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Flock genetics: Genetic selection improves performance

by Wyoming Livestock Roundup

Gillette – “Without genetic records, some bucks don’t look too bad to producers,” explained Lisa Surber, manager of the Montana Wool Lab and research scientist at Montana State University. “However, those bucks can cause more harm than good in an operation.”

Surber discussed the benefits of genetic records when purchasing sheep to improve flocks at the Northeast Wyoming Sheep Symposium held Sept. 4 in Gillette. 

Importance of records

“A poor quality ram is going to cost producers more than the purchase price of a good quality ram,” Surber said. “When purchasing a ram, producers should buy a ram with lots of records pertaining to traits that are important to their operation.”

Records allow producers to select animals based on visual appraisal, the animals’ own performance and the performance of its parents, siblings and relatives. 

“There are a plethora of traits that can be examined with records,” she continued. “Producers have to decide what is important to them and the bottom line of their ranch.”

Growth traits, such as weaning weight, and carcass traits, such as loin eye area, are associated with selection for terminal sires in breeds such as Suffolk and Hampshire. Reproductive efficiency, growth and wool traits are often sought in wool maternal ram breeds such as Rambouillet, Targhee and Columbia.

“Although a ram itself may perform well, it does not mean its progeny will do the same,” she added. “Examining genetic records will give producers a better idea of how the progeny will perform and know what to expect from that ram.” 

Ewe traits

Reproductive traits, such as number of lambs born, should be considered when selecting replacement ewes. 

“It is a common practice in many western sheep operations to select the heaviest ewe lambs as replacements without knowledge of type of birth,” she elaborates. “This selection results in reduced reproductive efficiency as the largest lambs are more likely to be born as singles.”

“Select replacement ewes from twin-born ewe lambs or from dams that have a history of twinning,” Surber continued. “Selecting replacement ewes from lambs that were born single results in larger ewes and decreased number of lambs.”

Research has shown that for every one-pound increase in weaning weight in lambs, there was a 2.5-pound increase in mature ewe body size. This increase in body size creates a greater demand for nutrients. 

“Bigger is not necessarily better in ewe body size,” she continued. “With the cost of feed being as high as it is we cannot follow the large carcass trend of the cattle industry. Even show sheep are trending towards larger sheep and larger carcasses, but that is undesirable in our range sheep because of the limited resources available to them.”

Profit selection index

One way producers can manage selection of traits is using a profit selection index. Scoring traits with positive or negative pressure, this method allows producers to select for multiple traits simultaneously. 

“The selection index is not a new concept,” she continued. “The original scientific paperwork describing the technique was published in 1943, but the sheep industry has been slower to utilize this.”

However, selecting using the index helps increase desirable growth, wool and reproductive traits. 

“In a project done by Montana State University, the top 20 indexing Targhee ewes weaned 239 more pounds of lamb than the bottom 20 indexing ewes,” Surber explains. “This is 12 more pounds of lamb per ewe.”

Growth, wool and number of lambs born are the selection traits desired by producers. Carcass quality is also a concern when selecting rams especially with the movement to lamb marketing cooperatives, such as Mountain States Lamb Cooperative. 

Eye muscle depth

Surber said that increased loin size gives American lambs a competitive advantage over imported lambs. The loin eye area (LEA) has a direct effect on dressing percentage and is the best predictor of carcass merit.

“American lambs average 2.9 inches of LEA compared to New Zealand and Australian lambs that average 2.1 inches of LEA,” Surber said. “However, improvements in LEA, if any, made in the United States over the last 10 years is likely due to increases in carcass size, not genetic improvement.”

Surber and Dave Notter from Virginia Tech developed a simplistic method to calculate LEA by scanning for eye muscle depth (EMD).

“We can predict area from depth very accurately,” Surber said. “This is one of the reasons that Targhee and other National Sheep Improvement Program (NSIP) breeds have gone to solely using depth to measure carcass trait in sheep.”

“As producers, we have to move from measuring LEA and start measuring depth like the rest of the world,” she continued. “We still report the LEA, but that is an area calculated from the depth measurement.” 

Twinning selection


For producers facing limitations on record keeping, Lisa Surber, manager of the Montana Wool Lab and research scientist at Montana State University, outlined short and simple methods that can be utilized to improve the genetic merit of flocks.

“Producers should ear notch or somehow identify twin lambs at birth and then sort their lambs at weaning,” she began. “Select at least half of the replacement ewes from twins and don’t select against twinning by keeping large, single-born ewes.”

“Identify the top quarter or half of ewes and breed them to the best rams. These ewes should be out of ewes with a long history of twinning,” she explained. “Then, if they choose, producers can breed the remaining ewes to terminal sires.”

“Simple steps such as these can help improve flocks without the producer making a considerable time commitment devoted to record keeping,” she added.

Kelsey Tramp is the assistant editor of the Wyoming Livestock Roundup and can be reached at 

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