19 September 1998


Chemical choices for potato growers desiccating crops can be very limited. Lucy de la Pasture reports on an environmentally-friendly alternative.

MANY potato crops require desiccation to achieve skin set or to stop tubers going over-size, especially where small potatoes are required and the parent crop is immature. Reglone (diquat) can be ruled out where ground conditions are over-dry and, if the crop hasnt started to senesce, Challenge (glufosinate-ammonium) also isnt an option.

That has left sulphuric acid as the best alternative, killing plants very rapidly and preventing further increase of tuber size. That is until the advent now of another effective and non-chemical alternative means of desiccation – the Greenburner.

Sulphuric acid is already banned in most of Europe and will be reviewed here in 2002. With increasing pressure to reduce the amount of active ingredient applied to the crop, Sainsbury recognised the potential of the Greenburner and provided essential funding for the project.

Matthew Sillifant, of Drackedon Agriculture Ltd which is introducing the Greenburner, explains the new technology: "The system is based on latent heat transfer, and the crop isnt burnt directly. When the burner passes over the potato haulm the plant cells are heated to an extremely high temperature. A period of just 2sec at 70íC is enough to rupture cell walls and cause plant death."

Propane gas feeds a row of 14 burner jets located under a hood insulated with the same material used to protect the exterior of the NASA space shuttle. When lit the flames heat a titanium grill which covers the inside surfaces of the hood and can achieve operating temperatures in excess of 850íC.

"Once the crop has been burned the immediate effect looks like frost damage," says Mr Sillifant. "The crop will then remain green for 6-10 days depending on weather conditions. If it is sunny the chlorophyll will bleach faster."

Although the crop doesnt initially look dead, bulking of the tubers rapidly ceases.

"To prevent any re-growth of the crop after desiccation, it is essential to kill the nodes on the stem closest to ground level," explains Mr Sillifant. "Chopping the crop in advance of the burner makes the stem bases more accessible."

Ideally, the chopped potato haulm should be thrown into the tractor wheeling to ensure there is no green material left on the potato row or bed. "If there is any blighted material that has not been irradiated and it then rains, spores will be washed into the wheelings and not onto the tubers," says Mr Sillifant.

The Greenburner unit is able to accommodate single beds with widths up to 80in or two standard rows. Outputs average about 4-6ha (12-15 acres) daily.

One important advantage of this technique is the machines lack of downtime. When the wind is blowing or its raining and the acid sprayer is parked up in the corner of the field, the Greenburner is able to keep on going. "There is extremely efficient transfer of heat energy under the hood and the extra steam produced when the crop is wet seems to help the process," adds Mr Sillifant.

Suffolk grower Bruce Kerr, of William Kerr (Farms) Ltd, is impressed by the Greenburner he has bought this season. He believes results have been on a par with acid and in some cases skin set has been slightly quicker.

Some 340 ha (850 acres) of potatoes are grown on the farm in total, with salad varieties Charlotte and Nicola a speciality. A reduced reliance on contractors to apply sulphuric acid at the right time or on small acreages is one of the key benefits, according to Mr Kerr.

"The Greenburner gives me complete control over the function of desiccation – timing is critical with salad crops and double cropped varieties," explains Mr Kerr. "Having a sprayer parked up because its too wet or windy can be a disaster as being too late can result in the crop being over the specified size."

However, the Greenburner hasnt eliminated the use of acid entirely on the farm. It is now used more strategically on the ware potatoes where there are usually large acreages to desiccate at any one time.

The cost of operating the Greenburner with a front-mounted topper is in the region of £70/ha (£28/acre), comparing favourably with acid application which can top £92/ha (£36.80/acre) if contract applied.

Daily output is much less than for a sprayer, but well up to coping with the farms harvesting capability. "The Greenburner can operate seven days a week, for 16-18 hours a day provided ground conditions facilitate travel," adds Mr Sillifant. "The propane does need a rest period during the day to prevent freezing problems in the tank."

Purchase price of the Greenburner is £8,500. Growers also have a lease option which, for 1999, works out at £6.25/ha (£2.50/acre) on a 40ha (100-acre) potato farm with no capital outlay.

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