Technical manager Mark Palmer of crop protection consultancy Agrovista said: "The latest time for linuron application is pre-crop-emergence and the maximum dose is reduced to 600g, so it is very much a mixer product now.
"The residual herbicides available to go with it have to be applied pre-crop-emergence too - except for Sencorex, but that can't be used post-emergence on all varieties."
The Potato Council began a project last year to help growers plug the weed control gaps left by restrictions on linuron and the loss of paraquat, an ingredient in PDQ.
Last year's trials compared potential paraquat alternatives and looked at residual partners for mixing with linuron. The most promising materials will be further trialled.
Potato Council knowledge transfer manager Rob Clayton said: "In future, growers will need to tailor weed control more specifically to their own weed populations.
"They will need to be more aware of individual varietal differences in crop safety to different herbicides."
None of the alternative products tried last year was as broad spectrum as PDQ.
Potato specialist John Keer of Agrochemex, which is carrying out the trials, said: "Strengths and weaknesses need not be a problem if known and suitable mixing partners are identified."
The trials also showed the limitation of linuron at the new lower rate but results suggested that residual annual meadowgrass control could be bolstered by adding Artist, Sencorex or Defy.
A report of the Potato Council's trial is available to levy payers.