Farmers affiliated with NFU Cymru have held a protest outside the Tesco base at the Royal Welsh Show.
The campaigners accuse supermarkets of stocking too much meat from outside the UK and going back on pledges made after the horsemeat scandal.
ITV News asked Tesco representatives for a response, but they declined to comment.
The Royal Welsh Show begins today at Llanelwedd, with warnings over the future of the beef industry here on the agenda.
Meat Promotion Wales says supermarkets and other retailers are stocking too much beef from outside the UK, having gone back on pledges to stock more British meat after the horsemeat scandal.
It says the amount received by farmers in Wales and around the UK has fallen meanwhile, with some paid up to £300 less per animal compared to last year.
The Welsh Retail Consortium, which represents retailers here, has responded by saying that beef prices are related to wider market conditions, and are projected to rise later in the year.
“Large quantities of imported beef products are now present in some UK retailers and we must presume – for want of a better explanation - it’s because it’s a penny or two cheaper than our own produce....The big danger for our industry is that some farmers will feel they have little choice other than to get out of beef altogether and that will be a tragedy, not only for our industry, but also for the consumer....Our message to the entire retail trade is simple - use our products or lose them."
Welsh farmers say they hope supermarkets will finally sit up and listen to their message at this year's Royal Welsh Show. The event, at Llanelwedd, gets underway tomorrow amid concerns that retailers are stocking less British produce than they did last year. Hannah Thomas sent this report from the showground.
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Final preparations are being made before the 2014 Royal Welsh Show gets underway at Llanelwedd, in Powys, tomorrow.
Organisers hope close to quarter of a million visitors are going to descend on the showground between Monday and Thursday.
Last year saw a new record attendance of 241,781. Visitors came from 40 different countries, and included new grandfather Prince Charles, and the Duchess of Cornwall.
Organisers the Royal Welsh Agricultural Society say this year's show is going to be just as impressive.
The livestock classes and special awards have attracted an unprecedented 8,000 entries, and there will be more than 1,000 trade stands.
Displays will include the King's Troop of the Royal Horse Artillery, with six horses pulling First World War saluting guns, to commemorate the centenary of the conflict.
A family from just outside Cardiff who have farmed their land for more than 200 years are fearful they could lose it.
The land comes under a development plan for 5,000 new homes in Radyr.
Now, three generations of the Rees family are questioning whether they have a future in farming at all.
Wales can set its own wages for farmworkers - that's the ruling of the Supreme Court today.
The Bill, which gives the Welsh Government the power to set minimum salaries and benefits was originally passed - but was halted by the Attorney General.
However the move has divided farming unions, as Kevin Ashford reports.
Mick Antoniw, the Assembly member for Pontypridd who was a lead campaigner on the Agricultural Wages Board hails the Supreme Court's decision as a victory for some of the lowest paid members of our society
Shadow Welsh Secretary Owen Smith has claimed that the UK Government wasted "huge amounts of court time and taxpayers money on a pointless court case" when the Attorney General referred Welsh farm wages legislation to the Supreme Court.
I congratulate the Welsh Labour Government for standing up for social justice and Labour values in Wales, by fighting to ensure that low paid agricultural workers get a fairer deal. This second judgement in favour of the Welsh Labour Government is a humiliation for the Tory led UK Government. So determined were the Tories to slash wages of low paid agricultural workers in Wales that they allowed it to cloud their judgement.
Mr Smith also said that the ruling shows the need for Labour's proposal that the Assembly is put on the same "reserved powers" basis as Scotland. It would spell out what the Assembly could not do.
Meanwhile the Welsh Secretary, David Jones, gave his reaction, concentrating on how the judges have rejected a narrow interpretation of the Assembly's powers.
The Government is grateful to the Supreme Court for providing clarity on this issue. In its judgment, the court has favoured a broader interpretation of the provisions in the Government of Wales Act that govern the Assembly’s competence.
We now need to study the judgment in detail and consider its implications. The Government is committed to working with the Welsh Government for the benefit of Wales and will continue to make every effort to ensure that the legislative arrangements for Welsh devolution work effectively.
Unite, the union that represents farm workers, has welcomed the news that agricultural wages will continue to be regulated in Wales, as they are in Scotland and Northern Ireland. The union also wants the system restored in England, where it was abolished last year.
This is wonderful news for thousands of low paid agricultural and rural workers in Wales. It demonstrates that the robust stand taken by the Welsh Government has triumphed and it will give us extra impetus in our campaign to get the Agricultural Wages Board for England restored as the impartial arbiter of agricultural workers' pay.