Potato growers can tackle hard-to-control weeds by selecting product mixes and timing applications carefully, trials by Scottish Agronomy, have shown.
The company trialled new techniques after the removal from the market of pesticides such as PDQ (diquat and paraquat) made weed control more challenging.
Senior agronomist Eric Anderson said annual meadow grass (AMG), black-bindweed, cleavers, fumitory, knotgrass and volunteer oilseed rape all now require a more considered approach. "But the good news is that with careful selection of mixes according to the weed spectrum in each field, and timely application, they can largely be dealt with."
Growers should aim to mix one contact product with one residual herbicide and apply two to three weeks after planting, the trials showed.
Anderson said: "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 2cm to 5cm below the soil surface.
"Growers with larger areas need to start as early as possible to avoid getting caught by the weather," he added.
Scottish Agronomy ran trials last year to assess herbicides' efficacy against high AMG populations - with the dry spring conditions adding to the challenge.
"Experience in 2009 - the first season without PDQ - also showed just how effective it was against AMG, which is now the main problem weed for many growers," said Anderson.
The best results were from Artist (flufenacet + metribuzin) at 2.5 kg/ha, while the herbicides Defy (prosulfocarb) and pendimethalin provide very sensitive to lack of soil moisture.
Anderson said: "On heavier soils weed control should be 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 within fields," he added. "AMG growth stage is likely to be the main consideration in contact herbicide choice."
WEED PROBLEMS RECOMMENDED SOLUTIONS
Situation Residual herbicides
Challenging AMG Artist 2.5kg/ha + linuron* 1.2 litre/ha
Challenging cleavers Artist 2.5kg/ha
General broad-leaved Linuron* 1.2 litre/ha + 70% metribuzin 0.75kg/
haweeds (+Gamit (clomazone) 0.25 litre/ha
where sporadic cleavers)
Light/very light soils Defy 4 litre/ha + 70% metribuzin 0.25kg/ha
Sands Defy 4 litre/ha + linuron* 1.2 litre/ha
* Approved formulations of 500g/litre linuron