Hedge fund manager Jim Rogers voiced skepticism about Jeremy Grantham's prediction of a global commodities bull market that will last "forever" in remarks at the fourth annual Innovative Alternative Investments conference in Denver yesterday.

Rogers is a long-term bull on commodities but he believes the markets are in an adjustment period. But the reason he doubts Grantham's prediction is more fundamental in nature.

"I've never seen a bull market" in any asset class that goes on "forever," he said. "There may be one but I've never heard of it."

Rogers acknowledged that loose monetary policy and a government-engineered slowdown in China are contributing to a price "hiatus" in a number of key commodities.

But this presents opportunities. Sugar, he noted, was about 75% off its highs and is trading at levels not seen since the 1973-74 recession.