8 September 1995

No ideal alternative to IPU

RESTRICTIONS on the use of isoproturon-based herbicides could be tightened further if IPU continues to be found in water above the EU limit. ADAS is working hard to find ways to maximise its effectiveness should that happen.

It is hard to make a case against withdrawing IPU on the grounds of blackgrass control in the short term. But banning it completely would create long-term difficulties for UK cereal producers, says Jim Orson, ADASs head of cereal development.

"The big issue is resistance." The key point is that despite the availability of pendimethalin (Stomp) and more recently clodinafop (Topik), chemicals like IPU are needed to help slow the development of resistance in blackgrass and possibly wild oats.

Mr Orson says IPU is also needed to control several other weeds – meadow grass, for example – for which there is a usage which cannot be replaced by other weedkillers post-emergence. "IPU is also one of our most effective herbicides against brome."

Results from using it can be very variable, he admits. "It has its good and bad days." It is also clear that the control from a full UK dose alone (2.5kg/ha of active ingredient) is no longer as effective as it once was.

"In the early days, about 1971, we always got 99% control of blackgrass from an early post-emergence treatment. Now we average 85%. But we acknowledge that some of our trials are on difficult sites where the figure can be significantly lower."

Soil moisture key

Experience and extensive trials have highlighted factors affecting IPUs effectiveness. Soil moisture seems to be the key.

"Weve found that rain after application is increasingly important as blackgrass control gets more difficult." Without rain the chemical lies on the surface and can quickly break down to leave less than an adequate dose when it eventually reaches the weeds roots, explains Mr Orson.

But exceptional rain shortly after spraying can move the herbicide too far. The ideal is a consolidated seed-bed with just enough rain to take it to the blackgrass roots at a depth of about 13mm (0.5in). "Weve had some very good control where weve had about half an inch of rain in the week following application."

Early post-em not best

Contrary to trials in the 1970s, early post-emergence applications are not always the best. "We sometimes get better results from going in January, even on relatively large blackgrass. So think about the conditions," he advises.

To get the best from sequences of herbicides it is important to keep treatments close enough to avoid earlier ones running out of steam before the next, he adds.

"If you get the right conditions, IPU is very difficult to beat." Despite possible price hikes this season, its relative cheapness is a big advantage.

IPU is a key element in anti-resistance strategies, says ADASs head of cereals Jim Orson.