The government will begin buying grain backed up at U.S. ports because longshoremen refuse to load it for shipment to the Soviet Union.

Stuart Eizenstat, the president's domestic policy adviser, said the new program would help prop up farm prices, help farmers counting on grain sales to pay their bills and alleviate congestion at grain shipping centers.

Eizenstat told reporters at a hastily called news briefing yesterday that the timing of the White House announcement was " not geared to the Iowa caucuses." Carter acted because of "the continued unwillingness of longshoremen to load any grain to the Soviet Union," he said.

Earlier this week Carter asked Thomas Gleason, president of the 80,000-member International Longshoremen's Association to lift the ILA's 11-day boycott on grain shipments to the Soviet Union. So far, the ILA has refused to call off the boycott.

On Jan. 4 Carter announced a partial embargo on grain shipments to the Soviet Union in response to the Soviet invasion of Afghanistan. At Soviet Union.

Eizenstat said Carter has ordered the Commodity Credit Corp. to purchase corn that has accumulated at grain elevators and shipping terminal because of the ILA's boycott at major Atlantic and Gulf ports.

In addition, Eizenstat said the Food for Peace program will buy 450,000 tons of corn and soybean products in the next two years for distribution to poor countries.

Henry Owen of the National Security Council staff said the U.S.-purchased grain will be divided almost equally between sales to other governments and distribution to relief agencies assisting refugees in countries such as Cambodia and Pakistan.

The White House said the buy-up program will increase federal spending by $100 million over the two fiscal years involved.

It also said that Carter has urged Congress to "promptly" enact legislation to create a 4 million-ton food security reserve to support agricultural aid programs.

Eizenstat said the cost of Carter's latest grain purchase program is included in the earlier estimate of $1.5 billion to $3 billion as the cost to the United States of its embargo.

In a related announcement yesterday, the Agriculture Department said it was going to buy more chickens for school lunch and other food assistance programs to keep poultry market prices from falling because of the ban on chicken sales to the Soviets.