Canada's system of quotas and tariffs for dairy, poultry and eggs, known as supply management, is something of a sacred cow - all major political parties support it, and, given the value of existing quotas, farm groups erupt when changes are discussed.
Trudeau called the reasoning "frankly insulting and unacceptable".
The G-7 finance leaders agreed that the global economy is strengthening and that the expansion is likely to continue, though there are risks such as potential vulnerability in emerging market economies, which have seen their currencies sharply depreciate as a knock-on effect of interest rate hikes by the U.S. Federal Reserve.
"If we do not charge fees for selling their products, and they make us pay [duty of] 25%, 50% or even 100%, is unfair", said the President of the United States.
Asked if he was willing to give that, Trudeau replied: "We were moving towards flexibility in those areas that I thought was very, very promising". "This doesn't happen that often at G7 meetings, but it was US against everyone else", Aso said.
Trump said on Friday he might prefer to end NAFTA in favor of separate bilateral agreements with the two US neighbors.
US President Donald Trump talks to the media after meeting with North Korea's envoy Kim Yong Chol at the White House in Washington, US, June 1, 2018.
G7 finance ministers call USA out on divisive tariffs
That summit, which will be hosted by Prime Minister Justin Trudeau , will also mark Trump's first visit to Canada as president. Trump personally agreed with him previous year that it would be wrong to impose such a measure on Canada .
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EURO WOES: The euro, which has been in retreat for weeks, sank further, falling to its lowest level against the dollar this year. The euro fell against the Swiss franc, Japanese yen and USA dollar, nearing $1.15 and touching its lowest point since July.
The Trump administration triggered a major worldwide trade war last week when it imposed hefty tariffs on steel and aluminum from Canada, Mexico and the European Union.
The tariffs have landed just as Morneau presides over a G7 finance ministers' meeting in Whistler, B.C., where the event's pre-set agenda has been overtaken by talk of protectionism and fears of a protracted trade war.
Canada sent 84% of its steel exports, worth $9 billion, south a year ago, according to Statistics Canada.
"The president is going to defend this country", he added.
"This is something that the government is going to need to pay attention to".
Tariffs will hurt jobs and prosperity on both sides of the border, he added - and he wants Americans to understand that.
"The Prime Minister raised the USA decision to apply tariffs to European Union steel and aluminium imports, which she said was "unjustified and deeply disappointing", said the spokesman. "And I really hope people will take some time to reflect on the lessons of history, and not go down that path again".
He told MPs: "It is right to seek to defend our domestic industries from both the direct and indirect impacts of these USA tariffs.it's important that the United Kingdom and the European Union work within the boundaries of the rules-based worldwide trading system".