Sheep Industry News August 2025

“The U.S. is fast approaching the time where we can be declared scrapie-free as a country. Nearly all Polypay, around 93% of Katahdin and Suffolk, and about 84% of the Rambouillet sheep we genotyped were either resistant or rarely susceptible to scrapie. However, about 16% of Rambouillet were characterized as highly susceptible to scrapie. Selection to increase the amount of scrapie resistant genotypes across breeds and the nation is considered a key part of the National Scrapie Eradication effort in the U.S.” When asked how this knowledge could be best used, Brad re sponded, “Breeders can be tempted to test and select only animals with favorable genotypes. However, there is no ‘one size fits all’ approach. In part, it depends on the economic importance of the genetic condition to a sheep enterprise. Culling all animals with less favorable genotypes is seldom the best strategy. From our results, there are clear differences among breeds in OPP and scrapie susceptibilities. That information is useful for designing breeding objectives tailored to the needs and priorities of individual breeds and flocks.” He added “We encourage breeders to integrate this genetic con dition information alongside, rather than instead of, their current breeding objectives. One approach to save genetic diversity in the flock would be to expand and evaluate more sons than needed to obtain a more favorable genetic condition status. A focus on rams and changing the frequencies in the ewe flock over time is a reason able approach.” The Sheep GEMS team is also considering parasite resistance, particularly given increasing anthelmintic resistance in our flocks. Dr. Joan Burke is a Research Animal Scientist at the USDA, ARS, Dale Bumpers Small Farms Research Center. Among her contributions to Sheep GEMS is her knowledge of genetic and management tools to control gastrointestinal nematode (GIN) parasites. When asked what we have learned so far, Joan responded, “I recall over 20 years ago deworming sheep and watching the fecal egg counts, or FEC, skyrocket rather than fall to undetectable levels as expected. It led to examining relative differences in GIN parasite resistance among breed types. We also found variable responses within breeds.” “Selection for parasite resistance was recognized many years ago in Merino and Romney breeds, and more recently in the Katahdin breed in the U.S. The timing of measuring FEC differs among breed types due to differences in immune response and maturity and should be validated for each breed for optimal selection criteria. Because Katahdins mature more quickly than many wool breeds, FEC can be collected as early as 45 or 60 days of age around weaning, and between 90 and 150 days of age post-weaning. Beyond that, compared with other breeds, FEC often naturally declines, and little variability

exists within a contemporary group. In our Booneville, AR, ARS flock, we have heavily selected for parasite resistance. Most lambs never re quire deworming. In addition, we have simultaneously made progress on all economically important traits included in NSIP.”

Beyond resistance to GIN, there is interest in identifying so-called resilient sheep. I asked Joan “what is the difference between resistance and resilience? In addition to FEC, we are collecting FAMACHA scores in Sheep GEMS. Why so? Is it to help distinguish genetically resilient from resistant sheep?” “Resistance to GIN parasites in the animal or host is the ability to initiate and maintain an immune response to suppress establishment or eliminate the parasite. That is, the animal resists becoming infected with GIN. The FEC remain low in the presence of GIN on pasture. Re silience, on the other hand, is the ability of the host to remain healthy and productive even when parasitized.” “Resilient sheep may have high FEC, but do not become anemic or lose body weight or condition like susceptible sheep in the same contemporary group. Resistance is relatively easy to measure by com paring FEC within a group of animals. Resilience can be more difficult to assess. The FAMACHA system examines the color of the lower eye membrane to estimate degree of anemia associated with the barber pole worm, a blood sucking GIN. Farmers can easily use this tool with proper training. GEMS flocks have been recording FAMACHA scores in their lambs. Even with its limited five-point scale, it is a heritable trait. FAMACHA can help us distinguish genetically resilient from sus ceptible sheep. Both may have high FEC, but the susceptible sheep will become anemic if heavily parasitized. Body weight is also an indicator of resilience and is already recorded routinely in lambs. Resilient lambs continue to grow, but susceptible lambs may grow more slowly.” Figure 3. Average estimated breeding values (EBV) per year in post-wea ning weight (PWWT) and weaning fecal egg counts (WFEC) in the USDA, ARS, Dale Bumpers Small Farms Research Center (DBSFRC) Katahdin flock and in NSIP Katahdin flocks overall

Dr Tom Murphy is a Research Geneticist who also is at USMARC.

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