Spray technique key to effective grassweed control

The available pre-emergence herbicides are the most effective products for killing blackgrass in wheat, but poor application can slash their efficacy. Farmers Weekly reports on making the most of this crucial spray.

Growers could be achieving significantly improved control of blackgrass from pre-emergence herbicide sprays in winter wheat by fine-tuning application techniques.

Distribution agronomy provider Agrovista has run trials on a heavily infested blackgrass site near Lamport, Northamptonshire, examining the effects of nozzle choice, water volume, spray pressure, adjuvant choice and boom height on blackgrass control.

See also: Beat blackgrass by turning to barley

The firm’s technical manager Mark Hemmant says achieving the right combination can make the difference between excellent control and failure, even in relatively dry conditions.

“With resistance to post-emergence chemistry and pre-emergence programmes alone costing around £100/ha for many farms, coaxing the best from pre-ems has never been more important.”

In a good year 80% plus control might be achieved in the field, but often 60% is the norm and much less in a poor year. While dry seed-beds may be to blame, poor application can also have a big effect.

“In our trials, control ranged from 10% to 95+% simply by changing the application method.”

Nozzle choice

Air-inclusion nozzles struggled and applying a pre-emergence spray through a set of blue 03 Guardian Air nozzles at 1.5 bar travelling at 10kph to apply 100 litres/ha produced the worst result, delivering 10-15% control at best.

Trial details

  • Background blackgrass population – 400 heads/sq m
  • Crop – winter wheat
  • Drilled 28 September
  • Seed-bed – dry, firm, small to medium clods
  • Chemistry – 240g of flufenacet, 60g diflufenican and 600g of pendimethalin/ha
  • Applied 2 October at 50cm boom height
  • No useful rainfall until week beginning 17 October
  • No post-emergence treatments applied


Key findings

  • 300 litres/ha better than 200 better than 100
  • Flat-fan nozzle better than air inclusion
  • Twin line better than single
  • Boom height critical

Doubling the pressure increased the number of droplets, improving soil coverage and control improved to about 60%.

“The effect of drift really kicked in at 4 bar travelling at 16kph, with fine droplets and increased turbulence meaning fewer drops reached the target,” says Mr Hemmant.

Adding Remix, a paraffinic oil that reduces drift, had a dramatic effect, boosting control to 85%.

The final air inclusion test used brown 05 nozzles to deliver 200 litres/ha at the Lerap three-star rated application (1.5 bar, 8.5kph) and control was better than the 03’s at 100 litres/ha, but still poor.

Flat fans gave much better results. Even a basic set-up of red VP 80-04 nozzles at 200 litres/ha (3 bar/10kph) angled vertically, outperformed any of the air inclusion nozzles in the trial, producing about 75% control.

Halving the water volume using a VP 80-03 blue nozzle compromised control by at least 20%, despite angling nozzles alternately forward 30 degrees and straight down to minimise shadowing of the spray by clods.

“We used the same pressure, but speed rose to over 14kph. Going faster with the finer spray inevitably created more drift.”

Twin lines

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

“This doubled the volume to 200 litres/ha and also produced lots more droplets. In this trial we probably got over 90% control and because of the higher speed, we increased work rate significantly compared with the red nozzles.

“But by increasing the water rate to 300 litres/ha by reducing the speed to about 10kph the result was stark – the increased water volume and the large number of droplets optimised coverage and achieved almost 100% control from a pre-emergence only,” explains Mr Hemmant.

Hit new weed control heights

Boom height can have a critical effect on herbicide performance, especially when used without an effective adjuvant to help reduce drift.

Applying Liberator (flufenacet + diflufenican) at 0.6 litres/ha through vertical flat-fan nozzles (3 bar/10 kph/150 litres/ha) set at 1m above the soil gave a very poor 20% control, says Agrovista’s Mark Hemmant. Lowering the boom to 50cm hugely improved performance due to reduced drift.

The effect was less clear where a good adjuvant was used. Remix stood out at the 1m boom height, boosting control to 90%. It also gave a significant benefit at 50cm.