The Senate last night approved a 5 percent reduction in the recommended 1991 appropriation for its operations and those of related agencies, including the Library of Congress, the Congressional Budget Office and the General Acounting Office.

Ignoring a plea by Sen. Robert C. Byrd (D-W.Va.) to "stop posturing," senators passed the amendment by Sen. Don Nickles (R-Okla.) on a voice vote after defeating, 60 to 32, a motion to table it.

Among the many Democrats who joined a nearly solid bloc of Republicans in support of the cuts were eight senators up for reelection on Nov. 6. Only two Republicans, Sens. James McClure (Idaho) and Ted Stevens (Alaska) sided with Byrd in voting to shelve the Nickles initiative.

"It's time we started taking some pride in this institution and stop posturing," said Byrd, who added that when he came to the Senate 32 years ago, it was made up of "men, not boys."

The total legislative appropriation for Congress and its agencies comes to $2.16 billion, but the Senate had before it last night only the $829.3 million for its operations and another $679.2 million for the Botanic Gardens, Library of Congress, Architect of the Capitol, Government Printing Office and General Accounting Office.

The House had earlier voted to cut the agencies by 2 percent. The 5 percent cut on the total $1.5 billion measure before the Senate would save some $75 million.

Sen. Patrick J. Leahy (D-Vt.) suggested that a better way of achieving Nickles's objective would be for senators to return to the Treasury the funds unused in their office accounts at the end of the year.

Beside the across-the-board cut in the Nickles plan, a proposal to reduce funds for Senate mailings to constituents from $35 million to $23 million in fiscal 1991 is also expected. Stevens warned the cut would leave insufficient funds in this account.

Senate floor manager Henry Reid (D-Nev.), opposing the Nickles cut, noted that 170 vacant positions now exist at the Library of Congress, along with a backup of 38 million items needing to be catalogued.