With post-emergence blackgrass sprays about to start on early-drilled crops, timely practical pointers on treating grass weeds were on the agenda at a recent HGCA/RRA weed management workshop, as Louise Impey reports

Small blackgrass plants are a challenging target to hit with spray applications, making them sensitive to application technique, says Clare Butler Ellis at NIAB TAG’s Silsoe spray applications unit.

“Their size and shape makes then awkward, especially when you consider that they are vertically orientated. Horizontal targets are far easier to hit.”

Small droplets have shown to work best, she adds. “Various studies prove that big droplets aren’t well retained on small targets. So nozzle choice is important at the early post emergence timing.”

Spray volumes also have a role, she continues. “As volumes are increased, you get less active ingredient on the target. There’s a declining relationship between deposit on the target and volume.”

However, it’s important to use the appropriate volume for the intended herbicide, she says.

Horizontal droplet movement is necessary for good spray deposits on vertical targets, such as small grass weeds, advises Dr Butler Ellis. “This can be achieved with wind speed but there are implications for drift. So a good alternative is to angle your nozzles.”

Angling gives droplets the required horizontal velocity, she notes. “It has been shown to give higher deposits and improved efficacy. Angling them forward is more effective than backward, as it stirs things up more.”

Forward spraying speed is another consideration, as it might increase deposition on small vertical weed targets, she says. “But it has other implications and the droplets tend to dissipate quickly at higher speeds.”

Looking ahead to spring herbicide applications, Dr Butler Ellis relayed the results of recent findings on penetrating a crop canopy. “There’s a common assumption that high volumes and large droplets are needed to get spray down to the bottom of the plant at the later growth stages. But that isn’t the case.”

Factors influencing penetration include crop structure and density, droplet velocity and trajectory and air movements around the canopy, she explains.

Two years of trials on the effect of volume on penetration shows that increased volumes aren’t needed to penetrate a cereal canopy. “There were no significant differences between 400 litres/ha and 100 litres/ha. Volume doesn’t help, particularly with small grass weeds.”

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Targeted applications

Good levels of volunteer potato control have been achieved in onion and carrot crops by using real-time image analysis techniques to apply herbicide to detected weeds. The rows help to identify which of the plants are crop and which are weed, allowing just the weeds to be treated. Cameras identify weeds as the tractor moves up the field, while specially designed nozzles ensure that a defined area is sprayed.

A prototype system has been successful in the field, giving good weed control and minimal crop damage, reports Dr Butler Ellis. “We’ve recorded 95% control of volunteer potatoes in onions and 75% in carrots and parsnips.”

Another project is looking at techniques for confining sprays to between the rows in oilseed rape. “Improved targeting is needed to help maintain existing weed control options – many of which are under threat – and to reduce the amount of chemical used,” she concludes.

“Unfortunately, the amount of crop damage in this project means that there’s still a great deal of work to be done,” said Dr Butler Ellis.

Automated weed mapping

A system that can scan whole fields and automatically create a weed map is to be developed following a research project undertaken at the University of Reading, with support from Masstock Smart Farming and the Technology Strategy Board.

Comprising a portable cab-mounted unit with cameras, GPS receiver and onboard computer, the system is expected to help produce application plans for treating weed patches.

Researcher Paul de la Warr believes it could replace manual mapping methods, done with either a hand-held GPS receiver or while combining.

“This development means that images will be captured automatically by machine vision during field operations. So far, effort has been on identifying black-grass.”

Work has proved that the weeds can be identified accurately, he adds. “That means it could be extended to other weed species that occur in stable patches.”