The U.S. economy grew at its softest pace in two years during the first quarter this year, slowing to a 3.1 percent annual rate of expansion as consumers and businesses curbed spending in the face of rising prices, the Commerce Department said on Thursday. The expansion in gross domestic product, which measures total output within U.S. borders, was the weakest since a 1.9 percent pace during the first quarter of 2003 . Wall Street economists had forecast first-quarter GDP would grow at a relatively more robust 3.6 percent rate.