Whether or not U.S.-Soviet relatiions worsen, one thing is already clear to American policymakers: the Soviet Union will continue to require substantial quantities of U.S. grain and it is taking pains to keep this grain trade out of politics.

Agriculture Secretary Bob Bergland, just back from a European trip that also took him to the Soviet Union, said yesterday the Russians wanted to talk about agriculture, not about Africa or Cuba.

The only political reference he said he heard was from Premier Alexei N. Kosygin, who expressed hope for an agreement on strategic arms reductions.

Other than that, the Soviet officials Bergland saw made clear that they wanted to keep buying U.S. grain to feed to the expanding livestock herds that are essential to improving the diet of the Soviet people.

The Soviet Union's purchases from the 1977 U.S. wheat and corn crops - now totaling 13.8 million metric tons - are slightly more than the 13.7million tons it bought in 1972. That year, the Russians negotiated secretly with international grain companies and bought about one-fourth of this country's wheat crop before the government was alerted.

Since then, the United States has covered about half of the Soviet Union's annual grain-import requirements of between 25 million and 30 million tons.

Although foreign grain amounts to less than 15 percent of the grain used each year in the Soviet Union, it is a crucial component because the Russian climate and soil are not well suited for growing animal feedgrains such as corn. Therefore, the Soviet government's program to provide more meat, poultry and diary products depends on these imports The recent Soviet purchases have been taking place under the guidelines of a five-year Soviet-American grain agreement that expires Sept. 30, 1981.

The agreement requires the Soviet Union to buy at least 6 million tons of wheat and corn, but no more than 8 million tons without getting the approval of the U.S. government. Last fall, following bumper wheat crop in this country, the Agriculture Department authorized the Russians to increase their grain-buying to 15 million tons.

Bergland said yesterday that he received no indication on his trip whether the Soviets would ask for permission to exeed the 15 million-ton limit.

Several U.S. officials said yesterday that they viewed the five-year agreement as binding. The only escape clause gives the United States the right to revoke the pact if the harvest here fails disastrously.

Administration officials, seeking to respond in some way to what they perceive as Soviet troublemaking in Africa and elsewhere, spoke this week of "groping for leverage." However, one senior official said that any attempt to use the grain accord to apply diplomatic pressure would be likely to backfire.

He noted that the Soviet Union has become a major grain market for American farmers and added that any abrogation of the agreement would be "costly in the long run" because it would demonstrate that the United States was not reliable supplier of grain.

The Ford administration's temporary embargo on grain sales to the Soviets in 1975 infuriated farm organizations in this country.

The Soviet Union has steadily expanded its cattle, hog and poultry numbers since the mid-1960s. Cattle have increased from 74.2 million head in 1970 to 112 million now. These cattle require large amounts of corn for calories and soybeans for protein.

Bergland said he was impressed with modernization in Soviet agriculture but added that the Siberian wheatlands lack adequate rainfall. "They are never going to be able to feed those animals with home-grown feedstuffs in the quantities which I think they are building up to," he said.