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ARS Home » Southeast Area » Booneville, Arkansas » Dale Bumpers Small Farms Research Center » Research » Publications at this Location » Publication #404184
Research Project: Sustainable Small Farm and Organic Grass and Forage Production Systems for Livestock and Agroforestry

Location: Dale Bumpers Small Farms Research Center

Title: Genotype by environment interaction and heteroscedasticity influence the expression of parasite resistance in Katahdin sheep

Author
item ARISMAN, BRIAN - University Of Nebraska
item Burke, Joan
item MORGAN, JAMES - Round Mountain Consulting
item LEWIS, RON - University Of Nebraska

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2023
Publication Date: 7/5/2023
Citation: Arisman, B.C., Burke, J.M., Morgan, J.L., Lewis, R.M. 2023. Genotype by environment interaction and heteroscedasticity influence the expression of parasite resistance in Katahdin sheep. Journal of Animal Science. https://doi.org/10.1093/jas/skad228.
DOI: https://doi.org/10.1093/jas/skad228

Interpretive Summary: Sheep producers increasingly rely on breeds that express resistance to gastrointestinal parasites because anthelminthic drugs are often ineffective. An example is Katahdin sheep, which are raised in various climates and management systems in the U.S. These factors can be combined into eco-management clusters to describe production environments more holistically. Our objective was to determine if a genotype by eco-management cluster (environment) interaction (G x E) affected performance levels, particularly for traits indicative of parasite resistance. Body weights (BW), fecal egg counts (FEC), and FAMACHA scores (FAM) were collected at 90-d in 3,527 Katahdin lambs delineated into 9 eco-management clusters. Heritabilities of BW, FEC, and FAM were 0.36, 0.31, and 0.26, respectively. Genetic correlations of BW with FEC and FAM were low to moderate and favorable (negative). When estimated within eco-management cluster, heritabilities were 0.30 to 0.37 for BW, 0.18 to 0.41 for FEC, and 0.07 to 0.32 for FAM. For FEC and FAM, these differences corresponded with heterogenous variances (heteroscedasticity) across environments. Furthermore, G x E explained 13% (FAM) to 20% (BW) of the variation in a trait. In genetic evaluation of parasite resistance, G x E and heteroscedasticity should be considered to improve the efficacy of the breeding program.

Technical Abstract: Increasingly, sheep producers are choosing breeds that express resistance to gastrointestinal parasites due to reduced efficacy of anthelminthic drugs. One such breed is Katahdin. Katahdin are raised in various climates and management systems in the U.S., which can be combined into eco-management clusters to describe production environments more holistically. The objectives of this study were to determine if genotype by environment interaction (G x E) and heteroscedasticity existed across these eco-management clusters for traits indicative of parasite resistance. Body weights (BW), FAMACHA scores (FAM), and fecal egg counts (FEC) were collected at 90-d in 3,527 Katahdin lambs delineated into 9 eco-management clusters. A tri-variate animal model including birth-rearing type, sex, and dam age (as a quadratic covariate) as fixed effects, and eco-management cluster, direct additive, uncorrelated maternal environmental (for BW), and residual as random effects, was fitted with ASReml. Heritability estimates for BW, FEC, and FAM were 0.36 ± 0.07, 0.31 ± 0.07, and 0.26 ± 0.05, respectively. The genetic (additive) correlations between BW with FEC was 0.26 ± 0.08 and with FAM was 0.16 ± 0.08, and thereby favorable. Heritabilities were also estimated univariately within eco-management clusters and ranged from 0.30 ± 0.07 to 0.37 ± 0.05 for BW, 0.18 ± 0.05 to 0.41 ± 0.12 for FEC, and 0.07 ± 0.03 to 0.32 ± 0.08 for FAM. Significant genetic and phenotypic heteroscedasticity among eco-management clusters was detected in FEC and FAM. A sire by eco-management cluster interaction term was added to the initial model fitted to evaluate G x E. This interaction defined substantial variation (P < 0.01) in all traits and explained 12% (FEC) to 20% (BW) of the phenotypic variation. Accounting for G x E and heteroscedasticity in the design and implementation of breeding programs may introduce operational challenges. Still, doing so would improve the efficacy of selection programs to improve parasite resistance.