Sheep Industry News August 2025

Attaining goals through selection indexes

Tom Murphy, U.S. Meat Animal Research Center Ron Lewis, University of Nebraska-Lincoln T he internet, now super-charged with artificial intelligence, has given us the ability to quickly find answers. With just a few keywords, we instan taneously have information at our fingertips. But these tools have limitations. Often, they favor more recently published material and overlook those from the past. Sometimes it’s good to set aside the smartphone and crack open an old book or have a conversation with a more “sea soned” person to reassure ourselves that we stand on the shoulders of giants. An excerpt from “Quantitative Genetics in Sheep Breed ing”, published by Australian authors Helen Turner and Sydney Young in 1969, summarizes an age-old concept in sheep breeding. “Any sheep enterprise has a greater chance of success if its aim is clearly defined…Aims are not necessarily static; they will be modified in the light of increasing knowledge, or possibly, for sheep-breeding, of changing end-use…”. It’s easy to get distracted by the latest trends and lose sight of our long-term goals. We are only human. That’s why it’s important to have a reference point to maintain the straightest path toward our ultimate breeding goals. Breeding objectives help serve as that reference point. What is a breeding objective? A breeding objective is simply the long-term goal of our selection program. Sometimes our breeding objective is strictly economic: “I want to improve the profitability of my flock.” At other times our breeding objective might focus on traits that we can’t easily measure directly: “I want to improve lean muscle growth for traditional lamb markets.” What is essential is that we define our breeding objective clearly. There are many traits that we can measure that will im pact our ability to achieve our long-term goal. These may include the number of lambs a ewe weans, the quality of her fleece and her parasite resistance, as well as the growth rate and carcass conformation of her lambs. Importantly, we can combine these measures to construct selection indexes that best align with our breeding objective.

Constructing selection indexes Selection indexes are built by weighting the estimated breeding values (EBV) for the traits contributing to our breeding objective by their importance. This results in a simplified single index value or score that we can use to compare animals for their overall genetic merit for our breeding objective. A selection index could consist of only two or many traits. However, the more traits we put in a selection index, the less genetic progress we make in any individual trait. Additionally, some traits are easier to improve and have greater genetic variability than others. Determining which traits to include in a selection index and their weight ing factor is a process that melds scientific principles and pragmatism to best approximate real-world scenarios. Current selection indexes in the National Sheep The NSIP technical committee developed two maternal productivity indexes (MPI) for use in hair and maternal wool breeds (e.g., Katahdin and Polypay, respectively). Their breeding objective is to maximize the total weight of lamb weaned per ewe lambing (TW). Since TW isn’t measured directly in NSIP flocks, these MPI predict TW from component traits on which EBV are generated. There are slight differences between these breed groups in the relative emphasis placed on individual traits in the MPI, but their interpretations are similar. The MPI place a slight emphasis on increasing wean ing weight (4 to 5%), a moderate emphasis on increasing maternal weaning weight (i.e., ewe milk production and mothering ability; 9 to 21%), and a strong emphasis on increasing number of lambs weaned (67 to 71%). It also places some emphasis on reducing number of lambs born (7 to 16%), which may initially seem counterintuitive. The reason, though, is we designed MPI to identify ewes that are genetically capable of weaning large litters without los ing lambs. Selection on the MPI, therefore, will improve TW and, thus, reproductive efficiency in maternal sheep flocks. Carcass plus index The NSIP carcass plus index (CPI) was developed in Improvement Program (NSIP) Maternal productivity indexes

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