"The MX missile system," the majority leader of the Senate said in profound, deliberate tones, "is essential to the strength of our deterrence triad. Tree-ad. Try-ad. Somebody better tell me how to pronounce that word."

And so, with a characteristic blend of city profundity and country personality, Majority Leader Robert C. Byrd (D-WaVa.) yesterday launched his weekly kaffeclatsch with Washington reporters, a wide-ranging seminar on the issues of the day on Capitol Hill.

At yesterday's session, Byrd had bad news for President Carter on Zimbabwe-Rhodesia but good news on the strategic arms limitation treaty (SALT). He promised major new initiatives on energy matters and propounded a new, revisionist assessment of Energy Secretary James R. Schlesinger.

The senator, a slight man with a high pompadour wave in his silver hair, seems at first glance out of place beneath the high, arching ceilings of his ornate Capitol office. But he yields to none in his zeal to master the complicated issues that come before Congress, as he demonstrated yesterday in discussing the prospects for Senate approval of SALT II.

Byrd said Carter had significantly improved chances for success in the Senate by his decision Thursday to move ahead with the biggest option for the MX missile system, a $30 billion weapon system that can be built despite the arms-limiting agreement Carter wants the Senate to accept.

In fact, the majority leader said, SALT would almost surely have failed in the Senate if Carter had not decided in favor of the new missile.

Byrd said he is still undecided about how to vote on SALT.In order to vote intelligently, he said, he is conferring regularly with weapons experts in and out of government and will probably visit the Soviet Union this summer.

The majority leader said the Senate probably will not vote on SALT before October.

The Senate will consider this week whether to lift economic sanctions against Zimbabwe-Rhodesia. Despite the president's hope that sanctions will be kept, Byrd said "the sentiment in the Senate as of today would be to overturn them."

Byrd said the Senate would start floor debate on the issue Monday. The administration will present its view at a Foreign Relations Committee hearing Tuesday. Byrd said a floor vote probably will not occur until after the administration has presented its side.

Asked about domestic energy problems, the majority leader seemed anxious to forstall criticism that Congress has not dealt adequately with shortages and prices. "It ought to be said that Congress is moving," he said.

Byrd said major new legislation in the energy field would be introduced this week and that he and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) would expedite consideration of energy-related bills.

Finally, the majority leader, who is sometimes in the forefront of new opinion trends in Washington, offered a more sympathetic assessment of Schlesinger, the oft-criticized secretary of energy.

All this spring Byrd has joined those who were critical of Schlesinger and the agency he heads, although the majority leader has stopped short of siding with those demanding Schlesinger's resignation.

Yesterday, though, Byrd said "the man takes a lot of criticism that might properly be aimed at others. This man is on the hot seat all the time.

"We ought to have some mercy," Byrd said, "as we condemn a man."