There is something exceptionally meaty about lamb. I don’t mean it has a lot of meat on its bones—though it can. I mean the experience of eating it is what eating meat should feel like. It’s often on a bone, and has a strong, deep flavor exactly opposite that of a sterile little chicken tenderloin.
Like beef, not all lamb is the same, but there is less variation in the grades of lamb you’ll see at the grocery store or market. According to meatscience.org, “[m]ore than 90 percent of lamb in the U.S. will grade USDA Prime or Choice.” There are, however, three more grades—Good, Utility, and Cull. Much like beef, the grade a lamb gets has a lot to do with the amount of fat present, though age and muscle “thickness,” also affect grading.
Meat from young lambs (between six and eight weeks old) that is “thickly muscled throughout,” with flank fat streakings that are anywhere from “small” to “abundant,” is considered Prime. But as the sheep ages, the need for fat increases. An older lamb needs “modest” to “abundant” streakings to be considered Prime; a yearling mutton must show at least moderate fat streakings.
Once the sheep ages into “mutton territory” (older than 20 months), the meat can no longer be graded Prime, regardless of how streaked with fat it is. (See this graph from the USDA for a visual representation.) All Prime lamb cuts will be tender, either due to its youth and/or the amount of fat present, but I still like to give the meat a once over at the store and choose the chops or steaks with the most marbling.
Young lamb with “trace” or “small” amounts of flank fat streaking is considered Choice, but the amount of fat needed to keep that rating increases as the lamb ages. If you’re buying mutton from a grocery store or butcher, it will most likely have a Choice grade, meaning it possesses a “modest” to “abundant” degree of flank fat streaking. Since you don’t really have any way of knowing exactly how old the sheep was, it’s a good idea to look for the fat, and choose cuts with the most streaking or marbling.
Lamb with a Good grade is “slightly thin muscled throughout” and can be anywhere from “practically devoid” of flank fat streaking to possessing a “moderate” amount, depending on the age of the animal. (Again, this graph is really helpful.) You will most likely not encounter whole cuts of lamb labeled as “Good,” as this meat is usually ground or processed to make other products.
Utility and Cull
These two are the bottom of the sheep barrel, so to speak. Utility can be “devoid” of streaking (providing the lamb is young), and mutton with this rating can be “practically devoid” or possess as much as a “modest” amount of flank fat streaking. “Cull” is used to refer to meat that does not meet even these meager qualifications. All Utility and Cull graded meat is processed into other products, though I cannot tell you which grade goes into which items. (I bet some makes its way into dog food.)