Boom height, nozzle choice and forward speed all have an effect on pre-emergence and early post-emergence weed control results, says Syngenta’s application specialist Tom Robinson, who stresses that it is possible to improve blackgrass control through application.

“The work we’ve done over the last two years has compared different nozzles in field situations, running them at different boom heights and forward speeds, to find out which combinations perform best on the pre-emergence spray,” he reports.

“It’s been a straightforward experiment, which was designed to give operators the practical advice they need to get the best from any herbicide application.”

Angled nozzles had already shown themselves to be superior in terms of coverage and minimising drift in previous Syngenta studies, he continues.

“We knew that the Defy nozzle worked well,” says Mr Robinson. “But we also knew that growers had an issue with the recommended boom height of 0.5m above the ground. It had proved difficult to adhere to, especially as they went faster.”

He points out that booms have got wider and sprayer technology has improved rapidly since the first Syngenta application work was started.

“Travelling at 12km/hour is considered slow today. And although there are better automatic boom height control devices now, forward speed can have quite an impact on spray quality.”

Four nozzle types – Amistar, twin outlet air induction, fan jet and Defy – were tested on a pre-emergence spray at a boom height of 0.5m above the ground, then again at a boom height of 1m.

The results (see table) showed that although the best outcome did come from the low boom height, it is possible to use a higher boom height with the Defy nozzle and still get good herbicide performance.

“All of the nozzles did better at the lowest boom height,” confirms Mr Robinson. “And it is still true to say that the optimum boom height is 0.5m. But there is some scope to raise it, providing you are using nozzles which aren’t prone to producing drift.”

The spray quality produced by fan jet nozzles is the finest, he explains. “So when they’re being used close to the ground, they work well. But any higher and their performance drops off rapidly.”

That’s why raising the boom height to travel faster can give disappointing results, he warns. “The final difference can be as much as 20% in weed control terms. It’s better to go steady and do everything right, especially where you have a high blackgrass population.”

Angled nozzles always did better, he continues. “Make use of angles, both forwards and backwards, to get the best spray quality. They’re least affected by boom height.”

His advice for this coming season for the pre-emergence treatment is to use Defy nozzles at 100litres/ha water volume, at a forward speed of 12-14km/hour and a boom height of 50-75cm above the ground.

“This advice also applies to the early post-emergence spray. The blackgrass is still a small target at this stage, so weed coverage is important.”

Seed-bed preparation must also be spot on, he ends. “The actual application of herbicide comes after everything else has been done to get maximum blackgrass control. Cultural control techniques should still be used to bring weed numbers down, just as adjustments should be made to the sprayer to get the best performance.”

Defy nozzle

The Syngenta Defy nozzle is a high-energy, narrow-angle fan jet nozzle which can be used pre- or post-emergence.

Producing half the drift of a conventional fan jet nozzle, it is inclined forwards and backwards at 40 degrees and was designed to allow growers to spray effectively at a 70cm boom height.

However, it can still be used down to 50cm and at forward speeds of up to 16km/hour.

“It was developed to give good performance without spray drift,” recalls Mr Robinson. “It came about when we wanted to improve the Hawk nozzle and found that the angle of inclination was important.”