U.S. wholesale inventories grew in March while sales posted the largest monthly decrease in four years, the Commerce Department said Thursday. Inventories at the wholesale level rose 0.4 percent in March from the prior month to 503.1 billion U.S. dollars, and were up 4.7 percent from the same month a year ago. It followed a 0.3 percent decline in February. Sales at the wholesale level tumbled 1.6 percent to 414.7 billion dollars, the largest monthly drop since March 2009. It followed a 1.5 percent gain in February. Changes in business inventories is a key component of the changes in gross domestic product. Total private inventories contributed more than 1 percentage point to overall economic growth in the first quarter this year, the Commerce Department said last month.