CHICAGO—Dale Hadden cannot find any spare tires for his combine harvester. So the Illinois farmer told his harvest crew to avoid driving on the sides of roads this autumn to avoid metal scraps that could shred tires. New Ag Supply in Kansas is pleading with customers to order parts now for spring planting. And in Iowa, farmer Cordt Holub is locking up his machinery inside his barn each night, after thieves stole hard-to-find tractor parts from a local Deere & Co. dealership. “You try to baby your equipment, but we’re all at the mercy of luck right now,” said Holub, a fourth-generation corn and soybean farmer in Buckingham, Iowa. Manufacturing meltdowns are hitting the U.S. heartland, as the semiconductor shortages that have plagued equipment makers for months expand into other components. Supply chain woes now pose a threat to the U.S. food supply and farmers’ ability to get crops out …