The impact of sire fecal egg count estimated breeding values on indicators of offspring gastrointestinal nematode infection, and relative impact of lamb estimated breeding values on sale value of ram lambs

Genetic selection of sheep for resistance to gastrointestinal nematodes (GIN) has become a priority for pasturebased production of lambs to minimize the need for deworming. The 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 containing fixed effects of year, sex, age at sampling (fitted as a repeated measure), and their interactions, and continuous effects of sire FEC EBV. The FEC and PCV of offspring was positively (P < 0.001) and negatively (P < 0.001), respectively, related to the sire WFEC and PFEC. A second objective was to examine effects on sale prices of breedingquality ram lambs from four farms, including ARS, of lamb EBV for WFEC and a ewe productivity trait (EPT) designed to identify ewes with superior maternal ability. Sale price effects were dominated by sale type with a premium of $1086/head relative to direct sales from the farm for animals in NSIP or Katahdin Hair Sheep International Expo sales. Producers likewise assigned a premium of $107/head to animals retained for use in their own breeding herds. The next most impactful variable was the EPT EBV. A 1-SD increase of 2.3 units in EPT yielded an extra $39/head whereas a similar 1-SD decrease in EPT resulted in a $32/head discount. Lambs born in fall were discounted by $6/head relative to those born in winter. Lambs that were not dewormed received a $29/head premium over lambs that were dewormed and was presumably taken as a phenotypic signal that the lamb was resistant. A +/- 1-SD change in WFEC had no effect on sale value, which changed by less than $0.01/ head. This result may have been confounded with whether or not the lamb had been dewormed. Thus, while more parasite-resistant sires produced offspring with lower FEC, the sale value of these ram lambs appeared to have been more strongly associated with other EBV such as EPT or with a desire to obtain ram lambs with balanced EBV rather than elite EBV for parasite resistance.