Sheep Industry News August 2025

Australia with the breeding objective to improve lean muscle growth (LMG). Although developed for Austra lian markets, past research has determined that the CPI provides a reasonable assessment of carcass value for U.S. terminal sire breeds. Since LMG cannot be easily measured directly, the CPI predicts LMG from component traits that are directly recorded. This index places a moderate emphasis on increasing weaning weight (14%), and strong emphasis (40%) on increasing both post-weaning weight and ultrasound loin muscle depth. It also places some emphasis (6%) on reduc ing post-weaning backfat depth, which is to limit, but not drastically reduce, carcass fat cover. Therefore, selection on the CPI will improve overall LMG in flocks generating terminal sires. Western range index The NSIP technical committee designed the Western range index (WRI) for use in dual-purpose breeds (e.g., Targhee and Rambouillet) with the breeding objective to improve overall flock profitability. The WRI considers the costs (e.g., purchased feed) and returns (e.g., wool, market lamb, and cull ewe receipts) of a typical western range flock, which are reflected in the emphasis placed on component traits. The WRI places a slight emphasis on increasing milk production and mothering ability (6%) and on yearling greasy fleece weight (10%), and a strong emphasis on increasing post-weaning weight (27%) and number of lambs born (38%). It also places slight emphasis on reduc ing yearling fiber diameter (4%). Lastly, it places a moder ate emphasis on reducing yearling weight (15%), which is to limit, but not drastically reduce, adult size. Therefore, selection on the WRI will increase lamb production while maintaining wool production and mature ewe size to im prove enterprise profitability. Current developments You may have noticed that current NSIP indexes do not directly consider traits associated with health (e.g., inter nal parasite susceptibility, survival, etc.) and functional ity (e.g., longevity, udder structure, etc.). This is because component traits included in indexes are limited to those that are presently recorded by NSIP breeders. However, current projects led by the NSIP technical committee are focused on practical ways to record novel traits with the goal of making them available as EBV while investigating how they may fit into larger breeding objectives. Let’s step back, though, and consider health. Internal

parasitism is particularly important to hair sheep produc ers. However, so is their current focus on maternal pro ductivity. We, therefore, have begun designing a selection index with the goal of increasing parasite resistance while still improving TW. To do so, we constructed indexes that placed varying emphasis on post-weaning fecal egg count (PFEC) relative to TW. We then predicted rates of genetic gain in TW and PFEC. We learned that placing 79% of the selection emphasis on TW, and thereby 21% on PFEC, made for a good balance. Under this scenario, 98% of the yearly gain in TW was retained while PFEC was favorably reduced by as much as 8% per year. Although work remains before we can launch a new index for producer use, we have demonstrated that we can incorporate parasite resistance into a breeding ob jective with positive impact on both maternal productivity and fitness. Future considerations A common saying among scientists, credited to the statistician George Box, is “all models are wrong, but some are useful”. Biology is complex, and economics change and are often unique to individual sheep operations. We attempt to remedy this by considering market trends over several years and by evaluating how model outputs change when inputs and management systems vary. Still, no model is perfect, and it would be impractical to develop selec tion indexes for every breeder. Nevertheless, by employing selection indexes, NSIP breeders have been making steady progress at improving multiple traits simultaneously for several years. To date, nearly every selection index across livestock species focuses on the general breeding objective of im proving profitability or increasing the quantity of market able products. After all, we need to make money to be sustainable. But notice in the earlier quote from Turner and Young they don’t define “success” in purely economic terms. They also consider our aims and recognize that the util ity of our flocks may change in the future. Some of us have already witnessed these changes as flocks find themselves in landscape management and agroecological roles where maximizing productivity, or even profitability, is seldom the primary goal. The methodologies of selection indexes are flexible. As production systems, end-uses, and our definition of success change, so too will the genetic tools available to sheep producers to ensure steady progress toward our goals.

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