Trudeau's first official visit to Mexico followed a two-day stop in Washington.
Three people who were briefed on the matter said that the US rules of origin proposal would put these materials on the North American Free Trade Agreement's auto parts tracing list for the first time.
"Our challenge lies in ensuring that everyone benefits from economic growth".
But he acknowledged that overall, NAFTA has been particularly good for the US agriculture industry. Canada-Mexico two-way merchandise trade amounted to $40.8 billion in 2016, an increase of eight per cent over 2015.
American negotiators want Canada to be "more transparent" about its supply management system for dairy, according to a source with direct knowledge of the NAFTA talks.
Canada and the USA have different views on the dairy industry.
Trump on Wednesday repeated his warnings that he might terminate the pact and said he was open to doing a bilateral deal with either Canada or Mexico.
What that might look like remains under development; at a forum for business executives in Washington earlier this week, Trudeau noted that such a chapter could be "aspirational, it can be concrete".
Trudeau says in a speech to the Mexican senate that support for NAFTA "depends on the fruits of this trade being fairly shared".
Trump has made no secret that he prefers bilateral trade deals, and skeptics wonder whether the US demands are part of an "America First" strategy created to ensure the current talks fail.
"Violence against women and girls is prevalent in all facets of life, from the studios of Hollywood to the digital public squares, our halls of Parliament", he said.
He received a standing ovation for his remarks and cheers not just from the senators but from onlookers in the chamber. They are also seeking to halt the shift of vehicle production and jobs to Mexico from the United States.
"Canada and Mexico are not only strong partners - we are close friends who share common goals and work shoulder-to-shoulder to advance them. And even in the face of unpredictability and change, we must remember that".