Weaver, Andrew R.Wright, Donald L.Greiner, Scott P.Bowdridge, Scott A.2023-06-302023-06-302023-070921-4488106984http://hdl.handle.net/10919/115607Significant genetic variability exits within sheep breeds for gastrointestinal nematode (GIN) resistance. There-fore, selection may be an important tool to combat anthelmintic resistance in GIN populations. To better un-derstand the effect of selection based on the fecal egg count (FEC) estimated breeding value (EBV) on lamb GIN resistance in a pasture-based system, a divergent mating scheme was established. Over two years, Katahdin rams with exceptionally high (High FEC; n = 5) or low (Low FEC, n = 5) FEC EBV were mated to random groups of Katahdin ewes at the Southwest Virginia Agricultural Research and Extension Center (Glade Spring, VA). Lambs were born mid-March and managed as one contemporary group (Weaning: June 4). In Year 1 (YR1), FEC was collected on all lambs June 26 with no prior anthelmintic treatment. In Year 2 (YR2), beginning at 60 days of age, body weights and FAMACHA scores were collected weekly and FEC biweekly. Anthelmintic administration occurred based on FAMACHA >= 3 in YR2. Lamb survival determination excluded first 7 days of age. Statistical analysis was performed using SAS (SAS Institute, Cary, NC) with fixed effects of sire type. Lamb FEC EBV cor-responded to sire type validating the mating scheme. Lamb FEC was similar and variable prior to and shortly after weaning. After this point, High FEC-sired lambs had greater FEC compared to Low FEC-sired lambs (P < 0.05) and anthelmintic treatment corresponded to FEC EBV type (P < 0.05). In YR1, death losses were greater for High FEC-sired lambs (P < 0.05) and those lambs that died had greater FEC EBV than those that survived (P < 0.05). In YR2, post-weaning FEC EBV difference between High FEC-sired lambs that survived to 120 days of age and those that died was significant (73% vs. 138%, P < 0.01). Therefore, selection for improved GIN resistance using FEC EBV is effective and the FEC EBV is also associated with lamb survival in a pasture-based system.application/pdfenCreative Commons Attribution-NonCommercial-NoDerivatives 4.0 InternationalParasite ResistanceSelectionSheepArticle - Refereedhttps://doi.org/10.1016/j.smallrumres.2023.1069842241879-0941