CHICAGO, Nov 21 (Reuters) - Tyson Foods (TSN.N) will close a major beef plant in Lexington, Nebraska, with about 3,200 employees in January after U.S. cattle supplies dropped to their lowest level in nearly 75 years, the meatpacker said on Friday. The closure in the heart of cattle-feeding country signaled that supplies will remain tight, forcing meatpackers to pay steep prices for cattle to process into steaks and hamburgers. Beef prices have set records due to low supplies and strong demand, raising costs for consumers. President Donald Trump said last month that he was working to bring down prices. Tyson...