(Reuters) - A collapse of the North American Free Trade Agreement (NAFTA), which U.S. President Donald Trump has threatened to scrap, could create the most profound disruption for U.S. farmers who produce grains, meats and dairy products sold to Canada and Mexico.Blake Erwin, a third-generation American who raises cattle, corn and soybeans in Dixon, Nebraska, said on Saturday that he is not closely monitoring the negotiations, but that he hopes the outcome will support U.S. farmers who are struggling to make a living due to low commodities prices, rising healthcare costs and high property taxes."A trade agreement has to be fair for the United States, but we also want to keep those exports going for the farmer," said Erwin, 34. "We don't want to mess up any good things we got going."Erwin spoke to Reuters over the weekend as U.S., Canadian and Mexican negotiators met in Montreal for the sixth of seven planned rounds of talks to revamp the 1994 pact.U.S. farmers and exporters are fighting to preserve their exports at a time when Canada is finding customers in new markets. They also face strained relations between the United States and Mexico, a major buyer of U.S. corn, wheat, beef, pork and dairy products.