ENVIRONMENTAL DETERMINATION OF GASTROINTESTINAL PARASITISM IN KATAHDIN SHEEP

Gastrointestinal nematodes (GIN) negatively affect the performance and wellbeing of sheep and are difficult to control due to anthelmintic resistance. Producers are therefore choosing breeds naturally resistant to GIN infection, such as the Katahdin. Katahdin are raised in various climates and management systems in the U.S., and those environmental factors can be combined into eco-management clusters. We hypothesized that GIN challenge varies predictably based on the characteristics of these clusters. Katahdin producers from across the U.S. were surveyed for management information. Body weights (BW), fecal egg counts (FEC) and FAMACHA scores (FAM) were collected on 3,527 Katahdin lambs around 90 days of age. The FEC were log-transformed. Management and climate data were combined into clusters using multivariate analysis. Lamb performance data aligned with nine clusters. A model including systematic effects, including cluster, was fitted by ANOVA. Cluster defined variation in each trait (P \u3c 0.05). When analyzing GIN challenge, clusters corresponding with hotter temperature, greater precipitation, and pasture born lambs suffered from a greater parasite challenge. Conversely, the clusters with lambs turned out to pasture at older ages benefited from a reduced challenge. Genetic parameters for parasite resistance were estimated, heteroscedasticity quantified, and genetic by cluster (environment) interaction (G x E) investigated. A tri-variate animal model was fitted using ASReml. The random effects were cluster, direct additive, and residual based on model selection. Heritabilities estimates for BW (0.36 ± 0.07), FEC (0.31 ± 0.07), and FAM (0.26 ± 0.05) were moderate, with a negative and thereby favorable additive correlation of BW with FEC (‑0.26 ± 0.08) and FAM (0.16 ± 0.08). Heteroscedasticity was evaluated by estimating variance components within cluster. Additive and phenotypic heterogeneity were detected for FEC and FAM (P \u3c 0.01). A random sire by cluster interaction term was added to the model to test for G x E. A significant G x E was found for all three traits and explained at least 12% of the phenotypic variation. Since traits indicative of GIN parasitism were affected by heteroscedasticity and G x E, accounting for them in genetic evaluation would improve the efficacy of selection for parasite resistance. Advisor: Ronald Lewi