Friction between the U.S. and Mexico over trade is starting to cut into sales for U.S. farmers and agricultural companies, adding uncertainty for an industry struggling with low commodity prices and excess supply.

Over the first four months of 2017, Mexican imports of U.S. soybean meal—used to feed poultry and livestock—dropped 15%, the first decrease for the period in four years, according to data from the U.S. Department of Agriculture. Shipments of U.S. chicken meat fell 11%, the biggest decline for the period since 2003....