The American Farm Bureau Federation has given up the ghost on its long-standing attitude toward free trade, which it usually feels as strongly about as the flag and apple pie.

Reversing past policy, the Farm Bureau now is calling for the federal government to subsidize U.S. farm exports as a way of combating what it feels is unfair competition by the European Economic Community.

And, in another break from its recent past, the Farm Bureau wants the Department of Agriculture to announce a paid diversion for 1983 crops -- that is, direct payments to farmers not to plant basic grain crops. The USDA is planning a partial-payment plan for voluntary diversions.

Bureau President Robert B. Delano, a Virginia farmer, told Agriculture Secretary John R. Block in a letter this week that his organization believes it is time to fight European fire with American fire on the subsidy question.

In the wake of new projections of another bumper basic commodity crop, continuing depressed markets, lower net farm income and a decline in export sales, the Farm Bureau's directors this week voted in favor of a series of steps they said would prop up the U.S. farm economy.

Chief among them would be offsetting subsidies on farm exports, which were authorized last month in the congressional budget reconciliation package with the establishment of a special fund for such activities.

Although openly critical of EEC trade subsidization practices, Block and the Reagan administration have been reluctant to embrace a policy of direct export subsidy.

But Delano said that efforts to persuade the Europeans to change their ways have failed -- a Farm Bureau trade delegation met with no success on a spring visit to the European Community -- and that the United States must take a tougher stance.

"The American farmer is ready and willing to compete with European farmers, but we cannot sit by and let our overseas markets be taken over by EEC products that are lower in price only because their government subsidizes them," he said.