The downside of upsized sprayers

High sprayer speeds and the resulting hit and miss coverage don’t happen by accident. It’s a deliberate trade-off.

Bigger farms require heavier sprayers capable of travelling faster. Farmers know higher speeds reduce coverage, but they willingly make that sacrifice to spray all their acres on time within that all-important window.

It’s expensive when the window slams shut and rain moves in.

The window scenario is a logical argument, but it does not account for liabilities in the equation, according to spray specialist Tom Wolf of Agri-metrix Research and Training in Saskatoon.

The momentum of the big sprayer trend continues gaining speed, despite the heightened awareness of their negative impact.

Wolf said it’s a complex situation, comprising five components that have negative implications for the soil, the environment and quality of the spray job:

  • Fluctuations in spray pressure.
  • Spray drift.
  • Boom height.
  • Uniformity of product deposit on foliage.
  • Wheel tracks.

“Weight is a detriment in every ag machine except the tractor. The soil is taking a bad hit from us.”

Wolf predicts that prairie farming practices will eventually hit the wall with size, bad economics and an inability to function in the field because of excessive weight. The five chief factors should be taken more seriously, he added.

“Spray pressure is the single most important parameter that makes a nozzle work well,” Wolf said.

“We all understand that the faster you go, the more difficult it becomes to get your nozzles into the right pressure range.”

The topography of the glaciated Prairies forces farmers to run their sprayers at different speeds within a lot of fields. Some regions have big square flat fields, making it easier to do a good job of spraying. However, the terrain found on the rest of the Prairies creates more of a challenge for sprayer operators.

“If you want to push more liquid through a given nozzle because of higher ground speed, you need more pressure. Then you get to areas in the field where you need to slow down, but you can’t stop to make nozzle changes to match the terrain in the middle of a spray operation,” he said.

“You’re committed to a certain spray volume by virtue of what you’ve already put into the tank. You’re tied into a fixed situation, and that means you have to drive a certain speed in order for your pressure to remain within a certain range.”

The non-farming public pays a lot of attention to off-target chemicals.

Wolf said the scrutiny will not stop, which is why producers have a huge responsibility to prevent pesticides from landing off target. Every time there’s an incident, it gives more ammunition to the people who want tighter regulations.

“It’s not a question of whether or not a stray pesticide is harmful,” he said. “It’s simply the fact that it’s somewhere it should not be. That’s the whole issue in the eyes of the non-farming public.”

The old pull type sprayers with outrigger wheels could be set at 20 inches and were then good to go.

“Now we have suspended booms, and we simply cannot set them down at 20 inches because they’ll hit the ground and destroy themselves,” Wolf said.

“So we buy sophisticated boom height control systems. They work, but not 100 percent. And you’re still nervous about hitting the ground so you raise the boom to 30 inches. (Farmers) are telling me they just cannot go below 25 inches. It’s simply too risky. Well, I tell them it’s risky because they’re going too fast.”

Aerodynamics are a function of the sprayer’s ground speed. The faster the speed, the more air is disturbed with the boom, the tires and the tractor.

“Gary Moffat in Alberta has done a lot of work documenting the impact speed has on the quality of a spray job,” Wolf said.

“He’s finding all kinds of problems created by air displacement, especially by the tractor unit. Bigger machines and faster ground speed displaces more air and creates unfavourable turbulence. It pushes droplets to places they’re not supposed to go.

“If you want to apply a rate of one litre per acre of product on every square foot, there are going to be places where you’re putting down half that dose and other places you’re putting down one and a half times that dose. That’s not efficient.

“It’s hard to prove the negative economics of high sprayer speeds because the problem is masked so well by recommended herbicide rates that are a little too high to begin with in order to allow for such problems. Second, bad economics of fast spraying are masked by fantastic chemistry of the products. The chemicals really are a miracle.

“Until there’s a proven economic loss, the situation is not going to change. It’s a difficult thing to measure, but when I show farmers the graphs documenting deposit uniformity, they’re genuinely alarmed by what they see.”

Wolf said farmers will see worse wheel tracks and more crop damage and soil compaction as the trend toward bigger sprayers, bigger tires and faster speeds gains momentum.

“We need the agronomy industry to speak up and say, ‘hey, we’ve got wheel track issues, inconsistent spray coverage and soil compaction.’ These are issues we cannot overcome with our current line of equipment simply because of its size. We need people who have no vested interest to say those kinds of things. And those people are becoming more scarce,” he said.

“I’m asked by farmers all the time, ‘what can I do about wheel tracks?’ I answer in jest that they should get an airplane.

“Wheel tracks are a fact of life. Wheel tracks are there and they’re not going away. We can’t fix that problem. They’re just going to continue getting worse as the trend toward heavier faster machines gains more momentum.”

Farmers, along with sprayer and tire manufacturers, all want to believe that a silver bullet solution will be found in better tires and inflation management technology.

“That may help. We’re finally starting to see sprayers benefit from the low pressure tire technology we’ve had for a while on big four-wheel-drive tractors,” Wolf said. “

We’re seeing sprayers on rubber tracks, although tracks are not light. But it’s a move in the right direction in terms of reducing soil compaction.”

As well, the boom is located behind the rear tires, which means the wheels creates more air turbulence as speed increases. This disturbs the spray pattern and retards the ability of the product to land uniformly on clean foliage.

A front mounted boom eliminates that problem for three reasons:

  • The ability to see what’s happening eliminates mistakes.
  • Air turbulence does not disturb the spray pattern.
  • Operator fatigue is reduced, resulting in better overall performance of man and machine.

For information, contact Tom Wolf at Twitter @nozzle_guy or email at


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