Controlling weeds before potato crops emerge will be a key part of herbicide strategies following the withdrawal of the widely-used paraquat, experts advise.

Unlike traditional PDQ (paraquat + diquat) mixes, most of the main alternatives have to be applied pre-emergence of the crop and need to be targeted more closely to specific weeds, says independent agronomist, John Keer.

“There are other good reasons for spraying a bit earlier you will get more help from the residuals in controlling grassweeds if they are still small or yet to emerge,” he says. “Also, soil conditions can be chosen to enhance residual performance as opposed to panic-spraying under poorer soil conditions just to get on before the crop gets too big.”

There are three main alternatives to paraquat that could be used alongside residual partners, such as metribuzin or linuron. These are Basta (glufosinate-ammonium) from Bayer CropScience, plus Retro (diquat) and Defy (prosulfocarb) from Syngenta.

“Basta is not a direct replacement for PDQ,” Bayer’s Neil Thompson says. “It will control weeds that are there, but it needs to be applied pre-emergence of the crop.” The product offers one of the broadest spectrums of weed control, with efficacy against 30 broadleaf and seven grassweeds, he says.

For the full spectrum of grass and broadleaved weed control, including larger grasses up to four leaves, he recommends the full 3litres/ha rate, whereas 2 litres/ha gives good results against broadleaved weeds and annual meadowgrass. “When tank mixing with residuals, such as metribuzin, a lot will probably go with the 2 litres/ha rate.”

Like Basta, Syngenta’s Defy also has to be applied pre-emergence, the firm’s Simon Parker says. “In our trials, 4 litres/ha Defy, plus 0.5kg metribuzin or 1 litre of linuron worked well, particularly against cleavers, fumitory and black nightshade.”

In contrast, the new contact herbicide, Retro, can be used in a similar vein to old PDQ mixes, he says. “It can be used up to 10% emergence for earlies or 40% emergence for maincrop. It’s the only PDQ alternative that’s got that flexibility.”

Maximum dose rate is 2 litres/ha, although a wetter should be used when mixed with other products, he says.

Paraquat withdrawal

• Growers have until 11 July 2008 to use up paraquat stocks
• Range of alternatives available, but need accurate timing
• Pre-em applications more important
• Robust residuals needed
• See Crops, 8 March for more PDQ alternatives advice