U.S dairy farmers already struggling with low milk prices worry President Donald Trump's talk of renegotiating the North American Free Trade Agreement could harm trade to Mexico, its biggest export market. About 15 percent of dairy production in the United States is exported with one-third valued at $1.2 billion going to Mexico in the form of milk powder, cheese and whey protein, according to the National Milk Producers Federation, which says trade with Mexico is its top priority. That's nearly double the value exported to Canada, the country's second biggest export market, according to the United States Department of Agriculture."The income of dairy farmers, whether they're in Vermont or California, depends heavily on our export to Mexico," said Jaime Castaneda, senior vice president responsible for domestic and international policy for the federation. Anything that happens that reduces the ability to trade products to Mexico will be troubling for dairy producers in the United States, said Scott Brown, a dairy economist with the University of Missouri."Can we renegotiate and get a better NAFTA deal? Only time will tell. Those trade agreements traditionally have taken a lot of time and effort to get everybody on the same page and we know we've benefited tremendously on the NAFTA agreement that we have," Brown said.