CHICAGO—After Russia’s invasion of Ukraine sent global wheat futures soaring, U.S. farmer Vance Ehmke was eager to sell his grain. Local prices shot up roughly 30 percent to nearly $12 a bushel, about the highest Ehmke could recall in 45 years of farming near the western Kansas town of Healy. Instead of reaping a windfall, Ehmke found a commodities market turned upside down. He and his wife Louise told Reuters they couldn’t sell a nickel of their upcoming summer wheat harvest for future delivery. Futures prices for corn and wheat had rocketed so abruptly that many along the complex chain of grain handling—local farm cooperatives, grain elevators, flour millers and exporters—stopped buying for fear they couldn’t resell at a profit. Others couldn’t afford an industry-wide risk-management strategy known as hedging that keeps global commodities markets moving. Missiles falling in Ukraine had rocked that system, sending middlemen scrambling to shore up …