378 Genetic Parameter Estimates of Post-Weaning Growth and Gastrointestinal Nematode Infection Traits of Katahdin Lambs
Authors: Hess Andrew , Murphy Tom W , Miller Jim E , Burke Joan M 
Published: November 2023

Abstract
The objective of this study was to estimate heritability of, and genetic correlations between lamb growth and gastrointestinal nematode (GIN) infection. Lambs were weighed and weaned at ~60 days of age and body weight (BW), fecal egg count (FEC), FAMACHA score (FAM) and packed cell volume (PCV) were recorded at ~90, 120, and 150 d of age. Lambs were not dewormed and FEC was transformed as: LFEC = log10(FEC+25). Average daily gain (ADG) was calculated from weaning to 90 days of age (ADG90) as well as 90 to 120 (ADG120) and 120 to 150 days (ADG150). Each trait (ADG, LFEC, PCV and FAM) was analyzed separately within each timepoint. The number of records per time point ranged from 1,980 (ADG90) to 1,283 (LFEC at 150 days of age). Traits were analyzed using a univariate model fitting fixed class effects of sex, birth/rear type, and season/system (fall or winter/conventional or organic), and fixed covariate of actual age at 90 days. Random effects included contemporary group, birth year, and an additive genetic effect whereby genetic relationships were captured through pedigree data. The heritability of FEC was 0.09 ± 0.03, 0.12 ± 0.03, and 0.07 ± 0.04, at 90, 120 and 150 days, respectively. PCV estimates were 0.22 ± 0.04, 0.32 ± 0.05, and 0.29 ± 0.06, at 90, 120 and 150 days, respectively. The heritability estimates of ADG were 0.09 ± 0.03, 0.12 ± 0.04, and 0.20 ± 0.04, at 90, 120 and 150 days, respectively and FAM were 0.15 ± 0.04, 0.10 ± 0.04, and 0.19 ± 0.05. Genetic and phenotypic correlations between GIN infection traits and ADG at the same time point were estimated using a bivariate model. The genetic correlation of LFEC with ADG was low for all timepoints analyzed and ranged from -0.07 ± 0.28 (150 days) to 0.07 ± 0.19 (90 days). Phenotypic correlations were similarly low and ranged from -0.02 ± 0.07 (120 days) to -0.16 ± 0.07 (150 days). LFEC showed a consistent negative genetic and phenotypic correlation with PCV at all timepoints, ranging from -0.17 ± 0.17 (90 days) to -0.57 ± 0.18 (150 days) and -0.32 ± 0.05 (90 days) to -0.20 ± 0.06 (150 days), respectively. Conversely, FAM was positively correlated with LFEC, with genetic correlation estimates ranging from 0.21 ± 0.22 (120 days) to 0.54 ± 0.20 (150 days) and phenotypic correlations ranging from 0.08 ± 0.06 (150 days) to 0.22 ± 0.05 (90 days). The genetic and phenotypic correlations of PCV with ADG increased over time, going from -0.17 ± 0.15 to 0.19 ± 0.18 and 0.08 ± 0.06 to 0.10 ± 0.06, respectively. The correlations of FAM with ADG were more variable, but generally low, ranging from -0.14 ± 0.06 (150-day phenotypic correlation) to 0.09 ± 0.18 (90-day genetic correlation). We showed that selection on PCV may be more effective than selection on LFEC to reduce the burden of GIN infection, although further investigation into its correlation with growth is warranted. This dataset will be used to investigate resistance, resilience, and susceptibility traits in Katahdin lambs.