The EU agriculture commissioner has said that Welsh farmers have a "better deal" as part of Europe.
On a visit to Wales, Phil Hogan said subsidies provided from Brussels represent a "safety net income for the farmers of Wales which is appreciated".
Support for farmers accounts for 40% of the EU budget with £240m in direct payments to Welsh farmers in 2014.
UKIP said it was a "diminishing slice of a crumbling cake, paid in a dodgy currency".
In some cases, European Union support can make up over half a farmer's income with reliance on subsidy three times higher in Wales than in England.
"This money is guaranteed between now and 2020," Mr Hogan said.
"There's no guarantee that farmers would be able to get money elsewhere if they were competing with nurses and teachers and schools in the context of any other spending review by the Welsh or UK governments."
Mr Hogan will go head to head with UKIP MEP and agriculture spokesperson Stuart Agnew at a referendum debate in Brecon, Powys, organised by farming unions.
Access to the single market was also a key issue for Welsh farmers, Mr Hogan claimed, with 92% of Welsh lamb and 93% of Welsh beef exported to EU countries.
"Welsh farmers appreciate the fact that they're part of a large European market of 500m customers."
"In the event of a Brexit, Wales like the rest of the UK would have to negotiate free trade agreements again. It would be quite an inconvenience," Mr Hogan said.
Mr Agnew told BBC Radio Wales that the money had gone down in real terms for Welsh farmers and it was harder to access.
"We've seen the best of this and Welsh farmers can draw comfort from the fact that George Eustice, Defra minister - who's on the Out campaign - has said he will support agriculture in the future.
"Even the prime minister, who's in the In campaign, has said we'll do so.
"Sixty countries across the world support agriculture and Britain did before we joined [the EU]."
But Brian Walters, a dairy farmer and vice president of the Farmers Union of Wales, said he was worried that payments would be cut by a third and concerned that access to markets could be lost, with nearly all beef and lamb going to Europe.
And Dai Lloyd Evans, farmer and former Ceredigion council leader, said he was sure there were markets which would welcome high quality food.
He said he was "fed up" with the bureaucracy and regulations involved.
"If we do anything that contravenes the regulations they impose a financial penalty," he said.