Genetic × environment variation in sheep lines bred for divergent resistance to strongyle infection

Drug‐resistant parasites threaten livestock production. Breeding more resistant hosts could be a sustainable control strategy. Environmental variation linked to animal management practices or to parasite species turnover across farms may however alter the expression of genetic potential. We created sheep lines with high or low resistance to Haemonchus contortus and achieved significant divergence on both phenotypic and genetic scales. We exposed both lines to chronic stress or to the infection by another parasite Trichostrongylus colubriformis, to test for genotype‐by‐environment and genotype‐by‐parasite species interactions respectively. Between‐line divergence remained significant following chronic stress exposure although between‐family variation was found. Significant genotype‐by‐parasite interaction was found although H. contortus‐resistant lambs remained more resistant against T. colubriformis. Growth curves were not altered by the selection process although resistant lambs were lighter after the second round of divergence, before any infection took place. Breeding for resistance is a sustainable strategy but allowance needs to be made for environmental perturbations and worm species.