The Senate Agriculture Committee, in an unusual one-day burst of speed and unanimity, gave quick approval yesterday to a legislative rescue package for the troubled Farm Credit System (FCS).

The bill, voted out of the committee on a 19-to-0 roll call, would authorize the sale of $4 billion in bonds with government backing and with interest paid partially by the government to prop up the financially distressed farmer-owned lending network.

The measure also would require a major reorganization of the nationwide farm banking system and, overriding objections of the Reagan administration, it would create a secondary market for farm real estate loans and provide new rights for Farmers Home Administration (FmHA) borrowers.

With one major exception, the bill was virtually the same as that written by a credit subcommittee and approved last week after more than 100 hours of hearings and mark-up sessions.

The exception was an amendment by Chairman Patrick J. Leahy (D-Vt.), which could make thousands of acres of land in the FCS inventory available to farmers who are eligible for FmHA loans. The new program would allow the purchase of up to $250 million worth of FCS farmland, with farmers receiving a 4 percent interest discount.

Leahy said the bill met most of his and other committee members' credit objectives. But he warned that aid for the FCS remains urgent despite rumors that the system today will disclose its third-quarter report that will show the FCS has moved into the black.

"Regardless of what we hear {from the FCS}," he said, "it is essential that we put in place a mechanism to aid severely stressed institutions of the FCS."

Leahy said he will try to move the bill quickly to the Senate floor for final passage, which would clear the way for a conference with the House over differences in their respective measures before Congress adjourns this year.