A major potential obstacle to congressional approval of new international agreements to lower trade barriers was removed yesterday when the nation's largest farm organization, overcoming earlier misgivings, endorsed the pact.

The American Farm Bureau Federation announced its support at a conference, called by the U.S. Chamber of Commerce, that turned into something of a pep rally for the new multinational trade rules being negotiated in Geneva.

Joining the federation in varying degrees of support for the trade package that will go before Congress later this year were the chamber, the American Retail Federation and the League of Women Voters.

Farmers, pressing for expanded exports of this country's agricultural abundance to Europe, Japan and elsewhere, have been viewed as a crucial element in the expected congressional battle over the agreements.

Originally, the federation, which represents 3 million farm families, had given what its president, Allan Grant, called a "low D rating," raised to a "C plus...or low B" after agreements were reached with Japan and other countries leading to expansion of U.S. farm exports.

The federation still objects to a pending wheat agreement envisioning an international wheat supply reserve, but, said Grant: "Being realists and recognizing that we can't have everything, I would say that as of now, the farm and ranch community generally, and the American Farm Burear Federation in particular, supports the trade package." The federation, Grant said, will "urge passage" of the pact by Congress.

In presenting the Chamber of Commerce position, president Richard L. Lesher also called on Congress to act promptly to extend a waiver on countervailing duties, which other countries have demanded as a condition for wrapping up the agreements. The waiver authority has expired, and legislation to extend it, which died in Congress last year, could get tied up in early skirmishing over trade concessions.

In a speech setting the stage for the fdderation's endorsement, Agriculture Secretary Bob Bergland said market access negotiations will mean a reduction in trade barriers on about$3 billion in U.S. farm exports, including $1.5 billion for Japan and $700 million for Europe. More importantly, he said, subsidy restrictions will enable this country to compete better with other trading nations in farm exports.

Bergland said the agreements will not increase the estimate of $29 billion in farm exports for this year but will "open the door a little wider" for the future.