Grain and soybean prices tumbled amid concerns about higher-than-expected crop supplies and a less-bullish outlook from Goldman Sachs Group Inc. 

Soybean prices slid 2.8% to a five-month low, and corn dropped by the same percentage to a six-week low, as money managers exited bets that prices would rise. The selloff was driven in part by traders who saw the price declines as signals more selling would ensue.