8 March 2002

Penetration poor if spray volume cut

CEREAL growers risk poor fungicide spray penetration and performance if they cut water volumes much below 100-200 litres/ha, warns BASF arable consultant Roger Allen.

Although sub-100 litre/ha applications increase work-rates, coverage of the lower canopy, of thick crops in particular, is likely to be reduced.

"Its no good coming down from the national standard of about 200 litres without getting the droplet size and pattern right."

Generally, 100 litres/ha is achieved using the smallest nozzle size, like a Lurmark 110í F12, at 2.5-3.0bar and travelling at 8-10kmh, he says. That combination produces a fine spray – average 120 micron droplets – with enough kinetic energy to penetrate well into the crop canopy.

But to get significantly below 100 litres/ha with the same size of nozzle the pressure must be dropped to 1.5-2.0bar and therein lies the problem, he says. "The result will be loss of spray penetration and quality – fewer droplets of the optimum size and more bigger, bouncy ones that are lost to the ground. This means the performance of most chemistry falls away."

Good spraying technique is essential to get the best from strobilurins such as Opera (pyraclostrobin + epoxiconazole), says Mr Allen. To obtain most from its curative activity – particularly against Septoria tritici – and fully exploit its physiological benefits, it must penetrate the crop well.

"Operas epoxiconazole component is upwardly mobile and has very good activity against Septoria, so it needs to be driven well down into the canopy in a mist of the correct sized droplets."

To get below 100 litres/ha, some growers use a coarser nozzle than the 110 degree fan type and a lower pressure than the normal 2.5-3.0bar. But spray cover would be better if they switched to the next nozzle size down, from a Lurmark F14 to an F12 for example, and raised the pressure 0.5bar to 2.5bar, says Mr Allen.

A common mistake, usually to avoid drift, is to set the boom too low, particularly for the T2 treatment. Unless it is 46-51cm (18-20in) above the crop the spray pattern and droplet size do not fully develop before hitting the target, he says. As a result, an excessive amount of the liquid is wasted because it runs straight to the ground. &#42