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Economically important traits for sheep evaluation that will be discussed are live weight, dressing percent, fatness, muscling, yield grade and quality grade.

Live Weight

Most lambs are marketed between 105 and 130 pounds. Ewes and wethers are marketed at similar weights.

Dressing Percent

Dressing percent is important because it indicates the proportion of live weight that is contained in the carcass. Sheep have the lowest of the three species discussed and more variation based off of wool. Use the following equation:

Dressing percent = (Carcass Weight/Live Weight) x 100

Dressing percent is affected by the fill, finish (increase in fat will generally increase dressing percent), fleece, sex and muscling. The normal range is 44-56 percent with an average of 54 for shorn lambs and 52 for unshorn lambs.

Fat Thickness

Measured at the 12th rib, it is the single factor used in yield grading ribbed lamb carcasses. The normal range is 0.05-.5 inches with an average of .25. 

Body Wall Thickness

The measurement taken approximately 4.5 inches form the midline of the ribbed lamb carcass. A lot of variation exists for this measurement based mostly off of age, breed and weight. The normal range is between 0.6 and 1.25 inches. 

Loin Eye Area

The cross-section of the longissimus dorsi muscle between the 12th and 13th rib. The normal range is 1.5-3.5 square inches with an average of 2.3.

Yield Grading

Yield grade is based solely off of measurement recorded opposite the ribeye between the 12th and 13th rib. It is ranked based on the following scale:

USDA 1 – Most desirable, minimum fat and heavy muscled
USDA 2
USDA 3 – Average
USDA 4
USDA 5 – Least desirable, fat and light muscled

This equation is used to calculate yield grade: (Back Fat x 10) + .4

Example

0.2 Back Fat = 0.2 x 10 = 2 + .4 = 2.4 yield grade

Quality Grade

Formulating a quality grade for lambs combines three separate factors that affect the palatability of the meat. They are:

• Maturity — The maturity insures that old or mutton sheep are not graded as young lamb. It is determined by evaluating the joint at the bottom side of the cannon bone; whereas mature lambs develop a fused "spool joint" and young lambs possess a "break joint". 
• Quality — The quality of sheep is evaluated by looking at intramuscular fat, much like in cattle, only this intramuscular fat is contained within the muscle of the flank. As the level of flank streaking increases, so does quality grade.
• Conformation — The unique portion of lamb grading includes a conformation factor. This insures that a carcass has a desired portion of red meat for its carcass.

Grades for lambs include:

Prime – Highest
Choice – Most lambs
Good
Utility

Sheep Practice