11 September 1998

Switch tactics, not rate

MODIFYING herbicide programmes rather than reducing rates of active ingredients is the best way to cut costs this autumn, says an independent specialist.

"With non-resistant blackgrass it is possible to cut herbicide rates, but I wouldnt do it," says Berks-based agronomist Laurence Sim.

He reckons blackgrass is the biggest threat to current rotational policy, and as such, not a weed to take chances with.

His approach to tackling the weed starts with assessing where, and to what level, it was present in the previous crop, and even the one before.

Ploughing may bury the seed for one year, but the following autumn about 30% of that seed is viable if ploughed to the surface. Hence the need to look back two harvests.

In the absence of resistance, growers anticipating only low to moderate infestations – that is less than 5 plants a sq m in Mr Sims opinion – can dispense with pre-emergence treatment.

Last season Mr Sims standard in this situation was a single application of isoproturon at 5 litres/ha plus trifluralin at 2 litres/ha, applied post-emergence but before the three leaf stage of the weed. But because of some disappointing results he anticipates using more Hawk (clodinafop-propargyl + trifluralin) plus oil, instead.

"We saw some poor results last year. The mild winter broke the residual chemicals down more rapidly and rain caused substantial leaching," he notes.

Timing is crucial. Treatments are best applied before Christmas and in a resistance situation, treatment any later is a waste of time, he says. Growers can be confident that few if any blackgrass plants emerge after the most advanced weeds reach the three leaf stage.

If high infestation is anticipated, even in a non-resistance situation, pre-emergence tri-allate at 22.5kg/ha is advisable, he believes. &#42

Even with non-resistant blackgrass, cutting rates is a risk independent agronomist Laurence Sim urges growers not to take.