Amy Chase started feeding Similac Organic infant formula to her second son, Amos, as soon as he was born in November 2006. “When I saw the organic at Publix, I bought it, no questions asked,” said Ms. Chase, a self-described “yoga mom” in Atlanta. Like Ms. Chase, many American parents have rushed to embrace Similac Organic formula, even though it sells for as much as 30 percent more than regular Similac. In 2007, its first full year on sale, it captured 36 percent of the organic formula market, with sales of more than $10 million, according to Kalorama Information, a...