Where's there's muck, there's brass for a long-established Norfolk family firm of agricultural engineers.

GT Bunning & Sons, of Gressenhall, has seen a boom in sales of heavy-duty manure spreaders.

One of the firm's largest machines, a three-axle spreader with 35-tonne capacity, has been exported to the United States. This first shipment took place just before Christmas.

Sales manager Chris Druce said the range, from 10.5-tonne to 35-tonne, was targeted at everyone from farmers to large-scale contractors.

The 12-tonne model, which was fitted with weigh cells and cost about �40,000, had proved popular; Happisburgh Farm Services had bought a machine, he said.

The company, founded in 1906 by Robert Bunning and named after his son, George Thomas, has specialised in manure spreaders since 1983.

While the range of distinctive blue trailers is known around the country, the spreaders are now being exported around the world.

Mr Druce, who joined the company a couple of years ago, is heading to North America next month to meet a distributor.

He said farmers were prepared to invest in the latest machines, fitted with load cells so that the right amount of manure could be applied to the land.

A demonstration of two machines, the 12-tonne and the 23-tonne, both fitted with weigh cells, was held at Kenninghall by agricultural engineers Ben Burgess & Co.

Farmer John Collen, from Gisleham, near Lowestoft, lent his 23-tonne spreader, fitted with a spinner deck, vertical augers and hydraulic doors that open and close depending on the type of muck to be spread.

Mr Druce said the application rate could be set automatically by the driver in the cab. A control box would dictate the speed of the spreader, which also computed data on the volume spread, width and total area.

With the increasing importance of measuring volumes of manure spread, sales of weigh cells had risen sharply in the past three years.

'We sold two of these machines in 2008, five in 2009 and 20 last year. Customers are taking it on board,' he added. And production of spreaders had gone from a total of 200 machines two years to 300 last year.

The even spread of manure, whether poultry or cattle, was essential. The density of the material was critical, so dry material might be spread 12 metres but wetter manure might be discharged across 36 metres.

'We're very busy, and I think the importance of spreading manure is being driven by the nutrient value of manures,' said Mr Druce.

He said GT Bunning & Sons, which employs 58 staff at its engineering base, had agents in New Zealand, Australia, South Africa and Canada.

'I'm off to North America next month to do a big show with our distribution company in Canada,' he added.

'We've got a good product line for spreaders, so we can built them cost effectively.'

Demand at the latest LAMMA Show at Newark had been tremendous, too.

'We had a phenomenal show: we could hardly cope with all the people coming on to our stand,' said Mr Druce.