The UK government has been accused of casting doubt on the future of post-Brexit farm payments by Plaid Cymru.
Environment Secretary Michael Gove said subsidies would have to be earned rather than handed out under his vision for a "green Brexit".
Plaid's farming spokesman Ben Lake said it was an "economically toxic" threat to the livelihoods of food producers.
The Farmers' Union of Wales (FUW) wants a "realistic" transition period to avoid "devastation" for rural families.
Under the EU's current Common Agricultural Policy (CAP), farmers are paid based on the amount of land they farm.
In a speech on Friday, Mr Gove stressed the UK government would continue to pay the £3bn a year currently granted to British farmers until 2022.
But he pledged to change the rules after that, claiming the CAP rewarded rich landowners, encouraged waste and failed to recognise "good environmental practice".
Mr Lake, Plaid's newly-elected MP for Ceredigion, welcomed the plan to cut payments to rich landowners, but was worried about the other conditions.
"We know that for any business, whether on the high street or the family farm, uncertainty is economically toxic," he said.
"When food security is at stake, the British government should know better than to make broad-brush statements when there is no plan.
"Farmers in Wales rely on payments for 80% of their income. It's these people who need clear assurances."
Simon Thomas, Plaid's agriculture spokesman in the assembly, called for post-Brexit rules to be agreed by all four UK nations.
He called Mr Gove's speech "a shot across the bows of the Welsh national interest and the powers of the Welsh Government".
FUW president Glyn Roberts voiced concern about Mr Gove's "strong focus on environmental schemes, which neglect the need for food production".
"His vision also does not recognise the role our farmers are already playing in maintaining the countryside," he said.
"It is not only farming families which are at stake here; there are countless other businesses and jobs which rely on agriculture, so we need to be as sure as we can that well-meaning policies will not result in devastation."
Wales' Environment Secretary Lesley Griffiths said it had been "inappropriate" for Mr Gove to mention Welsh farming in his speech, given the responsibility had been devolved.
She said she had been working to develop "a way forward which supports our farmers to continue to produce high quality food and manage the land in a way that delivers wider economic and social benefits".
"It is not about trading off the environment and the economy, it's about both," she added.