SAO PAULO, July 29 (Reuters) - Brazilian beef-packers' lobby Abiec, which represents companies including JBS and Marfrig (MRFG3.SA), opens new tab, on Tuesday estimated losses of $1 billion if the United States applies a 50% tariff on Brazilian exports The U.S. is Brazil's second biggest beef export destination after China. Roberto Perosa, head of Abiec, said Brazilian beef companies had expected to sell some 400,000 tons by yearend to the U.S., but imposition of a 50% tariff would make sales "inviable." No market can immediately replace the U.S. based on the sheer volume demanded by the importers and the price...