Farmers and farm groups urged Congress yesterday to act quickly to tighten federal grain-inspection standards to defuse growing overseas complaints about U.S. grain quality and to help regain lost export markets.

"U.S. grain producers believe that U.S. wheat and other grains are dirtier than grain exported by our competitors, and these producers want Congress to do something about it," said Dan McGuire, director of the Nebraska Wheat Board.

McGuire and other witnesses told a Senate Agriculture subcommittee that current standards are so lenient that export firms can make huge extra profits by mixing low-quality grain and foreign material into shipments that still meet official grade requirements.

"There is no mystery as to why the large grain exporting firms and the grain trade oppose changing the standards," McGuire said. "It is primarily a profit motive."

The North American Export Grain Association, whose 33 member companies account for more than 90 percent of U.S. exports, agreed that minor changes in the standards would benefit world buyers but argued that the quality issue has been "grossly exaggerated" as a factor in declining overseas sales.

Association President Myron R. Laserson, an official of Continental Grain Co., defended the industry by citing statistics showing wheat leaving export terminals in cleaner condition than when received from farmers.

"This suggests that dockage [material that can be removed] originates at the producer level, not in the grain-handling system. I repeat . . . any change in grade standards will affect the whole system," he said.

But farmer representatives said they are ready to bear their share of any additional cost of sending cleaner grain overseas.

"I believe a problem exists and that we need to tighten the standards," said North Dakota farmer Charles Ottem, president of the National Barley Council. "I also know the American farmer will pay the bill."

Ottem said poor quality was a major reason for a decline in barley exports from 100 million bushels in 1983 to 18 million this year. He told of foreign buyers who said they could find cleaner barley in other countries and said a U.S. barley sample he was shown in Japan was so infested "I would have been ashamed if that had come from my farm."

The exporters said they would support a bill by subcommittee Chairman Mark Andrews (R-N.D.) that would bar addition of material to export grain, because "it is fully consistent with existing proper elevator practice," as Laserson put it.

But Andrews said his proposal is "a skeleton to which we intend to add some muscle." Andrews and other senators said they are increasingly concerned about laxity in standards that gives buyers the impression they are receiving higher quality grain.

"Most of our standards are based on 1916 laws," Andrews said. "We are liable to be tarred with the label that we are overreacting, but many countries that used to be our customers are turning to our competitors. I want to move as soon as possible with this legislation."