By Martin Cassidy BBC Northern Ireland rural affairs correspondent

Organic butter production is set to begin in Northern Ireland as farmers look for a market to soak up a growing milk surplus from dairy herds.

Farmer Rex Humphry from Coleraine in County Londonderry is one of six members of United Irish Organics which markets the milk from more than 600 cows.

For the past four years they have been running their farms on organic lines but for all the froth about organics, the supply of milk has outstripped the growth in the market.

The market here is reckoned to be just a third of 1%, and that compares with an estimated 3% in parts of Britain.

Organic herd are given less antibiotics and home-produced feed

"We were led to believe that there was an enormous market for organic milk which simply wasn't there," says Rex.

United Irish Organics members are currently producing 250,000 litres a month, but only 40,000 litres is being sold into the premium organic milk sector.

The rest is sold at regular milk prices which are around 20% less than for the organic product.

A two litre pack of organic milk currently retails for about �1.29 and that compares with a conventional price of around �1.03.

Rex Humphry says the market for organic milk is continuing to grow, but that there is still a long way to go to find a market for all the organic milk currently being produced.

We also use more home-produced natural feed and of course we don't use artificial fertiliser

Robert Shaw Farm manager

"A lot of our milk is going into the conventional market, but that is going to change. As soon as the cows go off the grass we will make butter," he says.

"That will take up another 20 to 30,000 litres a month and we are about to make a soft cheese and that will take up another 20,000 litres a month. It is gradually building."

This is a big week for Rex's herd. After a long winter inside, the cows are being turned out onto the lush spring pasture.

No artificial fertiliser is used and the farm relies on farmyard manure and slurry to help maintain soil fertility. Clover too plays an important role on organic farms.

The plant has special nodules on its roots which fix nitrogen in the soil. Rex says the organic rotation also includes wheat and beans which provide high quality winter feed.

Farm manager Robert Shaw is involved with the day-to-day running of the 160 cow herd.

I wouldn't go back to conventional farming even if there was no premium

Rex Humphry

"The big differences would be the lack of the routine use of antibiotics. We also use more home-produced natural feed and of course we don't use artificial fertiliser," he says.

The move to organic production has brought a change in the type of cattle being kept. This year's crop of calves are mostly from a shorthorn bull.

It is a breed which was common in Northern Ireland at one time, but which was largely squeezed out by the higher yielding Friesian and Holstein type cows.

Rex reckons that these crossbred Shorthorn/Friesian calves will suit the organic system with its reliance on feed produced on the farm.

Despite his disappointment over the relatively slow growth in demand for organic milk here, Rex has no regrets about going organic.

"I wouldn't go back to conventional farming even if there was no premium. It is a much nicer way to farm," he says.