The National Retail Federation, the biggest U.S. retail association, cut its forecast for retail sales growth in the country this year, citing an unexpected slowdown in growth in the first half of the year. The NRF now expects U.S. retail sales to grow 3.5 percent in 2015, lower than the 4.1 percent growth it forecast in February. ” … A deflationary retail environment has been especially challenging for retailers’ bottom lines,” NRF Chief Economist Jack Kleinhenz said in a statement.