The Soviet Union has raised questions about insect and weed contamination and the quality of the U.S. grain it is buying, but a top Agriculture Department official indicated yesterday that the questions do not appear to jeopardize sales.

Agriculture Undersecretary Daniel G. Amstutz, reporting on two days of U.S.-Soviet grain-trade consultations ending yesterday, said the two countries had agreed to assign teams of experts to study the insect, weed and quality problems.

Although the Soviets previously have expressed concern about the baking quality of the wheat they buy here and about weed seed and residue in U.S. grain, Amstutz said there was greater discussion this time about insect infestations.

"Their records show there is more infestation this year," he said, "but it is explainable . . . . The Soviets are concerned about the use of some fumigants that we in this country have no problem in using. We are encouraging them to use stronger fumigants."

Amstutz said the insect problem is magnified by large shipments of grain during warm weather, because fumigants that kill insects do not destroy larvae that incubate in transit or as ships wait to unload in Soviet ports. The Soviets launched a major buying spree last summer.

Amstutz described the wheat baking-quality and weed-seed issues as "a nagging problem for quite some time." But he added, "I refuse to believe these can't be solved."

He said the Soviet delegation, here for twice-yearly consultations under the long-term bilateral grain agreement, was "satisfied" with a visit here by Soviet quality specialists last March and a joint study done on a fumigant not previously used on U.S. export grain.

Amstutz said the Soviets gave the U.S. team "no specific figures" on their needs for more American wheat, corn and soybeans. But he said he expects them to continue large-scale purchases under the second year of the agreement. The United States has pledged to make 22 million tons available in the fiscal year that began Oct. 1.

USDA figures released yesterday showed that the Soviets purchased 14.5 million metric tons of wheat, corn and soybeans last year -- their largest buy here since 1978-79 -- and that they have ordered 11.5 million tons of wheat and corn so far in the current year.

The agreement requires Moscow to buy at least 9 million tons a year, with the option of going to 12 million tons before seeking further U.S. approval. Earlier this fall, the Reagan administration -- eager to reduce the U.S. wheat surplus -- said the Soviets could buy as many as 22 million tons this year without further consultation.

"When we offered the 10 million additional tons we were confident that would be sufficient for this year," Amstutz said.