The Associated Press reports that DuPont Co., the nation''s No. 3 chemical firm, has announced that it is cutting back on certain research and plans to let some jobs go unfilled as it moves to cut costs in the face of tough economic conditions. The Wilmington, Del.-based company cited a "structural shift in the global industrial economy" for the changes in a prepared statement providing details on a broad plan outlined last month. In October, DuPont said it was reacting to higher material costs, slower U.S. growth and competitive difficulties for U.S.-based natural gas-dependent businesses. DuPont said it does not plan layoffs, but does expect to streamline its workforce through a combination of employee attrition, "contractor displacement" and assigning employees to different projects. The company predicted that such cost-cutting efforts will save $1 billion over the next three years.