Without fines, there’d be no drift. Worries about nearby cotton, surface temperature inversions, low or high wind speeds, downwind buffer zones or stories on the ABC would be a thing of the past, says FMC’s technical extension specialist, David Johnson.
“So, using an ultra-coarse (UC) spray quality with FMC’s drift reduction adjuvant, ‘On Coarse DRA’ in the tank, seems the obvious point to start a spray plan.”
Dr Johnson said a ‘fine’ is by practical definition a spray droplet that drifts –but if a droplet is large enough to get directly to ground from the nozzle, then it cannot move out of your control.
“With the boom at 1m or more above the ground, a spray droplet needs to be at least 200 microns in diameter to get directly to ground. If too much smaller than this, then its trajectory will be horizontal before reaching the ground making where it eventually ends up subject to the whims of the wind.
“And what’s more, it only takes a thermal updraft of just 2.5 km/h to counteract the fall rate of even a comparatively large 200µm droplet.
“So rule number one – drop the boom from 1.0m to 0.5m to halve the at-risk volume – not only is the safety of the ground that much closer to the droplet’s origin point, now all of those droplets between 150 and 200µm are big enough to get to ground directly.
“If you set up a boom at 1m height with 02 nozzles at 4 bar to travel at 20 km/h targeting a very coarse spray quality (VC) – the minimum spray quality needed to apply 2,4-D according to label, then between 3.3 to 6.6L out of that 55 L/ha is at a big risk of drift.
“Spraying this way with 800 ml/ha of a 625 2,4-D amine can make enough drift to ruin up to 85ha of cotton. This bears repeating – the fines made by spraying just 1ha of fallow according to the 2,4-D label, is enough to wreck 85ha of cotton.
“But it gets even worse.
“In the real world, it is very easy to turn VC into medium (M), or UC into VC, just through ‘tank mix effects’.
“According to FMC’s extensive wind tunnel testing at UQ’s CPAS facility at Gatton campus near Brisbane, a tank mixture of 3.8% Roundup Ultramax and 1.6% of Amicide Advance 700 in 2% Liase, is sufficient to fine up most low-pressure air-induction nozzles from ‘very coarse’ to a ‘medium’ spray quality because of impacts on the spray’s behaviour.”
Dr Johnson said even high-pressure air induction nozzles commonly go two categories finer as well.
“That means instead of ‘only’ about 6.6L of potentially driftable fines emitted from the 55 L/ha spray mix at a 1m boom height using what is thought to be a ‘very coarse’ spray, an applicator can be inadvertently putting out almost double that – 11 L/ha at a droplet size subject to the whims of the wind.
“That’s potentially enough 2,4-D to damage 142ha of cotton!
“The ‘fining up’ effect of the nozzle behaviour arises through the action of the wetters in a lot of formulated products. Wetters act on the dynamic surface tension (DST) of the droplet to have their adjuvant effect. Unfortunately, this property also affects the way spray droplets are formed, promoting a lot more fines.
“After boom height, this makes UC the obvious next consideration in any spray planning for 2,4-D application. And when paired with On Coarse DRA to ensure even fewer fines and better retention of larger droplets on weed leaves, you’re well on your way to a spray plan for controlled effectiveness.
“With an UC combined with On Coarse DRA, only about 0.7L out of the 55 L/ha, is applied as a fine from a 0.5m boom height which greatly diminishes drift.
“Even if you’d simply used On Coarse DRA with the original VC nozzle at 0.5m, then the ‘effective’ nozzle rating comes back from M to a genuine VC, reducing the drift risk faction from 11L to only 2.4 L/ha out of the 55L applied. A big improvement in 2,4-D control.”
Dr Johnson said On Coarse DRA is an adjuvant formulated around the active ingredient guar. It counteracts the property of wetters that change the fan break-up pattern at the nozzle orifice.
“It does not act on the DST for its effect, instead modifying the visco-elastic properties of the tank mix, making it less favourable to form a fine, while also acting like a ‘shock- absorber’ on leaf impact, enhancing droplet retention on the plant.”
“So why is there so much reticence about UC nozzles? Because of concern about an apparent loss of efficacy it might bring.
“So, let’s consider more closely some of the ‘finer’ points of UC plus On Coarse DRA, and why it’s a good platform for spraying with controlled effectiveness as listed below:
- Using On Coarse DRA, more than 99% – or 54L of our 55 L/ha, gets to the target under control, compared with as little as 44L from a ‘VC’ nozzle
- 99% of droplets have some vertical trajectory avoiding more capture by standing stubble
- a shorter path to ground so less opportunity for droplets to get smaller through evaporation while in flight
- fewer droplets detraining from the spray fan by wind or speed of the tractor
- less impact from boom and tractor wake effects
- Recent efficacy trials conducted by the GRDC showing knockdown efficacy of up to 97% (from extremely coarse) against small leafed annual ryegrass – a target usually cited as the reason to go finer with spray quality. This proved it is possible to get very good results with extremely coarse nozzles, even on hard to kill weeds, while at the same time effectively managing spray drift.”
Dr Johnson said some general rules-of-thumb with coarser spray qualities are:
- increase spray volume by either a higher spraying pressure and/or lower speed. Using On Coarse DRA gives you more pressure options to achieve this without breaching the VC minimum
- optimise water quality – if pushing the efficacy envelope by using a coarser spray quality then water that’s too hard, or full of bicarbonates can easily cost you another 20% efficacy, so use rainwater, reverse osmosis-treated water or add SOA.
“Also, consider if you have On Coarse DRA in the tank mixture, you can get the effective benefit of a lower boom height because it is taking a lot of fines out of the equation AND it is overcoming the fining up effects of the tank mixture.
“For example, if you were spraying unintentionally with a medium spray quality and a 1.2m boom height, with On Coarse in that mixture you’d get a genuine very coarse spray quality – the same effect on downwind drift risk as dropping the boom from 1.2m to about 0.6m
“Using UC spray quality with On Coarse DRA, lowering boom height and slowing down – are the fundamentals of spraying with controlled effectiveness – and reduced drift.
“Simply put: Fewer fines = less trouble.”
