It's an annual tradition whose history traces the evolution of East Anglian agriculture – as well as setting the tone for the rental value of the region's grazing land.

But it also marks a pivotal day in the calender for the family firm which has overseen it for more than a century.

The marsh lettings auction, run by property agents Durrants at The Bell in St Olaves, between Great Yarmouth and Beccles, will give graziers the chance to secure grassland to use throughout the summer months.

This year's event, from 1pm on March 30, will make 900 acres available for let on a seasonal grazing licence from April 1 to October 31.

Director Nick Durrant is the fifth generation of his family to be involved in the lettings. He said his father George Durrant, 88, had given him first-hand accounts of how the auctions had changed over the years.

'Between the wars all the dairy farms around here were small dairy farms, of about 20 cows,' he said. 'The cows were housed on the uplands adjoining the marshes and they would come down here to graze during the day.

'When the number of cows at the farms grew bigger, it was no longer practical to take them down to the marshes, because it was too far to walk. That was before livestock transport, so logistically there was only so far you could walk dairy cows between milkings.

'Once livestock lorries became the norm, then people from further away could come and rent these marshes. That is what we still find now.'

Mr Durrant said bidders would travel from as far afield as north Kent and south Essex – and the competition could often become intense.

'There is obvious rivalry, because there are certain farmers who always have the same marsh and they will pay whatever they need to keep it – because they have their own traditions and their own way of doing business,' he said.

Nick Rudge, another director of the company, will be the auctioneer on the day. He said: 'There will be over 50 people here. It will be standing room only.

'It is tradition. It sets the tone and sets the market rent for that year for a huge number of private lettings.

'We have been letting marshes here since the early 1900s in this location, but we don't know exactly when we started. We let marshes from the Swan pub in Norton Subcourse in the 1880s. The pub is no longer there, but that is the earliest we can trace letting marshes back.

'There are fewer dairy cows now, as the number of dairy herds is reducing. It is principally beef cattle on there now, but quite a lot of the marshes are able to be grazed by horses, so that land will make up to half as much again.'

Mr Rudge said the ownership of land had changed as well.

'Ten or 15 years ago we probably had 500 acres, and there would have been 12-15 owners,' he said.

'Now there are eight or nine people owning 900 acres. On the land we let, very few are owned by working farmers. Most are bought by people who want to let them out for income, and they invest in them over time.'