14 July 1995

Latest weed-killer set to cure potatoes woes

A NEW potato herbicide which controls a range of problem grass and broad-leaved weeds is proving popular with German growers.

The light, sandy soils of Lower Saxony, Germanys potato capital, are infested with problem weeds, explains agronomist Friedrich Maykuss. Cleavers, fat-hen, polygonums, blackgrass and couch are abundant.

But chemical choice is limited. Atrazine was a favourite but has been banned for some years, as has paraquat. Linuron is no longer registered.

Standard mix has been Boxer (prosulfacarb) and Sencor (metribuzin) applied at crop emergence, which gives good broad-leaved weed control when applied to moist soils, says Dr Maykuss.

The problem is that soils are often dry at this time, which reduces efficacy. Boxer is quite "hot" on the crop, he comments. Bentazone (Basagran) can be used, but is also hard on the crop and not always reliable, he adds.

For these reasons, Cato (rimsulfuron), from Du Pont, was first used commercially in Germany in 1993. A popular method is a sequential treatment of Sencor at emergence followed 10-14 days later with Sencor plus Cato.

Control levels of up to 90% of cleavers and 70% of couch means "hotter" chemicals are no longer needed and it avoids the need to overspray with more expensive products, says Dr Maykuss.

The amount of Cato needed varies according to weed size and spectrum. A half rate is enough to control fat hen, but up to 50g/ha is needed for big cleavers and couch grass, says Dr Maykuss. A 30g rate sufficient for smaller weeds costs the same – £50/ha (£20/acre) – as the Boxer/Sencor spray.

With Cato at the higher couch rate the mix costs £68/ha (£27.50/acre). But this is still some £54/ha (£22/acre) cheaper than applying an extra grass weed herbicide , Dr Maykuss reckons.

&#8226 UKapproval is awaited