Government rules for grading lamb and mutton are being eased to allow somewhat more of the animals to be classified as lamb, the more desirable--and more expensive--animal.

About 3.6 million carcasses qualified for the lamb grade in 1980, the latest year for which the Agriculture Department has figures. An additional 36,000 to 72,000 carcasses will make the lamb grade under the new rules, USDA officials said yesterday.

Carcasses graded under the new system probably will reach grocery stores within a week after the rules change Oct. 17, according to Paul Fuller, deputy director of the USDA's livestock, meat, grain and seed division.

But Fuller said that shoppers may not notice any difference.

"This won't have any effect on the price of lamb, and it won't affect the quality," he said. "We are not going to get any leaner carcasses; it will be as juicy as ever."

The new standards also provide for a change in the procedures that USDA uses to determine whether the carcass is prime or choice. Under the old system, the agency inspector examines the amount of fat streakings in two different areas: in the rib cage and inside the flank (that part of the carcass between the rib cage and the hindquarters), Fuller said.

But he said research now shows that inspectors can accurately judge prime or choice by examining just the flank.

"There won't be an increase in the number of lambs classified as prime or choice under this change; the change will just make it easier for us to apply the standard and determine what is choice and what is prime ," he said.

The department has proposed a more extensive grading change for beef, however.

Under a proposal made last year, the requirements would be reduced for marbling -- the flecks of fat that traditionally have denoted beef juiciness, flavor and tenderness in prime, choice and good grades. Consumer groups attacked that proposal, saying it could lead to higher prices for lower quality cuts.

A final decision on the beef grading proposal is expected within the next two weeks.

Like the beef grading changes, the changes in lamb and mutton grading were sought by producers who said the old system unfairly prevented some carcasses from being classified as lambs. Their major complaint was with the requirement that when the legs were separated from the feet in slaughtering, the breaks in two joints had to be uneven, rather than smooth, for an animal to qualify as lamb. The carcass was graded as mutton if one leg joint break was smooth--traditionally a sign of an older, more mature animal.

But under the new rules published in yesterday's Federal Register, the carcass with one smooth joint break and one uneven joint break can be graded as lamb rather than mutton, provided that the other characteristics of the carcass are those of a lamb.