Continental Grain, one of the country's leading grain exporters, yesterday pleaded guilty to federal charges that over a two-month period in 1975-1976 it had systematically misrepresented the type of grain it was sending abroad.

Attorneys for the New York-based, privately owned company yesterday conceded in federal district court in Norfolk that the company had knowingly caused the issuance of false or incorrect U.S. grain certificates by changing the composition of grain bound for export from its Norfolk grain elevator.

U.S. District Court Judge J. Calvitt Clarke levied the maximum fine of $18,000 on Continental.

U.S. Attorney William Cummings said yesterday the guilty plea by Continental to the six-count misdemeanor information represents the first time a company has been prosecuted for changing the composition of grain to keep U.S. inspectors from obtaining proper samples for grading.

Continental Grain was fined $500,000 by a federal district judge in New Orleans in 1976 after it pleaded guilty to charges it had inaccurately prepared grain export declarations at the company's New Orleans grain elevator, a company spokesman said last night.

"In New Orleans, they deceptively loaded as to weight. Here, they deceptively loaded as to grade," assistant U.S. attorney Ted Greenberg said yesterday.

According to the information filed yesterday by the government, Coutinental Grain employes had worked out a system for dumping low-quality, broken kernels of corn in the elevators when U.S. grading inspectors "were not observing the loading."

The government did not say how much broken corn had been improperly graded. One federal prosecutor estimated that the company was saving an average of 68 cents for every bushel of broken corn it exported as whole corn.

Continental Grain, in a statement issued in New York, noted that the recent installation of automatic, computer-run sampling equipment precluded "the likelihood of any recurrence" of deceptive grade loading practices.