Application advice for blackgrass weed control

How quickly do you drive the sprayer when applying herbicides? What height is the boom? Are you using the best nozzle? Are the nozzles angled forwards or backwards?

These are all questions a sprayer operator should be asking before applying any part of the blackgrass herbicide programme.

With the declining efficacy of most herbicides within the programme as enhanced metabolism resistance really starts to bite, concentrating on the finer nuances of application is becoming ever more critical.

And it is something Agrovista has concentrated on more and more at its 4ha dedicated blackgrass trials site at Maidwell in Northamptonshire.

Most of the work, understandably, has been concentrated on maximising the performance of Atlantis, or its liquid counterpart, Horus (both mesosulfuron-methyl + iodosulfuron-methyl-sodium).

Efficacy from the chemistry has started to decline with the rise of enhanced metabolism resistance, in particular, while last year’s slow autumn growth and cold, dry spring did not favour good control.

Agrovista’s research started by comparing nozzles spraying Atlantis, says the firm’s Mark Hemmant. “A blue flat fan nozzle angled forward at 30°, spraying at 150 litres/ha water volume at a forward speed of 10kph has proved most consistent in the trials over the years.

Agrovista post-emergence application advice

Nozzle: Blue flat fan angled forward 30°
Speed: Preferably 10kph
Boom height: 50cm above target
Adding Companion Gold can help increase speed and lessen boom height efficacy loss 

“In any given year another nozzle might be best, but that one has been most consistent.”

For example, in one year’s trials a yellow flat fan and the variable pressure Hawk nozzle worked more effectively. Both produce a finer spray quality than the blue flat fan. Low wind speed at application was critical to their success, Mr Hemmant says. “The next year, they were less effective when it was more windy at application; it was too drifty.”

Too much drift, particularly with finer sprays, means the chemical fails to reach the target. But a little bit of drift can also be useful for efficacy, Mr Hemmant reckons. “Some movement at the target level, particularly with flat fan nozzles, can increase the dose that hits the target increasing weed control.”

Managing drift, however, has to be a priority for sprayer operators, he stresses. One easy way to reduce drift is to use an air-inclusion nozzle, but the Agrovista trials quickly showed the coarser spray quality produced poorer control.

Slowing down will reduce drift, and increases grassweed efficacy. “Our trials suggest a 5% drop off in going at 14kph and 100 litres/ha than at 10kph and 150 litres/ha.”

But adding the firm’s drift retardant, Companion Gold, to Horus, the liquid version of Atlantis, has produced comparable efficacy with the faster speed and lower water volume, he says. “But you can only mix it with Horus, not Atlantis, and you still need the Biopower adjuvant.”

This year, the firm has investigated in more detail the interaction between boom height, nozzle choice and efficacy.

“Paul Miller at NIAB TAG has shown that raising the boom from 50cm to 70cm increases drift four-fold and tenfold if you’re spraying at 1m.”

But not much research has been carried out on what that means for efficacy. To find out Horus + Firebird + Biopower was sprayed using either a blue flat fan or the Syngenta Defy nozzle at three boom heights.

The results with the blue flat fan, sprayed at 14kph and 100 litres/ha water volume, suggested a 5% drop off by raising the boom from 50cm to 70cm, and 8% to 1m.

A similar drop off was recorded with the Defy nozzle, sprayed at a similar speed and water volume, when raising from 50cm to 1m, but control was maintained at the 70cm boom height.

“Syngenta brought out the Defy nozzle, which is a variable pressure Hawk nozzle but with a fan angle of 80 degrees that also angles the spray forward 40 degrees, for growers to use where they struggled to get the boom height down to 50cm,” Mr Hemmant says.

“Reducing the fan angle reduces drift, which is probably why there is no drop off in performance at 70cm boom height.”

The trial also included Companion Gold with each treatment, and as with forward speed, its drift reduction properties helped minimise efficacy drop off at higher boom heights. For example, with the blue flat fan there was only a 1% loss in control at 70cm, while with the Defy nozzle there was no drop off from moving to 1m from 70cm.

