As the blackgrass menace spreads and the cost of controlling it spirals, wheat growers need to ensure they are eking out the best possible performance from their herbicide programmes.

More reliance is being put on pre-emergence chemistry as once reliable post-emergence products come under increased pressure from resistance.

Actives are being stacked three, four or even five high to try to clear out as much blackgrass as possible to take the pressure off or, in some cases, even replace the final post-em overspray.

But it is not cheap – some growers are spending upwards of £100/ha on pre-ems alone. And big bills don’t always mean better results. In a good year, more than 80% control might be achieved on a field scale, but usually 60% is the norm and much less in a poor year.

Even in combination with good non-chemical control and a post-emergence follow-up, that can leave overall control far short of the 97% needed to prevent populations increasing.

Dry seed-beds are often to blame for poor pre-em performance. But application technique can also have a huge effect, even in relatively dry conditions, judging by the latest results from Agrovista trials on a heavily infested blackgrass site at Lamport, Northamptonshire.

While a fresh seed-bed looks an easy enough target to hit, results suggest otherwise. A few tweaks can make the difference between a field fit for silage and one that could stand up to scrutiny in any roadside field, says Agrovista technical manager Mark Hemmant.

“When Atlantis worked, no one took too much notice of the difference good application techniques could make. Now people are starting to pay a lot more attention to get the best out of the pre-ems they possibly can.

“In our trials, control ranged from 10-95% plus, simply by changing the application – everything else stayed the same.”

The trials examined the effect of nozzle choice, water volume, spray pressure, adjuvant choice and boom height on blackgrass control. All trials (apart from the last) used a boom height of 50cm. The chemistry consisted of Trooper at 2 litres/ha and Herold at 0.3 litres/ha, putting down 240g of flufenacet, 60g DFF and 600g of pendimethalin/ha (see panel for further details).

Nozzles

Air-inclusion nozzles struggled throughout the trials, especially at three-star Local Environment Risk Assessment for Pesticides settings, which were included to show the effect should such restrictions be imposed on blackgrass pre-ems.

Blue 03s Guardian Air nozzles, angled forward and backwards alternately along the boom, were tested at 1.5 bar and 10kph to deliver 100 litres/ha. “This produced the worst result of the whole trial,” says Mr Hemmant. “Control was about 10-15% at best.”

Doubling the pressure to 3 bar, more in line with on-farm practice, had some benefit. “This increased the number of droplets, improving soil coverage. There was more drift due to the higher speed of 14.5kph needed to maintain the 100 litres/ha spray volume, but control did improve, rising to about 60%.”

The effect of drift became more apparent when pressure was raised to 4 bar and speed to 16kph. “Control started to fall away – the droplet spectrum was too fine and turbulence was also increased, so fewer drops reached the target.”

Adding Remix, a paraffin oil made up of long-chain molecules designed to reduce spray drift, had a dramatic effect when added to the sprayer tank.

“It transformed control, boosting it to about 85%,” says Mr Hemmant. “Remix improves the spray pattern, giving a more even distribution of herbicide onto the soil, exactly as it is designed to do.

“Its long-chain molecules also help to bind the herbicide active to clay and organic matter, keeping herbicide in the surface layer for longer.”

The final air inclusion test used brown 05 nozzles to deliver 200 litres/ha at the three-star-rated application (1.5 bar, 8.5kph). “Control was much better than the 03s at 100 litres/ha, but still not very pretty.” Increasing the pressure had little subsequent effect on efficacy, he adds.

Flat fans

Much better results were obtained from flat fans. Even the basic setup of red VP 80-04 nozzles at 200 litres/ha (3 bar/10kph) angled straight down outperformed any of the air inclusion nozzles in the trial, producing about 75% control, says Mr Hemmant.

Halving the water volume using VP 80-03 blue nozzles reduced control by at least 20%, despite angling nozzles alternately forward 30deg and straight down to minimise shadowing of the spray by clods, he says. “We used the same pressure, but speed rose to more than 14kph. Going faster with the finer spray inevitable created more drift.”

Adding another line to the sprayer using the same nozzles, pressure and speed transformed the result. Front nozzles were inclined forward 30deg and the back line set vertical.

“This ended up doubling the volume to 200 litres/ha, but sprayed lots more droplets,” explains Mr Hemmant. “In this trial, we probably got more than 90% control. And because of the higher speed, we also increased work rate significantly compared with the red nozzles.

“We then took this a stage further and applied 300 litres/ha by pulling the speed down from 14kph to about 10kph. This result was incredible – the combination of extra water volume and the large number of droplets optimised seed-bed coverage, and we achieved almost 100% control from a pre-em only.

“Compare that with the control achieved with the air inclusion nozzles. The only difference is the application – and you don’t have to pay much to achieve it.”