Chicago | Reuters — U.S. corn, wheat and soybean futures fell on Friday, weighed down by fresh concerns about demand after U.S. President Donald Trump threatened to place tariffs on imports of goods from Mexico in response to illegal immigration across the U.S. border.

All three commodities also faced pressure from profit-taking after posting big gains for the month and the week. Heavy rains throughout May have caused severe delays in U.S. soybean and corn planting. The excessive moisture also may have cut into the quality of the Plains wheat crop that will be harvested soon.

For the month, the most-active Chicago Board of Trade corn futures contract rose 17.9 per cent, its biggest monthly gain since June 2015. Soft red winter wheat futures rallied 17.7 per cent during May, their biggest monthly gain since June 2017, and soybean futures were up 2.9 per cent, snapping a streak of three straight losing months.

Trump said on Thursday he will impose a tariff on all goods coming from Mexico starting at five per cent and ratcheting higher until the flow of illegal migrants ceases.

“The U.S. government continues to throw out banana peels for the grain bulls to slip on,” Matt Zeller, director of market information at INTL FCStone said in a note to clients.

Mexico is the top foreign buyer of U.S. corn and wheat and the second largest buyer of U.S. soybeans.

“Mexico imports 19 million tons of corn a year, most of it from the United States, and with South America premiums weak enough to export to the United States, I can tell you there are two countries down there frothing at the mouth to muscle into the Mexican market,” Charlie Sernatinger, global head of grain futures at ED+F Man Capital, said in a note to clients.

CBOT July corn futures settled down 9-1/4 cents at $4.27 a bushel (all figures US$).

“Geopolitics is also back to centre stage with tensions between the U.S. and Mexico caused by immigration,” crop consultancy Agritel said.

Corn received some support from the Trump administration’s announcement that it had lifted restrictions on the sale of higher ethanol blends of gasoline, potentially boosting demand for the yellow grain. The announcement allows gasoline stations to sell blends containing up to 15 per cent corn-based ethanol year-round, ending a summertime ban.

CBOT July wheat was 11-1/2 cents lower at $5.03 a bushel. CBOT July soybeans were down 11-1/4 cents at $8.77-3/4 a bushel.

— Mark Weinraub is a Reuters commodities correspondent in Chicago; additional reporting by Naveen Thukral in Singapore and Gus Trompiz in Paris.