Copper prices were on the rise Wednesday as traders worried that a strike at a major producer would threaten supplies of the metal. Benchmark contracts for copper were tacking on 4 cents at $3.75 a pound in recent action on the Comex division of the New York Mercantile Exchange. The PowerShares DB Base Metals ( DBB) exchange-traded fund, which tracks the value of copper and other industrial metals, was ahead by 1.3%. Workers at a Southern Copper ( PCU) facility in Peru are staying off the job for a second day and the action is raising concerns that an already tight market for the metal will get worse, explains William Adams, an analyst at BaseMetals.com in London. Shares of Southern Copper were recently edging lower by 0.2%. In precious metals, silver prices were up a penny at $13.46 an ounce, while gold prices were off 40 cents at $735.90 an ounce. Among agricultural products, cocoa prices were shedding 2.6% at $1,867 a ton, while coffee prices were rising 1% at $1.36 a pound. As for the commodities-producing stocks, Bear Stearns upped its rating on shares of smelter Kaiser Aluminum ( KALU) to outperform from peer perform, helping lift the shares almost 7%. RBC Capital Markets raised Kinross Gold ( KGC) to an outperform rating from sector perform, sending the shares higher by 1%. Piper Jaffray took coffee seller Green Mountain Coffee Roasters ( GMCR) to outperform from market perform, a move that boosted the stock more than 7%.