gear.

Similarly, the main self-propelled sprayers have chassis suspension enabling them to operate at higher speeds.

But do agrochemicals work as well when operators apply them at 20 to 30km/hr as opposed to 12km/hr? Does speed alter the droplet spectrum, droplet size, biological efficacy of the treatment or all three? Its a subject the industry is just beginning to explore and Chavtrac is one of the first .

On behalf of the company, Morley Research Centre has conducted trials with the Spra Coupe. Chavtracs Richard Price takes up the story: "Basically, the research shows that spray coverage on crops is as good, if not better, when the sprayer is travelling up to 32km/hr."

The experiment compared a range of spray volumes from 55 litres/ha to 210 litres/ha using different forward speeds with the Spra Coupe. The results were not taken to yield because disease failed to develop.

However, the fungicide deposits applied with XR 04 nozzles at different volumes and speed were compared on Riband wheat.

The speed of 32km/hr applied at the lowest volume of 55 litres/ha gave the greatest spray deposits on horizontal pipe cleaners placed in the crop. At 8km/hr there was a coverage of 600 microlitres/section of pipe cleaner compared with 1,300 when applied at 32km/hr.

The reason, concluded the researchers, could be a change in angle of spray penetration, droplet size or turbulence. They even went so far to suggest that there may be no fall off and possibly an increase in activity from fungicides applied at higher speeds. But there is no evidence yet to suggest this is the case.

Mr Price offers a scientific explanation for the results. "When the sprayer travels at a slower speed the agrochemical droplets are likely to fall close to the vertical – heading straight for the ground.

"However, at faster speeds of 20 to 30km/hr the spray droplets end up on a glide path of about 18 to 45í. So instead of the droplets passing, and impacting, on just two or three plants, they are likely to be intercepted by 10 or more plants. So there is a much greater chance of the spray being intercepted."

Clearly this has implications for cutting dose rates. "It is not company policy to suggest rate cutting. But there is no doubt that the sort of increases in deposition seen in these trials would allow growers to reduce chemical."

Higher spraying speeds allow operators to cover 245 to 325ha/day (600-800 acres/day), which is three times conventional practice. So there is the potential to apply more timely treatments, rather than rely on the less frequent application of more persistent products, points out Mr Price.

As an ex-aerial operator, Duncan Jack has a natural leaning towards speed. But then the experience has also equipped him with a greater understanding of the way droplet size, water volumes and pressure interact.

For the past three years spraying at Brakelands Farm, Swalcliffe in Oxfordshire, has been on the fast side – 18 to 20km/hr. "I have always noticed a better coverage on the leaf by spraying faster. But success rests on using the right jets and the right pressure," says Mr Jack.

He points out that spraying at 200 litres of water, at 20km/hr, could do more harm than good because the concentration of active ingredient per droplet would be diluted.

Water rates of 80 litres/ha for fungicides are typical on his 520ha (1,300 acres) arable farm where cropping comprises winter wheat, winter oats, winter barley, oilseed rape, linseed and grass seed.

"We push water rates up to 100 litres/ha where we have to travel more slowly," he adds. "But theres no doubt that there is less coverage."

But as you go faster, the pressure increases and produces smaller droplets so you have to be careful to avoid drift, points out Mr Jack.

"Watch for vortices. Try going fast with a conventional square tractor and you are likely to miss a lot of the crop behind the machine. An airstreamed shape and stable boom are crucial for faster spraying."

Conversely, Mr Jack believes that slowing down the sprayer produces bigger droplets, which could affect target coverage and spray efficacy.

But is faster spraying confined to bigger fields? "Our field size ranges from 4 to 80 acres, so it isnt really a limiting factor," replies Mr Jack. "We achieve 50 acres/hour with a bowser andsomeone else mixing"

But understanding how chemicals work is crucial, he says. Reglone is one example. "We use it with 140 litres/ha of water. But it must be applied at night or in dull conditions because the product in my view is light sensitive. Knowing this helps us make low water rates, and higher speeds, work."

The next stage in speedier spraying? "We need to have the ability to have a constant pressure of water and vary the amount of chemical according to speed. Direct injection systems are very nearly there."

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