0 Drift versus volatility when spraying your crop

driftSpraying on a quiet, still morning is the best way to cut your drift risk, right? (Answer: No!)

By Cg Production Editor, Ralph Pearce
Published: March 20, 2015

An interesting insight on the problems of spray drift:-

Just when farming already seemed complicated enough, here are more misconceptions that science is disproving. Except this time, the new findings will help more growers get more value from their crop protection dollar. The best part is, you're probably one of those growers too.

Drift - Physical

Trying to paint drift as worse than volatility, or vice versa, is overly simplistic, experts say.

To be specific, drift can be broken down into two forms: particle drift and vapour drift. Particle drift is the more straightforward form, and mainly occurs at time of appliction when a strong wind blows some of the spray off-target.

Vapour drift, on the other hand, is linked to volatility, the situation where a pesticide, once applied to a surface, be it leaf or soil, can volatilize given the appropriate temperature and humidity conditions.

"All pesticides can result in physical drift - it's just a physical property and it doesn't matter what pesticide you're using - all of them will drift to some degree from physical movement of the wind," says Dr. Andrew Thostenson, pesticide program specialist at North Dakota State University in Fargo. "But very few pesticides will volatilize, and we know this because we can measure the volatility of the chemicals, and predict or estimate their ability to gas-off or vaporize after they hit a surface. And there aren't large numbers of pesticides that volatilize."

Tighter windows and tank mixing different chemicals cannot override the need for diligence and attention to detail in spray applications.

Still, there are conditions that can make physical drift more difficult to cope with, especially in this era of larger acreages and tighter application windows. But even there, there can be misconceptions.

Dr. Tom Wolf, sprayer specialist with AgriMetrix Research and Training in Saskatoon notes that many growers suspect the best way to prevent physical drift is to spray when their isn't any wind.

Not necessarily, Wolf says. "We do need some wind," says Wolf. Just because a field seems to have no wind blowing across it, that doesn't mean there aren't air currents. Calm conditions eventually produce airflows, and with these variable winds during a spray application, it's harder to control whatever amount of spray remains aloft. "On the other hand," says Wolf, "with a consistent breeze, you know what's downwind and you can protect that area."

Inversion figures into this too. It often occurs early in the morning (but can come at dusk as well). What the farmer sees is the sun rising, the air perfectly still - supposedly an invitation to responsible spraying. Yet spraying into a still atmosphere with zero turbulence means there's zero wind to help distribute the spray, so a portion may be left suspended in the air. When the winds do pick up, there's a tremendous risk for damage.

It's all why Wolf discourages growers from spraying in low-wind conditions or inversion conditions. Instead, he says, they should spray with some wind.


Vapour drift is a greater concern because it's a less-known, harder-to-predict risk of many sprays. Historically, vapour drift was the more serious of the two, but only because in the past, farmers used more highly volatile chemicals, and volatility was always much harder to control.

"Vapour drift can occur from plants or soils days after spraying is finished, and it depends entirely on the weather that follows," says Wolf, adding that regulators in Canada have scaled back on registrations for many volatile products. "That's one of the reasons why we're no longer seeing the dinitroanilines and trifluralins for new crops, even though they would have a fit. There's just too much vapour. Also there's the deregistration of the higher-volatile esters of some of the products like bromoxonil and 2,4-D that have been gone for a long time now."

Thostenson agrees, citing glyphosate as an example of a chemical that's widely used and has a very low potential to volatilize or vaporize. Once it hits a surface, it won't gas-off under hot or humid conditions.

There are other products and different formulations of 2,4-D and dicamba that will volatilize, however, although a lot depends on the particular product and formulation in question.

"If you look at the new 2,4-D choline formulation (used in the Enlist technology), it has been reported to have a much, much lower volatility potential," says Thostenson. "That's not to say that the 2,4-D choline will not volatilize, it's just much better at resisting volatility."

So if we return to the question of whether drift is worse than volatilization, it becomes clear why the experts are saying it may be the wrong question to ask.

"Is one worse than the other?" says Thostenson. "No, all pesticides will physically drift, but some pesticides will physically drift and they can also volatilize or result in vapour drift."

GMO debate

In mid-January, Monsanto gained registration of Roundup Ready 2 Xtend soybeans, with expectations the company will have sufficient seed containing its dicamba-resistant trait for the 2016 planting season. There's a similar expectation for Enlist seed technology's 2,4-D ester technology from Dow AgroSciences.

The launch of these technologies has many growers hoping their lives get simpler, letting them address their weed control and their weed resistance management needs in a practical, reliable way.

