The Soviet birth rate is so low that the U.S.S.R. may not have enough men to induct into its armed forces by the end of this decade, according to Dr. Murray Feshbach, former top specialist on Soviet demographic affairs for the Census Bureau.

In a major report published yesterday by the Population Reference Bureau, Feshbach said that Soviet birth rates have been falling and death rates rising for a generation, particularly in European Russia.

"The implications of this disparity for future labor supplies and military manpower must be of major concern for the Russians in the Kremlin," wrote Feshbach, who is now with the Center for Population Research at Georgetown University.

Feshbach said the Kremlin is aware of the problems and in 1981 launched a "pro-natalist" drive, paying women special subsidies for having babies, creating more day-care spots, barring women from 460 hazardous jobs, and increasing positions for part-time work. Nevertheless, Feshbach said, these efforts would probably be insufficient to change the demographic outlook much.

Feshbach said the Soviet armed forces are usually maintained at 4.8 million men, double the U.S. level. Most of the Soviet personnel consists of conscripts drafted for two years at age 18. However, he said, the number of youths reaching age 18 is expected to drop from 2.5 million annually in 1980 to only about 2.1 million in 1990 before starting to rise again.

Consequently, "military conscription may have to be extended from two to three years to compensate," said Feshbach.

Because of the drop in both birth and death rates among European Russians, population growth in some of the Moslem republics of Central Asia is expected to be 14 times the projected rate in the Russian Soviet republic by the year 2000.

The Great Russian nationality group, which made up 52.4 percent of total Soviet population in 1979, is expected to drop to 46.7 percent by the year 2000, while Moslems, now 16 percent, will make up 21 percent of the population. Ukrainians, Balts, Belorussians and certain others make up the rest.

Because of this, Feshbach said, a far higher proportion of the new recruits -- one-third by the year 2000 -- will of necessity come from non-Russian areas like Central Asia, Kazakhstan and the Transcaucasus.

"This dramatic increase in draftees from those areas poses a threat to the efficiency of the armed forces. Non-Slavic minority draftees have demonstrated a notoriously poor knowledge of Russian, which is the sole language of communication in this most Russian of all Soviet institutions.

"Military efficiency could also be impaired by rising ethnic tensions within the ranks," Feshbach said.

He said these problems had been recognized in a recent book by the Soviet chief of staff and "are likely to become more serious in the two decades ahead" as a result of the continuing demographic trends.

Feshbach said that in the industrial sphere, the population growth slowdown had already created 2 million job vacancies with European Russia particularly hard hit, and the gap has not been filled even though 55,000 foreign workers have been imported from Bulgaria, Poland, Vietnam and North Korea. (By contrast, the U.S. labor force is replenished by hundreds of thousands of legal immigrants and additional illegal entrants annually.)

The net number of new workers entering the Soviet labor force, which was 2.7 million a year during the 1970s, will drop to 300,000 a year during the mid-1980s before starting to rise again. The Soviets might try to meet the need by shifting Central Asian workers to fill the worst worker shortages. But Feshbach said these workers' "poor training and command of Russian," plus their cultural and family ties made them "resist offers of higher pay to migrate where the jobs are."

Feshbach, in his analysis of demographic trends, said fertility had been declining for a long while, as is the case with most developed countries, from 2.8 births per woman over her child-bearing period (15 to 49) in 1958-9 to 2.3 in 1980.

At the same time, he said, death rates, which plummeted to 6.9 per thousand a year in 1964, have started to rise, with poor health care a major reason, reaching 10.3 in 1980; the U.S. rate then was 8.8.

The result: the rate of population growth dropped from 1.8 percent a year during the 1950s to 0.9 percent during the 1970s and is now about 0.8 percent; but it is triple that in many parts of Central Asia.