“Companion Gold evens out the differences in efficacy between boom height and nozzles.”

Agrovista has also looked at whether the direction a nozzle is angled has any effect. Some previous research has suggested that angling can help coat the target more effectively. This year’s trial compared different configurations when applying Horus + Biopower in April with a blue flat fan.

The results were slightly surprising. Angling 30° forward using a cap improved control by about 10% over the normal configuration, while alternate forward and backward or just backward made no difference.

But the best configuration appeared to be alternating a normal positioned nozzle with ones either forwards or backwards. That added 20% to control over the normal configuration.

“It might be that configuration creates some movement of the spray at the target level. But it is the first time we’ve tried this, so we need to see whether it is repeatable.”

Residual sprays

It is just as important to apply pre-emergence sprays effectively, Mr Hemmant stresses.

The key with that timing is coverage, he says. “What is important is the space between droplets, so you get good coverage of the soil.”

Usually increasing water volume is the best way of increasing coverage, but for time-poor sprayer operators chasing a small window to spray pre-emergence treatments, cutting water volumes helps speed up the operation.

“Reducing water volumes can give good coverage, as long as at the same time you make the spray quality finer to increase numbers of spray droplets.”

Low water volumes of 100 litres/ha can give perfectly good coverage, in combination with a finer spray, he notes. “But it has to be balanced against the risk of drift, which can have a big influence on performance as there isn’t any crop at the pre-emergence timing to intercept the droplets, and hold them where they are targeted.”

Previous years’ trials have found that a flat fan nozzle has provided the best efficacy from a pre-emergence spray. “An air inclusion nozzle can give poorer efficacy because it gives bigger droplets and at low water volumes, in particular, it can mean there are large gaps between droplets letting blackgrass through.

“But it is a very practical nozzle for growers to use at an exceptionally busy time of year when weather can be breezy. So we looked at how we could make it work, and found that by adding an adjuvant, Grounded, we could equal efficacy of the flat fan.”

As with post-emergence sprays, the impact of drift on pre-emergence was also investigated, comparing a blue 110° flat fan nozzle applied at 3 bar with the Syngenta Defy nozzle at 2 bar, which has a fan angle of 80°, at three different boom heights.

The same flufenacet + diflufenican treatment was applied in all six treatments at a forward speed of around 14kph in 100 litres/ha of water. The nozzles were alternately angled forwards and backwards.

The results suggest boom height is more critical with pre-emergence sprays. Whereas with the post-emergence trials, drop off was only 7-8%, the difference between 50cm and 1m boom height with the blue flat fan in these trials was 20%.

The Defy nozzle also clearly gave more control at all relative boom heights than the blue flat fan, but again the there was a clear 10% difference in control as boom height was raised to 1m.

“The Defy nozzle looks interesting for a pre-emergence spray, but it is our first year of trials, so, again, we need to confirm the results,” Mr Hemmant says.

Pre-emergence advice
• Nozzle: Blue flat fan or air-inclusion nozzle plus Grounded adjuvant
• Boom height more critical than at post-emergence
• Defy nozzle promising

Agrovista trials spray techniques

Agrovista’s application trials are applied using either a spray boom mounted on a quad-bike or, for the first time last season, a purpose-built plot-sprayer.

The idea is to mimic farm practice, while still being able to do replicated and statistically valid plot trials, Mr Hemmant explains.

The quad-bike sprayer was able to replicate on-farm sprayer speeds, allowing the firm to use standard farm water volumes, pressures and nozzles, in contrast to normal small plot work, which typically only allows plots to be sprayed at 3.6kph.

That approach has been taken a stage further with a modified Agribuggy sprayer being developed which allows the firm to spray four different application treatments at once through a 12m boom split into four 3m sections.

“It means for each boom section we can vary pesticide, pressures, nozzles and nozzle angling. The only thing we can’t vary is forward speed for the four different treatments.”