13 December 1996

IPU should still be working – but check to avoid surprises

ISOPROTURON herbicides applied to cereals in early November should still be working well in most areas, says ADAS. But computer models suggest weed control on light soils and on some medium land in the south is likely to be slipping.

Much depends on how the active ingredient has broken down and leached, explains Wolverhampton-based Steven Bailey. A persistency model, based on temperature and soil moisture, suggests IPU degradation in November was much the same as normal in Shropshire, Devon, Sussex, and Suffolk. But in North Yorks estimated residues are slightly higher.

However, that is only part of the story. IPUs effectiveness falls as it gets washed below 4cm (1.6in). ADASs leaching model shows that on light land at all five sites, and on medium soils at Exeter and Gatwick, this has already happened.

Herbicides applied in the past three to four weeks are giving variable results, probably because of the cold which reduced weed and crop growth, says ADASs John Garstang. Continued checks on effectiveness are advised, but further treatments are unlikely to be needed before the new year.

Crops given a first full programme and showing some signs of weed control can have the gate shut on them until early spring, he says. "But a routine check will avoid unpleasant surprises."

If the winter is mild, a residual component such as IPU remains essential for fields not yet sprayed to prevent newly emerged weeds becoming too big. Wheat is less likely to be damaged than barley by spraying in adverse conditions.n

Andrew Blake