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Inside the carcass: Young producers learn to evaluate lambs for carcass traits

by Wyoming Livestock Roundup

Laramie – This fall, University of Wyoming (UW) teamed up with Oregon State University Extension, Washing State University Extension, Superior Farms and the American Sheep Industry Association’s Let’s Grow initiative to explore the marketability of lambs through a course titled Lamb 300.

Lamb 300 is a three-day workshop that allowed students the opportunity to learn, in-depth about producing and marketing high-quality, profitable lambs.

Whit Stewart of Montana State University said, “A lot of young producers haven’t been trained in sheep production. Though information has been passed on through their families, it hasn’t been extensive.”

“This program is a great way for those students to learn a lot more about lamb production,” he added.”

The workshop started with an program to assist producers in assessing carcass traits on a live lamb. Caleb Boardman of the UW Animal Science Department described how to assess back fat and yield grade in lambs.

Evaluating lambs

“It’s important to estimate and understand carcass traits based on live animals,” Boardman commented.

First, he noted that weight is an important factor.

“Live weights can range extensively, but we don’t want lambs that are too big,” he said. “Then, we evaluate dressing percentage.”

Dressing percentage of lambs ranges from 45 to 57 percent, and the percentage influences carcass value significantly. These percentages are calculated by dividing carcass weight by shrunk live weight.

“Fill affects dressing percentage,” Boardman noted. “That is based on whether the lamb has been off feed for 12 to 24 hours or if it is coming straight off the bunk line.”

He added, “The biggest factor is the weight of the pelt.  There can be differences among breeds, as well as whether they are shorn or not.”

Muscling and the degree of fatness also increase dressing percentage.

“The heavier muscled an animal is, the more dressing percentage will rise,” Boardman explained. “Heavy-muscled animals range from 54 to 60 percent, depending on their fat.”

Additionally, show lambs tend to have higher dressing percentage because  they are bred for heavier muscle.

Back fat

Back fat is measured between the 12th and 13th rib.

“When we try to estimate back fat on the live animal, we want to look at back fat in the middle of the ribeye area and take an average,” Boardman said. “We measure body wall thickness a little further down.”

Back fat ranges from 0.05 to 0.5 inches, with a lean lamb coming in at 0.15 inches and fat lambs at 0.35.

“0.1 inches of fat is not very much,” he commented, recommending using an old 4-H trick to assess depth of fat. “If we make a fist and rub our fingers over the knuckles, that’s about 0.1 inches back fat. The back of the hand, where we can feel the bones just a little bit would be about 0.2 to 0.25 inches.”

Finally, fatter lambs where a producer can’t feel the ribs likely have close to 0.3 inches of back fat.


Boardman noted that often first impressions are very important when it comes to evaluating livestock.

“I teach my students that they should turn around, look at the animal and give them a score of good, average or bad right away, instantaneously. First impression is important,” he said. “Our former meats judging coach used lean, average or fat.”

He continued, “Start with a range and then move from there. If it’s super lean, it might be 0.15. If the lamb looks fat, it’ll probably be 0.35.”

Fat, he noted, will make an animal  look smoother, wider and deeper.

“Fat can trick us into thinking that lambs have extra muscle, but fat is smooth, whereas muscle is shapely and round,” Boardman said. “We have to get our hands on the lamb to really know. We can feel the fat over the ribs.”

He summarized that muscle will be firm while fat is soft.

“It’s good to get a visual evaluation, but our hands are a great tool toward complementing what we see,” he commented.

Yield grade

Describing yield grade for lambs is more simple than evaluating yield grade of live cattle or hogs.

“Cattle have four factors that affect yield grade, and hogs have two factors,” Boardman said. “When we look at sheep, back fat is the sole factor that goes into yield grade of lambs.”

The formula to figure yield grade in lambs is back fat thickness multiplied by 10. Then, add 0.4 to determine the yield grade.

For example, a lamb with 0.2 inches of back fat would be a yield grade 2.

“Yield grade 2 is ideal,” Boardman said. “Yield grade 3 is concerning and Yield grades 4 and 5 get big discounts.”


As producers begin to evaluate their lambs, Boardman noted that muscle size across lambs – and any other animal – is highly correlated.

“Most muscles are correlated,” he explained. “As we look at a lamb, if they have a small forearm, they’ll have small muscles everywhere else.”

Visually, Boardman said that the leg shape is one of the easiest ways to look at muscles.

“We can give them a leg score to see how much thickness there is through the leg and stifle,” Boardman said, adding that the leg size can be a predictor for ribeye size.

Evaluating lambs for carcass traits can help producers to capture more value for their lambs overall.

Look for an overview on the Lamb 300 program in next week’s Roundup, including insight on the course from several attendees.

Saige Albert is managing editor of the Wyoming Livestock Roundup and can be reached at

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