President Carter's decision to hold back 17 million tons of grain ordered by the Soviet Union is certain to affect the diets of millions of Soviets.

As the president noted in his speech last night, the sharp reduction in grain shipments will not result in Russians going hungry, since most of the U.S. wheat and corn is fed to hogs, poultry and, to a lesser extent, cattle. u

However, U.S. officials said, it is likely to mean that the Soviet leadership's decade-long efforts to put more meat on Soviet dinner tables will be set back at least for the next year or two.

Soviet meat consumption increased between 1970 and 1975 from an average 105 pounds a year per person to 132 pounds, mainly due to a high-level decision to import massive amounts of feed grains.

This improvement came to a halt in 1975 as a result of a disastrous Soviet harvest. Until the president's announcement last night, Soviet agriculture finally seemed poised for a recovery, with hogs, poultry and cattle all at record levels.

However, U.S. officials last night predicted a "substantial" cutback in these livestock numbers and a corresponding decline of meat in Soviet markets due to a smaller supply of grain with which to feed animals.

Less certain was the overall impact on the people who have gained most from the booming Soviet grain market -- American farmers.

Carter pledged his determination to "minimize any adverse impact on the American farmer from this action."

At the same time, senior U.S. officials announced that the government was prepared to spend an additional $2.5 billion to $3 billion in the next two fiscal years to support farm incomes.

The 17 million tons of grain withheld from the Soviet Union represents 6 to 7 percent of total U.S. grain production and about 15 percent of estimated wheat and feed grain exports of 109 million tons projected for the current marketing year.

While the "farm vote" no longer plays a major factor in U.S. elections because of the declining number of farmers, the grain trade has become central to the agriculture-based economy of many western states.

A temporary embargo of grain shipments to the Soviet Union in 1975 was politically costly to Gerald R. Ford in the Midwest.

Iowa, whose corn and soybeans tend to move into export channels, is the scene of a Jan. 21 precinct caucus contest for 1980 presidential delegates.

For this reason, the handling of the grain export cutback is likely to be crucial for Carter's political fortunes in that state.

Administration officials last night outlined several major measures for preventing the cutback from causing a glut and depressing prices in the grain states. They include:

Purchase by the government of up to 4 million tons of wheat for an expanded foreign food assistance program. In his speech, Carter noted that increased amounts of grain would be used to alleviate hunger in poor countries. i

New incentives to persuade growers to place more of their grain in storage on their farms under a program already in effect. These incentives are likely to include an increase in the price at which this grain would be released for sale, once the grain markets stabilized.

Some 17.5 million tons of feed gains and 6.25 million tons of wheat are in this "farmer reserve," whereby the government pays the storage costs but the farmers continue to own the grain until prices reach the minimum release level.

Howard W. Hjort, the Department of Agriculture's chief economist, said it is possible to increase the volumes stored under this program as a "first line of defense" of farm incomes.

Under a five-year Soviet-American grain agreement running through September 1981, 5 million tons have been loaded for the Soviet Union this year. The Soviets have a right to buy up to 8 million tons without U.S. approval. The 25 million tons the Soviets had counted on getting from the United States is about 6 percent of total Soviet grain needs.