Milk price wars spoiled Dean Foods' bottom line.

Dean Foods, one of the largest dairy food manufacturers, said Tuesday bountiful U.S. milk supply and heightened competition among retailers contributed to a 45% drop in its net income in the second quarter. 

Shares tumbled 21% on Tuesday as it also missed earnings estimates and warned that the "macro and competitive" pressures will continue in the coming months. The Dallas-based company, whose brands include Land O'Lakes, TruMoo, and Friendly's Ice Cream, lowered its 2017 per-share earnings expectation.

"We faced a challenging and rapidly evolving retail environment," CEO Ralph Scozzafava said.

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