Rio Tinto is heading for sharp slowdown in first half profit after revealing sharper than expected falls in iron ore and coal production and exports in the three months to March.

In fact production and sales forecasts for 2011 show a sharp slowing in the growth of iron ore, coal, bauxite, alumina and aluminium production, while copper production and sales will be lower as will that of uranium.

Higher prices for iron ore, coal, copper and aluminium will go some way to offsetting the slowing production growth rates and the first quarter falls, but the current price levels are coming under pressure, especially for the company's most valuable resource, iron ore.

World copper prices continue to weaken, although gold and silver prices are still at high or near record levels.

Certainly the company won't repeat the surge in earnings it saw in 2010 when interim earnings tripled to $US5.85 billion from $US1.62 billion as higher prices for iron ore boosted the result.

The actual dollar profit is likely to be matched, but the growth will have gone because of the first half weakness and the plateauing in the price of some commodities.

Output of its most important commodity, iron ore, is forecast to be up just 6 million tonnes from 2010, or just over 3%.

Bad weather in Western Australian, Queensland and the top end of the Northern Territory cut production and sales of iron ore, coal and uranium, while the company's Canadian iron ore pellet business suffered a drop in production as well.

Copper production in South America, Indonesia and the US fell and is expected to decline this year, but output from the Northparkes mine in NSW again rose and is expected to be higher this year.

Chief executive Tom Albanese said in yesterday's