The National Farmers' Union has welcomed the idea of a temporary customs union with Europe after Brexit.
The UK government has put forward the idea to avoid the damage of a "cliff-edge" departure before a final deal.
Speaking at the Pembrokeshire Show, NFU President Meurig Raymond said sheep farmers could suffer most from any return of tariffs.
The Welsh Government said it was "disappointed" a permanent post-Brexit customs union was not explored.
The UK government's position paper, published on Tuesday, said the UK would want to negotiate its own trade deals during the transition period, something it cannot currently do as a member of the EU customs union.
Mr Raymond, who farms in Pembrokeshire, told BBC Wales: "We're delighted to hear the government will be arguing for a transition period post-March 2019.
"We cannot afford that cliff-edge departure - it would have a devastating effect on certain sectors, particularly sheep, if we ended up with tariffs on our exports to Europe.
"Obviously we need to be within the customs union through that transition period as I believe it would take at least two to three years to negotiate a bilateral trade deal with the European Union.
"We need to have a frictionless trade position where business can carry on as normal, at least in the short term."
Mr Raymond stressed the need for an agreement across the nations on how the financial aid to farmers promised by the UK government following Brexit would be spent.
"No farmer must be disadvantaged - whether you're in Wales, England, Scotland or Northern Ireland - because we're going to have to be competing against the rest of the world," he said.
"That money needs to be directed to farming so that it can become more efficient, more competitive, and invest for the future."
Stephen James, president of NFU Cymru, agreed with the idea of a temporary customs union, for the sake of the whole rural economy.
"We've got all these businesses that are dependent on farming and agriculture - for every pound of support we have, it delivers £7.40 in the wider economy.
"That's a good message for all governments, because that keeps an economy going."
The Farmers' Union of Wales (FUW), also represented at the Pembrokeshire Show, renewed its call for an agricultural summit of the various UK governments to agree the rules on how the sector should be run when EU policies no longer apply.
The UK government proposes an "innovative and untested approach" to customs checks once any transition period is over, using measures such as number plate recognition technology for lorries.
However, Guy Verhofstadt, the European Parliament's negotiator, dismissed the idea of "invisible borders" as a "fantasy".
The Welsh Government said it welcomed "the recognition that there will need to be an interim or transitional period with as little change as possible in terms of practical customs arrangements".
But a spokeswoman added that there had been "no attempt to seek input" from devolved administrations.
She said the Welsh Government was "disappointed" that the costs and benefits of establishing an independent trade policy had not been analysed, and that the idea of a new customs union with the EU post-Brexit had not been considered.
Plaid Cymru MP Hywel Williams said the idea of the UK keeping "the perks of remaining in the EU Customs Union whilst negotiating alternative deals with third countries is at best naive and at worst deluded".
"What was meant to be a reassuring step ahead continues to cause more uncertainty for businesses all over the country," he added.