Want to know the price of wheat? Have a look at what copper is doing.

The correlation between the two commodities has become "scary", indicating the role of funds in driving the sell-off in raw material markets, Australia & New Zealand Bank analysts said.

Their prices should, in theory, respond to different influences, with copper, used largely for electrical wires and plumbing, closely attuned to the world economy.

For wheat, which as the basis of everyday foods has more stable consumption patterns, supply factors – notably the impact of weather on yields - have a bigger role in setting prices.

The commodities' geographies diverge too, with the likes of Chile and Peru, the top copper ore exporters, and Zambia, the second-ranked (after Chile) shipper of the refined metal, playing only marginal roles in the wheat market.

'Scary' 

However, the prices of the raw materials have moved remarkably in step this year, and in particular during the sell-downs of the last two weeks, a sign of the pre-eminence of indiscriminate commodity sales by funds in setting prices, the bank said.

"The copper/wheat correlation is scary," ANZ said.

"Fund flows - outgoing - have driven this market over the last month.

"Increasingly this is looking like a stronger, or at least more consistent, deleveraging move in commodities than what we saw over February and March."

The comments follow a heightening on Thursday in a sell-off in raw materials, with Brent crude suffering its worst-ever sell-off, in dollar terms, silver plunged 12%, and the CRB commodities index dived 4.9%, its fifth-worst performance on record.

Twins 

Copper and wheat prices have moved particularly closely of late, with both shedding some 5% this week, as of Thursday's close, and 3% last week.

However, in only one week since the end of February have they moved in different directions.

And the correlation between the two commodities, on a six month basis, has risen above 80%, to levels last seen during the height of the global financial crisis in 2008, when markets were also suffering wholesale selling.

Late in 2009, the correlation fell near to minus 80%.