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Pointers for weed control

1 August 2011

New pointers to help potato growers achieve top-notch weed control following the loss of the PDQ/linuron mix are coming to light in time for this season.

The loss of PDQ in summer 2008 and the halving of the maximum rate
of linuron before the 2009 campaign caught the industry off guard, says development agronomist John Keer of Essex-based Agrochemex.

“The combination provided cost-effective control of all annual grass and broadleaved weeds on medium and light land for nearly 30 years. Suddenly it was gone.”

Several products were rushed out to try to plug the gap, but little was known about rates, weeds control or crop safety, Dr Keer notes. He successfully approached the AHDB Potatoes for funding to carry out much-needed work to identify effective alternative products and combinations.

Crop safety and efficacy were assessed over six replicated small plot trials in the east of England in 2008 and 2009. “The key finding was that the replacement contact materials didn’t have the same broad spectrum activity or high level of efficacy as PDQ,” says Dr Keer.

“This means you have to be careful how you tailor them in a mix with other products to achieve the best control, especially of grassweeds.”

Three contact materials – Harvest (glufosinate), Retro (diquat) and Shark (carfentrazone) – were compared against PDQ.

On crop safety, Retro applied at 2 litres/ha at 40% emergence was no better or worse than PDQ. Shark was more aggressive, leading Dr Keer to suggest it should only be applied pre-emergence. Harvest gave a similar result to Retro but currently has no post-emergence recommendation.

All provided good control of broad-leaved weeds. However, to improve grassweed control, increased rates of Retro (3-4l/ha) were examined.  Although some improvement was seen, levels were still well below that expected from PDQ, Furthermore, Shark provided no control .

 “Satisfactory post-emergence grassweed control can only be achieved by tank-mixing these contacts with another product,” says Dr Keer.

Linuron at the new maximum 1.2 litre/ha rate proved a good start. “It contributed useful levels of grassweed control and is the best residual for polygonum control,” he notes. “But it was not a strong enough partner on its own – we needed something more in the mix.”

He tested several additions, including Artist (flufenacet + metribuzin), Defy (prosulfocarb) and Gamit (clomazone). While all proved useful where cleavers were a problem, they bumped up the price.

In the end, the most cost-effective early post-emergence mix to control a wide range of weeds was Retro (2 litres/ha) plus linuron (1.2 litres/ha) plus Sencorex (0.5kg/ha).

However, light land growers wary of Sencorex’s reputation for causing crop damage on light soils might prefer Defy. “I have spoken to growers on light soils who have never had a problem using Sencorex at 0.5kg/ha,” says Dr Keer.

“I believe it is quite safe at that rate on light land, but was not able to prove it last season as a result of the exceptionally dry soil conditions.” Given the general suitability of Sencorex as a partner for linuron, further work is needed, he believes.

“While you can’t hope to come up with all the answers in a couple of dry years, I do believe these results do provide good pointers for recipes in certain weed situations,” says Dr Keer. “It may cost £5-10/ha more, but growers can still get effective weed control at the early post-emergence timing.”

The exceptionally dry spring and early summer of 2010 accentuated the weed control problems in potatoes.  A return to some of the older selective contact herbicides may be inevitable.  The problem is that these are not always very selective and the tolerance data are not available for currently grown varieties.

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