With great power comes great responsibility. Walmart, the world’s largest grocer, announced Thursday that the company aims to double the percentage of locally grown produce sold in its U.S. stores to 9 percent, by the end of 2015. In Canada, Walmart is more ambitious, planning to buy 30 percent of produce locally by the end of 2013. Walmart also pledged to sell $1 billion of food from farmers with fewer than 50 acres, and pledged to invest over $1 billion to improve its perishable supply chain. Perhaps most importantly, Walmart is developing an index to measure efficiency among its suppliers to determine whom they buy from and to label the environmental-friendliness of their products. When they developed a similar index for environmental efficiency five years ago, it prompted far-reaching changes in manufacturing: Proctor & Gamble, for instance, redesigned their packages that are carried by other retailers.