5 April 1996

Beat resistance by autumn spraying?

SWITCHING from spring to autumn spraying can help counter herbicide resistant blackgrass, a trial in Essex suggests.

The Ciba Agriculture experiment, at Elmdon, was set up 10 years ago, originally to test spring-applied broad-leaved herbicides in cereals.

In order to avoid complicating control assessments, grass weeds have always been tackled in the spring with contact materials, explains the firms Andy Pigott. For the first few years Hoegrass (diclofop-methyl) was used, but Topik (clodinafop) then took over.

Resistant blackgrass

"Three years ago we noticed a small patch of blackgrass building up," says Mr Pigott. "The control from spring-applied Topik dived." Tests confirmed that plants in the patch displayed the enhanced metabolism type of resistance.

To assess the impact of alternative tactics, partial tramline treatments across the patch have been tried for the past three seasons. The comparisons involved autumn-applied Topik, autumn-sprayed Hawk (clodinafop + trifluralin) and spring-applied Topik.

Encouraging results

The results are particularly encouraging, Mr Pigott believes. The two autumn treatments seem to have given very good control, whereas blackgrass density appears to be increasing in the spring-treated area.

The trial reinforces Cibas view that for maximum effect from clodinafop (on any blackgrass population), the material must be used early – though not before the weed has at least one leaf unfolded and is preferably at the two/three leaf stage, says Mr Pigott.

"It shows that by a simple change in the timing weve cured the problem."