Again, Thostenson and Wolf don't entirely agree. Thostenson cites glyphosate as a simple, non-volatile herbicide that replaced a variety of high-volatile pesticides that were used until the introduction of GMO crops, including phenoxy herbicides and dicamba. So the introduction of glyphosate tolerance actually reduced the amount of volatile pesticides being used.

"Now we're proposing to reuse some of these pesticides that we haven't used for perhaps 10 or 15 years on a broad scale," says Thostenson. This time, however, the difference is that the new products have lower volatility.

"Some of these phenoxy herbicides, are vastly different improvements over where we were using 15 or 20 years ago," Thostenson says. That said, however, he adds that "everybody is a little apprehensive because they think about the way it used to be, and they're wondering if that sort of problem is going to manifest itself with the introduction of these new-generation, genetically modified organisms that include these phenoxy-based herbicide resistance."

From his perspective, Wolf says there's another correlation between GMO technology and drift. He refers to it as "the Big Wake-up" when Roundup Ready was introduced in Saskatchewan in 1996. Prior to that, glyphosate was used primarily as a pre-seeding treatment while the land was still relatively dormant.

"Now we're spraying in the middle of the growing season, when everything's vulnerable, and that's when the wake-up began," says Wolf. He also started working with Monsanto, Bayer and other companies, trying to document how to mitigate the drift risk with coarser droplets. "That's how the whole movement to low-drift nozzles really gained traction and the air-induction technologies showed that these products could be applied safely with those types of nozzles. Those are still the first kinds of technology that people use to reduce drift."

But the GMO issue also bumps into shifts in weed species such as kochia, a very troublesome weed in Western Canada and parts of the U.S. Great Plains.

"Instead of just one kochia species, we now have three," says Thostenson, citing subspecies that are resistant to glyphosate and fluroxypyr (Starane). "What happens is that these resistant biotypes become a different genetic problem that we're having to deal with, which means we have increased our problems threefold in kochia. The resistance issue means that we're going to have more biotypes, which means we have more problems to deal with."

Do we have to go slower, smaller?

Against the backdrop of tighter planting and application windows, many growers are upping their sprayer speeds. But Wolf urges caution. Sprayer speeds are definitely a challenge, with both physical drift and vapour drift.

To Wolf, sprayer speed is among the biggest enemies of the modern spray operation.

"Aside from spray drift, which we can show increases with faster travel speeds, a lot of other things also become worse," says Wolf. "Wheel tracks become worse, uniformity of deposition from the boom becomes worse because of turbulence. And you have difficult situations in terms of pressure management - booms have to be higher for faster speeds - and all of those things work against the quality job that we're trying to achieve."

Wolf adds that if there's a reasonable wind but a fast travel speed, there's a large amount of wind being generated at the boom, and that removes the fine droplets that would otherwise be pushed to the canopy with the spray cloud. This allows them to drift free to move wherever they get blown, so a greater portion of the spray is at risk.

Thostenson admits slowing down may be impractical. In a perfect world, he says, it would be better if growers did drop their speeds, but it may be more realistic to focus on finding better windows for when and where to spray.

"If you slow down and you're much more prudent in your operations, there's much less of an opportunity for spray drift," says Thostenson, adding that it simply enables the grower to manage things better. Yet the "ideal" spraying conditions related to weather are limited, meaning there's a greater need to maximize efficiency with so few "good" days. "We also know that if you apply pesticides at extremely high ground speeds with an airplane - speeds in excess of 160 to 170 m.p.h. (roughly 255 to 270 km/h - you do get more breakup of the droplets and so there are more fine droplets available to be subjected to drift."

With ground applications of 24 to 32 km/h (15 or 20 m.p.h.), the same thing happens: the droplets break up more at that high rate of speed and so you get more driftable fines.

Impact of nozzles

A decade ago, Wolf started saying that the type of nozzle is the biggest factor in drift risk, whether it's the latest from Tee-Jett or an air-induction design. Instead, it was the operator's knowledge of water volumes, chemical makeup, weed species, air pressures and boom heights, among others.

That hasn't changed, Wolf says.

"What I have now is a renewed emphasis on communicating the exact spray quality that the producers are getting out of their nozzles," says Wolf, who's trying to develop some tools to help with that.

That means that producers need to get the numbers on their nozzles, which can take some digging through manufacturers' catalogues and other literature.

But he also urges growers to keep those numbers in perspective.

"Without exception, even the very best nozzle that's been designed to minimize fine droplets, in the hands of somebody who doesn't have it properly set up can make things very, very bad," cautions Thostenson. "It's absolutely critical, no matter what pesticide nozzle you're using that you use it within the parameters recommended by the nozzle manufacturer."

Article sourced from:- Country Guide

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