Because the U.S. economic outlook is "less buoyant," the International Monetary Fund staff has scaled back its economic growth estimates for global economic activity in 1985 made only six months ago, officials at the agency confirmed yesterday.

The figures are part of the IMF's World Economic Outlook report, which will be released in Seoul on Oct. 6 during the meeting of the IMF's Interim Committee, the agency's policy board.

Officials said the report will project the real growth rate of industrial nations at about 2.8 percent for 1985, down from the 3.1 percent forecast last spring, and far below the 4.9 percent growth achievement of 1984, which was the best in almost 10 years.

But for 1986, the staff sees a slight improvement over 1985 to a rate of 3.1 percent, reflecting an expected modest recovery in the United States next year. This is also a marginal pickup from the IMF's spring forecast for the industrial nations for 1986 of 3.0 percent.

On a global basis, the current IMF forecast for 1985 is a growth rate of 3.1 percent, down from 4.5 percent in 1984, and 0.3 percent, down from the spring forecast of 3.4 percent. For 1986, the global forecast is unchanged at 3.4 per-cent.

"There's more concern about the world economic outlook than there was in the spring," an IMF spokesman said. "Not only does the American picture look softer, but oil and other commodity prices are down. Everything is a little less positive."

But another IMF official offered a slightly brighter perspective. "In reality," he said, "what the staff is forecasting now is a steady 3 percent trend for the two years, 1985-86. That is reasonably positive, and I think shows quite a bit of strength in contrast to some of the gloomy forecasts you hear."

For the Third World countries -- heavily dependent on the trend in the industrial world -- the staff forecast is a decline to 3.5 percent real growth in 1985 from 3.75 percent in 1984, but a gain to 4 percent in 1986.

At the time the IMF made its forecasts for the spring meeting of the Interim Committee in Washington, the guess on U.S. economic growth for 1985 was 3.5 percent, down from 6.8 percent in 1984. But actual results for the first half of the year show that U.S. gross national product gained at an annual rate of only 1.5 percent. The new estimates by the IMF staff place the U.S. growth rate for all of 1985 at about 2.5 to 2.6 percent, with a pickup to 3.3 percent in 1986.

All of the figures may be adjusted when finally released in Seoul, reflecting early estimates of the U.S. third-quarter GNP rate, which will be published this morning.