The Archer Daniels Midland wet mill on the outskirts of Decatur, Ill., rises like an industrial behemoth from the frozen, harvested cornfields of Central Illinois. Steam billowed in the 20-degree cold last week, as workers turned raw corn into sweet, ubiquitous high-fructose corn syrup. Three miles away, a Primient mill, which sprawls across 400 acres divided by North 22nd Street, was doing the same. To Robert F. Kennedy Jr., President-elect Donald J. Trump’s nominee for secretary of health and human services, this bedraggled city — set deep in Trump country — is the belly of the agribusiness beast, churning out...