Higher doses of residual potato herbicide linuron will prove more tricky to replace than the contact herbicide, paraquat, according to John Keer of Agrochemex.

He had set up a Potato Council-funded project to compare residual herbicide mixes and the new contact materials “rushed onto the market” to replace PDQ following the removal of paraquat from the market.

A big proportion of the country had been using a 2.0 litres/ha linuron + 2.0 litres/ha PDQ mix for potato weed control for nearly 30 years, he said. “It is going to be a hard act to follow.”

In the trial at the site of the East Midlands Potato day the mix was the only one in the trial to give 100% control of typical silt land weeds, such as redshank, nettles mayweeds and knotgrass, he told growers.

John Keer

Polygonums like redshank will prove tricky to control with lower rates of linuron, according to John Keer

But with PDQ already removed from sale and linuron potentially to be restricted to a maximum 1.3 litres/ha dose growers were having to find alternatives, particularly as the trial demonstrated 1.3 litres/ha of linuron by itself was not strong enough on groundsel and redshank.

“You will need to add other residual partners, in particular for polygonum weeds.”

The problem was that all the potential options, Sencorex (metribuzin), Gamit (clomazone), Artist (flufenacet + metribuzin) and Defy (prosulfocarb) were even weaker against polygonums. “A common theme in the trial is redshank coming through.”

Linuron + Sencorex + Gamit was perhaps the best mix, he suggested. “I wouldn’t say Gamit’s strength is polygonums but it seems to be more than the sum of its parts.”

The second part of the trial compared the contact materials, Basta (glufosinate-ammonium), Retro (diquat) and Shark (carfentrazone-ethyl), available to replace PDQ.

Shark had the best activity on weeds, with only mayweeds coming through – an omission that could easily be taken care of in mixture, he said. “But the trial shows you cannot take liberty with late applications of it.”

Both Basta and Retro had left small nettles, and were no more detrimental to the crop than PDQ. “I don’t think there is much to fear from losing PDQ,” he concluded.