New forecasts of a bumper grain crop, both in the United States and in other major producing nations, may mean added relief from rising food prices later this year for hard - pressed American consumers.

Although some uncertainties persist, experts say food prices probably won't rise as much as had been expected during the rest of this year and early 1979 - and some may even level off, if predictions hold.

The crop bonanza is worldwide, involving both western agricultural nations and Soviet Union, which in recent years has been a "swing" country whose production and purchases influence grain prices throughout the world

On the negative side, the crop surplus may hurt taxpayers through increases in farm price subsidies. And if it pushes prices down too far, experts foresee renewed pressure from angry farmers, such as the American Agricultural Movement, for more costly federal help.

Main reason for the high yields this year has been uncommonly good weather throughout the world. The United States, for example, in contrast to the drought of the last few summers, had "a perfect July," one analyst says.

An Agriculture Department survey published last week predicts the likely corn crop at a record 6.5 billion bushels - far higher than an earlier forecast.

The outlook for wheat and soya beans isn't quite that good, but it still points to a sizable harvest.

The result has been to depress corn and wheat prices, particularly in so - called "futures" transactions, in which traders essentially are gambling on what prices will be months from now.

Howard W. Hjort, the Agriculture Department's chief economist, says there is "no question" that farm prices are on the decline, and the only uncertainty is how far they will slump.

The oversized harvest comes despite new efforts by the government to have farmers cut production - a push begun early last spring in an attempt to help reverse last year's grain - price declines.

Responding to heavy pressure from farmers. Congress and the administration established new acreage set asied programs designed to encourage farmers to plant less. But the good weather overwhelmed the government's effort.

Since crop prices are only part of what determines the supermarket price, their decline won't mean a dollar - for - dollar dip at retail.

However, Hjort and other farm economists say that, at the least, consumers will see prices rise less rapidly than the 10 percent annual increase predicted earlier, and may even find some leveling off.

The outcome depends on two factors:

How much grain the farmers decide to put under government loan rather than on the market over the next two to three harvest months.

How much U.S. grain the Soviet Union buys. Although the Soviets have a hefty crop this year, they could decide to buy outside grain as well to replenish reserves. If so, it will hold prices firm.

The Soviets must buy at least 6 million tons of U.S. grain a year under an agreement signed in 1974. Last year their purchases were considerably higher.

If prices plummet sharply, consumers may end up paying part of the difference through government subsidies. Programs enacted in the past year have increased subsidy levels substantially, but a real slump in prices could generate pressure for still more aid.

Carter administration policymakers, faced with the prospect of such a backlash, have begun talking about additional steps to limit the drop in corn prices, in an effort to strike a balance between farmers and consumers.

Planner now are considering changing the present reserve program to allow farmers to store more of their grain rather than market it, and possibly even to have the government buy up some of this year's harvest.

Meanwhile, the situation is causing some uncertainty about 1979 planting plans. Farmers have to decide in early fall on the size of their winter wheat crop. How they react could affect prices in late 1979.