The European Agriculture commissioner spent more than an hour with farmers in County Down to try to persuade them to remain in the European Union.
Phil Hogan visited Pat McKay's farm outside Warrenpoint on Monday.
He took questions from dairy, beef and sheep farmers as well as crop growers.
Mr Hogan told them that levels of farm income support and access to a market of 500m consumers could not be guaranteed if voters opted to leave.
There could also be implications for cross-border trade and the negotiation of wider UK-EU trade deals, he said.
He claimed it could take eight to 10 years to conclude new trade arrangements.
He also questioned whether a UK farm income support mechanism would be as generous as that under the Common Agricultural Policy (Cap).
"Farmers must remember that £8.70 out of every £10 sterling is coming from the European Union, because the markets are not good," he said.
"There are no guarantees from the Treasury."
Farmers attending the event voiced complaints with the commissioner about European red tape. They also raised issues about food imports from outside the EU undercutting their produce.
The commissioner said he understood their concerns. He also announced that beef would not be included in an EU trade deal currently being negotiated with a bloc of south American countries.
Farmers had voiced fears that the deal could see the European market flooded with cheap beef from countries like Brazil and Argentina.