The trade agreement negotiated in 2018 by the U.S., Mexico and Canada languished for more than a year as congressional Democrats pressed the Trump administration to extract concessions from Mexico on labor regulations and pharmaceutical patents. The amended USMCA, successor to the North American Free Trade Agreement, was signed this week, putting an end to 14 months of political wrangling. But to those of us who live in farm country, the pact means a lot more than politics.

To Sam Dobson, whose farm in Statesville, N.C.,...