156 The Impact of Sire Parasite Resistance on Offspring Gastrointestinal Nematode Indicators, and Relative Impact of Lamb Breeding Values on sale Value of Ram Lambs

Abstract Genetic selection of sheep for resistance to gastrointestinal nematodes (GIN) has become a priority for pasture-based production of lambs to minimize the need for deworming. The first objective of this experiment was to determine the impact of sire weaning or post-weaning fecal egg count (FEC) estimated breeding value (EBV; WFEC and PFEC, respectively) from the National Sheep Improvement Program (NSIP) on GIN infection in Katahdin lambs born in fall (Oct – Nov; n = 459) or winter (Jan – Feb; n = 378) of 2018 through 2021 at the USDA Agricultural Research Service (ARS). FAMACHA scores were determined, and blood samples and feces were collected from the lambs to determine packed cell volume (PCV), and FEC at 60, 90, 120, and 150 days of age, and lambs were selectively dewormed if anemic. Data were analyzed using mixed models. The offspring FEC was positively (P < 0.001) and the PCV negatively (P < 0.001) related to the sire WFEC and PFEC. This implies that offspring parasitism would be less and survival greater from highly resistant sires. A second objective was to examine effects on sale prices of breeding-quality ram lambs from four farms, including ARS, of lamb EBV for WFEC and ewe productivity trait (EPT) designed to identify ewes with superior maternal ability. Sale price effects were dominated by sale type with a premium of $1,086/animal relative to direct sales from the farm for animals in NSIP or specialty sales. EPT EBV was impactful yielding a-SD increase of 2.3 units or an extra $39/animal. WFEC had no effect on sale value. Thus, while more parasite-resistant sires produced offspring with decreased FEC, the sale value of these ram lambs appeared to have been more strongly associated with EPT, or with a desire to obtain ram lambs with balanced EBV rather than elite EBV for parasite resistance.