18 February 2000

A six-point plan for weed-free potatoes

Remember how speedily

potato crops emerged from

warm, wet soils in 1999 and

how often pre-emergence

residuals went on far too

late? David Hudson, Sutton

Bridge technical director,

offers some timely tips for

the coming season

POTATOES nearly always recover from damage caused by mistimed treatment, says Mr Hudson. "But the delay in canopy development could cost you earliness, baker content and yield."

For cheap, effective weed control it is worth considering several questions, he says.

Is a pre-emergence residual worthwhile?

"The larger your potato area and the heavier your soil the more a pre-emergence residual makes sense. Mechanical weed control is only a real option on light soils."

Is it always worth waiting until just pre-emergence to apply residuals?

"No! You will usually be too late, and it is always more effective to apply residuals to settled, wet ridges. Dont miss these opportunities even if the timing seems well before crop emergence."

If ridges are really dry and cloddy is a pre-emergence residual worthwhile?

"No! But this will require skilful use of pre-emergence paraquat/diquat and careful assessment of likely weeds and suitability of the very limited range of post-emergence herbicides. If your variety is intolerant of post-emergence metribuzin (as in Sencorex), it will be safer to use the cheapest pre-emergence residual just to avoid serious or difficult weed problems. Other post-emergence contacts based on bentazone (as in Basagran) or rimsulfuron (Titus) should manage to at least suppress remaining problems."

When is it wise to tank mix paraquat/diquat with the pre-emergence residual?

"Just about always!"

As a guide, he suggests:-

&#8226 Weeds very hard to find: Use 1 litre/ha paraquat/diquat

&#8226 Weeds easy to find: 2 litres/ha paraquat/diquat

&#8226 Weeds big and/or dense:3-4 litres/ha paraquat/diquat

"Never miss a chance to control weeds with cheap pre-emergence paraquat/diquat even when earlier herbicide applications seem to be failing."

Are there any really effective herbicides for broad-leaf perennial weed control?

"No! Control these in other crops. However, you will get suppression of some perennials with bentazone and rimsulfuron, which could be all you need."

Can weeds be controlled with herbicides alone on peats?

"Generally no! Incorporating some metribuzin will make mechanical weed control more complete and timing less critical. Ridges will also be less prone to drying out because you should need less mechanical control."

2000 GUIDELINES

&#8226 Use the cheapest pre-emergence residual. (Note LERAPS, soil limitations and post-potato cultivations and cropping restrictions).

&#8226 Tank mix paraquat/diquat.

&#8226 Use incorporated metribuzin on peats.

&#8226 Apply pre-emergence residuals well before emergence.

&#8226 Use post-emergence treatments as required, but treat broad-leaf weeds when small. Allow grass weeds to achieve enough leaf area to absorb graminicides.

&#8226 Consider canopy development as part of weed suppression.

Weed-free potatoes can be achieved with a little forethought, suggests David Hudson.