* ADM shuts four New Orleans grain elevators ahead of storm
* Cargill says two grain elevators will be shuttered
* Mississippi River transportation slowed by storm
* Traffic may be affected until Thursday
By Tom Polansek
CHICAGO, Aug 27 (Reuters) - U.S. grain elevators on the Gulf coast were shut and barges carrying grain and other goods on the lower Mississippi River were halted in preparation for Tropical Storm Isaac, which was due to make landfall late Tuesday or early Wednesday.
Archer Daniels Midland closed four grain export elevators in New Orleans, while Cargill said elevators in Westwego and Reserve, Louisiana, will be closed. Bunge Ltd is shutting down an export facility in Destrehan, Louisiana.
"We have activated our hurricane readiness plan and are taking precautions to ensure the safety of our employees and their families, as well as the security of our assets in the New Orleans area," ADM spokeswoman Jackie Anderson said.
Isaac, if it stays on its current track, is due to slam into the Gulf Coast between Florida and Louisiana by Tuesday night or early Wednesday, the seventh anniversary of Hurricane Katrina hitting New Orleans, according to the U.S. National Hurricane Center.
Isaac is forecast to become a hurricane on Tuesday.
In its latest advisory, the National Hurricane Center said the storm was not expected to strengthen beyond Category 1, the weakest type on the five-step Saffir-Simpson scale of hurricane intensity.
MAJOR CHANNEL FOR GRAIN EXPORTS
Grain companies battened down in key locations.
Bunge said it was closing its Destrehan, Louisiana, facility because of a mandatory evacuation order in the area and will watch forecasts to "make decisions as the situation warrants."
The Mississippi River is a major shipping channel to move grain produced in the Midwest farm belt to export terminals at the Gulf of Mexico, where it is then shipped worldwide.
Grain movement is set to pick up in the coming weeks as Midwest farmers -- which produce 75 percent of the country's corn and soybeans -- begin harvesting in earnest.
The harvest is already underway in southern areas of the United States, the world's top grain exporter, increasing supplies available for export.
Grain shipments and barge transportation on the Mississippi River were also disrupted in 2005 by Katrina.
"NOTHING MOVING ON THE RIVER"
The Crescent River Port Pilots' Association, in consultation with the U.S. Coast Guard, suspended barge traffic and the movement of vessels on the river from Baton Rouge, Louisiana, to the U.S. Gulf as of 9 a.m. CDT (1400 GMT) on Monday, said Capt. A.J. Gibbs, president of the association.
The suspension of barge traffic costs facilities that move cargo on the river system $300 million a day, said Gibbs, who predicted vessel movement will not return to normal before Thursday.
More than 100 vessels, many of them bulk carriers transporting goods such as grain, are affected by the restrictions, he said.
"You'll see nothing moving on the river at all," he said. "There is a significant impact on the country, not just on the ports."