Sheep & Goats

Immune competence at weaning is associated with growth performance in Merino sheep

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Abstract

Increasing evidence suggests selection in livestock on production alone may have detrimental effects on fitness or resilience within the production environment. Immune competence (IC) is one component of resilience measured in cattle as a potential avenue for genetic selection to improve both production and health traits simultaneously. In extensively grazed Merino sheep, preliminary studies indicated similar genetic relationships may exist, including a negative genetic correlation between weaning weight and IC, and further investigation is warranted. The objective of this study is to evaluate whether immune competence influences growth in Merino sheep. In total, 3,230 Merino offspring born over two years from 23 sires and 1203 dams were used to assess the heritability of IC as well as explore the phenotypic relationships between IC and growth. Bodyweight was recorded at birth, weaning and at 2-4 weekly intervals to yearling age (average of 14.5 weight records per animal). At weaning, animals were assessed for immune competence by measuring their response to a clostridial vaccine using both antibody and cell-mediated immune responses (IR) to calculate a composite index called ImmuneDEX (IDEX) weighted by their correlation and ranking concordance. Heritability (h2) was estimated using the Bayesian mixed model in MCMCglmm (v2.36) R package fitting birth/rear type (SS: born and reared single, MS: born multiple and reared single or MM: born and reared as a multiple) and contemporary group (concatenation of dam management group and sex) as fixed effects and animal additive genetic effects as random. Animals were divided into high and low immune responder status based on IDEX, with those in the top 50% classified as high IR and those in the bottom 50% as low IR. To assess changes in weight (kg) over time, a linear mixed-effects model was fitted using the lmer function from the lme4 package (v4.4.0) in R with REML estimation and the bobyqa optimizer. The model included fixed effects for time (age of animal in days), sex, their interaction (time × sex), birth/rear type, and immune response status. Random intercepts and random slopes for time were fitted for each animal to account for individual variation in baseline weight and rate of change over time. The results demonstrated that IDEX is heritable with h2=0.50 (95% HPD:0.37-0.62). Animals with high IR were lighter than those with low IR (-0.34 ± 0.09 kg per time unit; P < 0.001), suggesting a modest trade-off between immune function and growth. Random effects confirmed substantial among-animal variation in both baseline weight and growth rate. This study suggests that immune competence is moderately heritable and further study of the correlation between immune competence and growth are warranted. Future studies could investigate this relationship and determine the optimal application in a production system.

Keywords: 2026

How to Cite:

Acton, G., Bell, A., de las Heras-Saldana, S., Doyle, E., Driscoll, D. & Hine, B., (2026) “Immune competence at weaning is associated with growth performance in Merino sheep”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2285241. doi: https://doi.org/10.31274/wcgalp.23641

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