Nozzle selection and water volume can make a big difference to the results from autumn herbicide treatments, so choose carefully, urge specialists.

Depending on what you are trying to achieve, there are optimum combinations for both pre- and post-emergence spray applications, according to Syngenta’s Tom Robinson and Agrovista’s Mark Hemmant.

Syngenta doesn’t have extensive field knowledge for its new pre-emergence herbicide, Defy (prosulfocarb), says Mr Robinson.

But experience with pre-emergence sprays shows cloddy seed-beds can result in unsatisfactory control with any soil-applied product.”

That is because some spray is shielded by the clods and weeds germinate from underneath.

“In the application work we have done with Defy, we haven’t seen a difference in biological performance between water volumes of 100 and 200 litres/ha.

“So we are recommending either for this autumn, with a preference for the lower volume for the work rate benefit it brings.”
That can be up to 30% more output a day, he estimates.

“We’ve also seen a benefit on cloddy seed-beds from applying Defy through Hawk nozzles, which are inclined at 40 degree, pointing alternately forward and back along the boom.”

That is thought to be through better coverage of all clods sides.

To get the best control of blackgrass from post-emergence herbicide Hawk (clodinafop-propargyl + trifluralin), it should be applied at the two-leaf stage of the weed.

But two-leaf blackgrass is a small vertical target, stresses Mr Robinson.

“Trials and field use show that best results from Hawk come from applying it at 100 litres/ha through a forward facing Hawk nozzle.

“For twin-fluid atomiser sprayers, such as the Airtec, you should apply Hawk in 80 litres/ha, in as fine a spray quality as can be achieved without causing a drift hazard.

Agrovista has been experimenting to help growers fine-tune nozzle selection to autumn herbicides since 2002.

“Our work began in response to comments that results in fields never quite matched the performance of some products in small plot trials,” says Mr Hemmant.

“So we decided to test different nozzles and water volumes.”

The results showed that control from the same chemicals could be cut by 15% if the wrong combinations were used.

Best control came from 200 litres/ha through a flat fan for pre-em treatments and 100 litres/ha in a fine spray for post-ems.
Least effective for both applications were air-inclusion nozzles applying 100 litres/ha.

For the pre-em treatments Lo-drift nozzles gave better control, but neither was as good as using 200 litres/ha through flat fans.

The trials also showed including the adjuvant Grounded in pre-em treatments significantly boosted blackgrass control, irrespective of water volume, nozzle type or herbicide.

But the improved deposition achieved by the adjuvant reduced the differences between nozzles.

“In particular the performance of pre-ems using 100 litres/ha through the air-inclusion nozzle was similar to that of the Lo-drift nozzles,” says Mr Hemmant.