Aug 19 (Reuters) - U.S. stocks opened lower on Thursday on concerns the Federal Reserve could begin to rein in its massive monetary stimulus, while commodity-linked stocks sank as oil and metal prices tumbled to multi-month lows. The Dow Jones Industrial Average fell 86.02 points, or 0.25%, at the open to 34,874.67. The S&P 500 opened lower by 17.83 points, or 0.41%, at 4,382.44. The Nasdaq Composite dropped 102.75 points, or 0.71%, to 14,423.16 at the opening bell. (Reporting by Ambar Warrick; Editing by Maju Samuel) Our Standards: The Thomson Reuters Trust Principles.