20. The Gastrointestinal Parasite Resistance Screening and Breeding in Katahdin Sheep Flock.

Abstract Gastrointestinal nematode (GIN) parasite poses serious health risks and economic losses, which affect the small ruminant farming sustainability and profitability. The objective of this project is to evaluate the selection efficiency of GIN resistant genotype and elucidate correlative relationships of traits in the parental genotype and progeny. Katahdin is a typical hair sheep breed that is popular in Northern and Eastern United States, including Missouri. Six breeding sires and 72 mixed age ewes were screened and selected from 50 rams and 280 ewes available at Lincoln University experimental research station. Three six-month-old rams containing a pedigree of GIN resistance records were teamed for the selection group (SG), whereas three randomly selected rams from the experimental station flock served as control group (CG) sires. Ewes were randomly assigned to SG and CG by body weight and body condition score stratification. Each sire was mated to 12 ewes for two estrus cycles (35d) from November 15th to December 20, 2024. Ewes’ pregnancy status was diagnosed at about 45-50 d post mating and lambing was scheduled for April-May 2025. Ewes were recorded for breeding, post gestation, post lactation liveweight (LW, kg), body condition score (BCS 1-5 scale), Famacha® score (FAM, 1-5 scale), hematocrit (HCT%) and fecal egg count (FEC/g) at four intervals. Lambs were recorded for the ID, birth weight (BW), sex, birth rearing ranks (BRR), weaning (WW) and post weaning weight (PW), average daily gain (ADG), and fecal egg count (FEC). Data was analyzed and presented with R-statistics for both breeding ewes and progeny. FEC measured was at 1600/g for rams and 732/g for ewes in SG compared to 5600/g for rams and 1114/g for ewes in CG at breeding (P = 0.10). LW of ewes were 50.4, 53.1, 54.1, and 51.8 kg at four intervals respectively. There was no difference in LW ewes throughout the breeding to lambing period although ewes in SG remained slightly higher than ewes in CG. There was no significant difference but a similar trend in BCS (2.8 vs. 3.1), and FAM measured in ewes (1.6, 1.5, 1.5, and 1.7). The BW was lower in CG (P < 0.05) (3.5 vs.3.7 kg) but no difference was found in WW (22.4 vs. 22.7 kg), PW (26.6 vs. 27.0 kg), ADG (129 vs. 129 g), BCS (2.9 vs. 3.0), FAM (1.4 vs. 1.5) and HCT (34.7% vs. 34.3%) in progeny lamb between control and selection. There was significant difference in BRR (p < 0.001) of lambs for BW (single 4.1, twins 3.6, triplet 2.9 kg) and WW (26.5, 23.5, 18.9 kg). The GIN resistant selection resulted in culling off high GIN carriers and shaders and retaining GIN resistant animals for early age breeding while cumulating resistant genotypes in the flock.