The Soviet Union today officially reduced its economic growth targets for next year to 4.1 percent, the smallest planned increase since World War Ii. tHe cutback was brought on by a poor harvest and disappointing results in many key industrial sectors.

Warned yesterday by President Leonid Brezhnev in a closed-door session that the news would not be up to hopes, the figurehead Supreme Soviet (parliament) sat impassively in the Great Kremlin Palace on the results of 1980's efforts were made clear and the 1981 plan adopted.

State Planning Committee Chairman Nikolai Baibakov said 1980's industrial growth will be 4 percent, falling short of the planned 4.5 percent increase. And the autumn grain harvest will be about 181 million tons, he indicated in a complicated formula. That will be 54 million tons below the 235 million tons set for the year.

This shortfall of over 25 percent is the second successive poor harvest for the Soviet Union. It all but guarantees continued meat and milk shortages through the coming year.

Since their 235 million tons harvest in 1978, the Russians had experienced food shortages partly as a result of the U.S. grain embargo. Their 1979 harvest totalled 179 million tons, or a shortfall of 56 million tons. They managed to buy only half the amount in Western grain markets. The new setback coupled with embargo could further complicate Moscow's food situation.

In the five-year period just ending, 1976-80, overall economic expansion totaled 24.8 percent, Baibakov said. This is about one-third less than the 36 percent growth target set five years ago.

Crude oil production was set at 610 million tons for 1981, as 1 percent increase from this year's 606-million-ton record. In part because of massive oil exports, overall trade with socialist countries will climb to 55 percent of the Soviet total in 1981.

Baibakov said average salaries for factory and office workers will be raised 1.8 percent to 171 rubles, or $262, and 4.1 percent for farm workers to 123 rubles ($190). The leadership thus continues its efforts to equalize pay for rural workers to staunch the continuing migration from village to city which has had a damaging effect on agriculture.

Premier Alexei Kosygin, 76, who suffered a severe heart attack last year, was missing from the Politburo dais and Mikhail Suslov, 77, the party ideologue, moved over next to Nrezhnev to fill in the empty chair. Kosygin's functions as head of the government and thus, the economy, have been effectively taken over by Nikolai Tikhonov, 75, who was raised to the Politburo last year. The youngest and newest Politburo member, Mikhail Gorbachov, 49, who was elevated to full memebership yesterday to bolster his agricultural portfolio, looked very busy throughout the morning session, studying reports and conferring with his new collegues.