An extra 185/ha return.

That is what, on average, north Norfolk punnet potato grower Mark Thompson believes his decision to speculate on Accumulator-treated Maris Peer seed has been worth.

Next planting season three-quarters of the 300 acres of pre-pack punnet potatoes ultimately destined for Tesco and Sainsbury’s will be treated with packer Greenvale AP’s seed treatment – up from 150 acres last season.

“We’ve steadily built up year on year as the evidence for its benefits has become more compelling.”

The effects of the Accumulator seed treatment, which is applied in-store by Greenvale, were obvious four seasons ago, when just 8ha (20 acres) were grown.

“You could immediately see the difference in stem numbers when [conventional and Accumulator crops] were planted next to each other.”

Typically Accumulator crops produce an extra couple of stems on average, which support more tubers per plant.

“As a generalisation I’d say there are around four more tubers per plant.

Every field is different, and it is difficult to assess.”

With extra tubers to support, the crop takes longer to mature, he says.

“Accumulator crops also tend to be more uniform.”

That helps with yields.

To meet supermarket spec Mr Thompson harvests when 70% of his crop by weight passes through a 42mm riddle, with the other 30% above that size.

“If you’ve got an uneven crop it only takes two or three big tubers to send it above 30%.”

But if all the tubers are a similar size it takes longer for the crop to reach that 70:30 split, allowing it to put more weight on, giving a higher overall yield, he says.

With the crop in the ground longer, growing costs increase, he says.

“Nitrogen inputs need more attention – you have to be a little bit cuter with what you’re doing.”

On successful Accumulator crops he will typically put on an extra 25kg at flowering.

There is also an increase in blight sprays.

“Normally it is an extra couple, but we don’t have to go any more frequently.”

Not all of the increase in inputs can be attributed to Accumulator seed, he stresses.

“While we’ve been growing Accumulator, supermarkets have changed their specification from a 35mm down fraction, up to the below 42mm size.

That has also had an impact on crops being in the ground longer.”

Using Accumulator seed hasn’t been a 100% success, he admits.

“We’ve had one or two disappointments where the seed has behaved like normal seed, but it is difficult to know quite why – it was not related to seed lot or planting date.”

On those fields the benefit of the treatment, which attracts about 50/t premium over conventional seed, has been neglible.

“But on successful fields it is very worthwhile – it is not usual to see large numbers of evenly-sized tubers.

On average the return has been about 185/ha net of the cost of the Accumulator.”

mike.abram@rbi.co.uk