28 August 1998

Fine-tune spray use

Better weed control at lower

cost is possible by paying

greater attention to the spray

leaving the boom, particularly

with modern contact-acting

graminicides. Here we

provide some topical advice

LARGE numbers of blackgrass survivors mean growers need to use every weapon they can in the war on grass weeds this autumn.

The key factors for producers using modern contact-acting gram- inicides to consider are spray characteristics and timing, says Tom Robinson applications expert for Novartis.

Newer products give a greater response in final control to changes in spray characteristics than their residual predecessors, says Mr Robinson. "So its worth setting the sprayer up correctly.

"When using a contact material against blackgrass it is important to have droplets that stick. Blackgrass has small, spiky leaves. Coarse droplets that fall vertically stand little chance of hitting the target."

Operators should identify the days target before spraying, then fit nozzles that put the most droplets on, Mr Robinson advises. "Pinpoint accuracy is never possible, but tipping the balance of spray distribution certainly is. The ideal spray quality to use is the fine end of medium. I dont recommend fine sprays because of spray drift."

Fitting the correct nozzle for the job can improve blackgrass control from 90 to 99%, he says. "That 9% improvement gives a ten-fold reduction in the number of weeds. Achieving 99% control of 1000 blackgrass plants/sq m leaves 10 plants/sq m, while 90% leaves 100 plants."

UK sprayer operators have traditionally used conventional 110-04 fan jet nozzles running at 2.5 bar pressure and travelling at 8km/hr (5mph) to apply 200 litres/ha with contact herbicide Hawk (clodinafop propargyl and trifluralin). Mr Robinson says research shows these settings are fine, but increasing the pressure to 3 bar is even better, giving uniform coverage without excess drift.

Excess pressure needs avoiding. "Pressures above 3 bar may result in the spray cloud being easily moved by wind, and lead to uneven distribution."

Trials have also shown weed control is just as good using a spray volume of 100 litres/ha as 200 litres. "This way you can spray more acres/day and make sure you get the best timing – useful in autumn when windows for spraying are narrow. That is why we are enthusiastic about low volume spraying."

To achieve 100 litres/ha, Mr Robinson recommends either 110-03 or -02 fan jet nozzles. The -02 nozzles will deliver 100 litres at a forward speed of 8 km/hr, while -04 nozzles deliver 200 litres/ha at the same speed.

"In reality, with the advent of better sprayer suspension, people go at 12-13 km/hr. At these speeds, a -03 nozzle is a choice for 100 litres/ha at 2-3 bar.

"In experiments where we have measured spray volumes, lower volumes tended to put more product on the blackgrass. This was for three reasons – the finer spray increased retention, the higher concentration of chemical per droplet helped retention and because you get less run off at 100 litres/ha than at 200 litres.

"Airtec sprayers can go down to 70 litres/ha. The spray needs to be kept fine at this volume, but theres less drift with Airtecs. With air sleeve booms, use conventional nozzles of 110-03 or -02 and set the air to the minimum possible. Too high an air pressure can cause rebound."

Mr Robinson says the optimum boom height is 40cm. "Most people tend to run with booms set too high, which doesnt help drift. Silsoe Research Institute trials have shown that drift increases dramatically at boom heights above 70cm. This is because, at lower boom heights, a layer or turbulent air around leaves actually helps drag droplets into the crop."

As for adjuvant use, Mr Robinson favours those with high levels of surfactant, of 5%, to aid spray retention.

But avoid low drift nozzles, he says. They deliver a coarse quality spray. "With low drift nozzles, you can expect a 3-5% drop off in blackgrass control, slightly more with air inclusion nozzles. This isnt a problem if theres not much blackgrass, but I wouldnt recommend it for heavy infestation." &#42


&#8226 Aim for droplets that will stick.

&#8226 Fine-medium spray best.

&#8226 9% better control possible.

&#8226 100l/ha water OK.

&#8226 40cm max boom height.

&#8226 Adjuvants can help.

&#8226 Avoid low drift nozzles.

Modern contact-acting grass-weed killers demand greater attention to spray quality, says Novartis applications specialist Tom Robinson. Focus on droplet size, water volume and boom height, he urges.