Deep in the heart of the Agriculture Department, a group of bureaucrats is carving out a new set of beef grading standards that may change the amount of fat scattered through the different grades of beef sold by supermarkets and restaurants.

The action comes in response to a petition from cattle producers who want the government to reduce the requirements for marbling--those flecks of fat that traditionally have denoted beef juiciness, flavor and tenderness.

The National Cattlemen's Association (NCA) says that lowering marbling requirements would give consumers the leaner beef they want and help reduce production costs. Animals generally would cost less to raise because they wouldn't have to be fed as much grain to qualify for the top grades. The cattlemen also contend that less marbling wouldn't make beef any less tasty.

But some opponents argue that consumers could end up paying higher prices for lower quality meat if the standards are reduced. Others fighting the NCA plan--including some state cattlemen's associations--believe it would be better to create a new grade for lean beef than to lower the standards for the familiar prime, choice and good grades. Names suggested for a new lean grade range from "choice light" to "Lo-Calorie."

"It's always a sensitive issue when USDA is involved in a grade change," said Jim Wise, one member of the agency team assigned to review the proposals, counterproposals and comments from 19 groups, representing meat wholesalers, middlemen, restaurants, cattlemen and consumers.

"Everybody has a different interest in what they want the grades to do," Wise said. "And what may suit one may not suit another. We try to find a position that will satisfy as many as possible."

That position could begin affecting consumer palates and pocketbooks by this time next year. That is the minimum period in which USDA could define its position, get the proposed standards approved by the Office of Management and Budget, publish them in the Federal Register, listen to public comments and issue the final rules.

Buried in the stew over what the standards should be is a second issue of who makes the decision.

The Community Nutrition Institute, a consumer group, has expressed concern that the USDA official with jurisdiction over beef grading is C. W. (Bill) McMillan, a former lobbyist for the NCA, the group pushing for the grading change.

"I suppose some people could perceive it as such a conflict ," said McMillan, the assistant secretary for marketing and inspection services. "I don't perceive it that way." But to avoid the appearance of a conflict of interest, McMillan said he won't participate in the final decision-making on the proposal.

Since the USDA beef grading standards were first developed in 1916, they have been amended eight times. The last major change was made five years ago when the agency, in a case similar to the current one, granted an industry request to lower marbling requirements.

Now the cattlemen want the 1976 requirements reduced even further. Less marbling would enable producers to "respond to a growing consumer demand for more lean beef," NCA says in its latest petition. The group says that current grading rules have resulted in "considerable overfattening of some cattle" by producers who feed animals an additional 30 days to get them into the choice grade. The proposed grading changes would help shorten the feeding time--and thus reduce producers' costs--for animals to qualify for prime and choice labels, NCA said.

Some groups fighting the NCA plan say that savings in feed costs are more likely to be retained by the producer, packer or retailer than returned to the consumer. And they warn that the proposal would result in consumers paying " 'prime' prices for 'choice' beef," in the words of one trade group fighting the plan.

At present, the government rules require that beef have moderately abundant or slightly abundant marbling to qualify for the prime grade stamp that the government inspector places on the carcass after it has been slaughtered. Prime usually costs consumers the most and generally is considered the most tender, juicy and flavorful. Prime also generally costs more to produce because the animal often must be fed grain longer than choice grade animals.

The second most expensive grade for consumers is the choice grade beef, which must have moderate, modest or small amounts of marbling to meet federal choice standards. If the beef has only a slight amount of marbling, it falls into the "good" grade. Beef with traces or almost no marbling goes into a fourth grade known as "standard," the least expensive grade.

What the cattlemen propose is a shifting of some beef into the next higher grade: Beef with moderate marbling would move up one step from the choice grade into the prime grade; beef with a slight amount of marbling would move up from good into choice, provided the carcass has an adequate fat thickness and acceptable color, and beef that has traces of marbling or is "practically devoid" of it would move up from standard into good.

Among the groups that have written letters of protest to USDA is the New England Wholesale Meat Dealers Association, which said the proposal would "cheapen the quality of our beef . . . .Under these proposed changes," it added, "the traditional U.S. quality beef animal could eventually become an endangered species."