Senate-House conferees have voted to wipe out a major cost-of-living benefit that will trim the lifetime annuities of everybody retiring from government in the future.

This week they will deal with the question of whether to let U.S. retirees continue to get COL adjustments every six months -- in March and September -- or to limit them to a single inflation adjustment each year.

Conferees have already given the chop to a "look-back" feature that allowed employes, when they retired, to get the benefit of most of the previous COL raise, plus the full amount of the next COL adjustment.

The changes mean people could no longer retire just before a COL raise went into effect and get all of the raise. Both the look-back, and the look-ahead benefit will be wiped out of the Senate and House, as expected, approve the conferees' recommendations, which are part of the big, controversial budget reconciliation bill.

Cutting out look-back, and starting a system of prorating COL raises will mean smaller annuities for people retiring in the future. They will not be able to take advantage of the previous COL, and will get only a portion of the COL raise that becomes effective before they retire.

Thousands of federal workers, anticipating the congressional action, retired in late August. They got most of the COL raise that took effect in March of this year, plus all of the COL adjustment (of 7.7 percent) that went into effect Sept. 1, even though they retired before it was effective.

Under the new rules, future retirees will not benefit from look-back, nor will they get the full amount of the first COL adjustment that is effective before they actually retire.

Although that is a major loss to people facing retirement in future, the big item -- preserving or killing the twice yearly COL adjustments is yet to come. The Carter administration favors the one-shot COL, on grounds that it is unfair to give U.S. retirees two inflation-linked raises a year when most other retirees, and persons under Social Security, get only one COL raise per year, if that. Independent presidential candidate John Anderson has also come out in favor of the one-shot COL adjustment for U.S. retirees.(GOP candidate Ronald Reagan has not taken a stand on this issue).

The item before the Senate-House conference committee is a resolution already approved by the full Senate that would skip the March COL adjustment next year. The House has instructed its conferees to block any attempt to change the twice-yearly COL system. But conferees from the House Budget Committee are trying -- and they may have the votes -- to eliminate the March 1981 COL raise.

At this point the calendar is the best ally federal-military retirees have. Congress is due to recess in early October for the elections. It will not come back until the week after the elections. If the conferees reject the COL cutback, or if they simply do nothing, retirees will probably keep their March and September adjustments. This week should tell the story.