New recommendations for weed control in potatoes

Weed control has been more challenging over the last two seasons and in response Scottish Agronomy have been trying new techniques with growers and conducting trials to find the way forward.

Scottish Agronomy’s Eric Anderson cites annual meadow-grass, black-bindweed, cleavers, fumitory, knotgrass and volunteer oilseed rape as weeds that now demand a more considered approach. "The good news is that with careful selection of product mixes – according to the weed spectrum in each field – and timely application, they can largely be dealt with," he says.

"Experience in 2009 – the first season without PDQ – also showed just how effective it was against annual meadow grass (AMG); it is now the main problem weed for many growers."

Last year Scottish Agronomy ran trials to assess herbicides’ efficacy against high AMG populations; dry spring conditions added further challenge. Best results were from Artist (flufenacet + metribuzin) at 2.5 kg/ha while the herbicides Defy (prosulfocarb) and pendimethalin proved very sensitive to lack of soil moisture.

Based on their trials work and the advisory team’s recent experiences, Eric advises growers to now aim for control with one contact + residual mix applied two to three weeks after planting.

"Post emergence options are very limited so the main thing is to go early. Allow the ridge to settle and time the spray when the largest sprouts are still 2 to 5cm below the soil surface. Growers with larger areas to cover need to start as early as possible to avoid getting caught by the weather," he advises.

"On heavier soils weed control should be fairly straightforward using metribuzin or Artist based programmes. In challenging AMG or cleavers situations the flufenacet component of Artist adds the extra residual activity needed.

"However, growers must take account of varietal and soil type restrictions; on light to very light soils and sands you need to be aware of soil textural variation that very often occurs within fields." Scottish Agronomy’s recommended residual herbicide options for 2011 are summarised below.

Situation Residual herbicides

Challenging AMG Artist 2.5 kg/ha + linuron* 1.2 L/ha
Challenging cleavers Artist 2.5 kg/ha
General broad-leaved weeds Linuron* 1.2 L/ha + 70% metribuzin 0.75 kg/ha
(+Gamit (clomazone) 0.25 L/ha where sporadic cleavers)
Light/v.light soils Defy 4 L/ha + 70% metribuzin 0.25 kg/ha
Sands Defy 4 L/ha + linuron* 1.2 L/ha

*Approved formulations of 500 g/L linuron

"AMG growth stage is likely to be the main consideration in contact herbicide choice," Eric continues. "For small AMG 2L/ha of diquat should be sufficient. If it’s reached three leaves, reduce the rate to 1.5 L/ha and mix with Shark (carfentrazone-ethyl) at 0.33 L/ha or, for the strongest AMG activity, consider Harvest (glufosinate-ammonium) at 2L/ha if you’re confident in getting six hours drying time post application."

His final word of advice is; "Don’t skimp! The extra cost of a robust mix is a very small part of the total cost of growing the crop; not getting good enough control will certainly come back to haunt you with horrendous problems at lifting."