Genetic parameters and genomic regions associated with horn fly resistance in organic Holstein cattle

摘要

Horn flies (Haematobia irritans [L.]) contribute tomajor economic losses of pastured cattle operations,particularly in organic herds because of limitations oncontrol methods that can be used. The objectives of thisresearch were to determine if resistance to horn flies isa heritable trait in organic Holstein cattle, determineassociations with yield traits, and to detect genomic regionsassociated with fly infestation. Observations of flyload were recorded from 1,667 pastured Holstein cows,of which 640 were genotyped, on 13 organic dairiesacross the United States. Fly load score was determinedusing a 0 to 4 scale based on fly coverage from chine toloin on one side of the body, with 0 indicating few to noflies and 4 indicating high infestation. The scoring systemwas validated by counting flies from photographstaken at the time of scoring from 252 cows. To mitigatethe effect of our data structure on potential selectionbias effects on genetic parameter estimates, survival tosubsequent lactations of scored animals and herd-matesthat had been culled before the trial was accounted foras the trait stayability. Genetic parameters were estimatedusing single-step genomic analysis with 3-traitmixed models that included fly score, stayability, anda third phenotype. Model effects differed by variable,but fixed effects generally included a contemporarygroup, scorer, parity, and stage of lactation; randomeffects included animal, permanent environment, andresidual error. A genome-wide association study wasperformed by decomposing estimated breeding valuesinto marker effects to detect significant genomic regionsassociated with fly score. The rank correlation betweenthe subjective fly score and the objective count was0.79. The average heritability of fly score (± standarderror) estimated across multiple models was 0.25 ±0.04 when a known Holstein maternal grandsire wasrequired and 0.19 ± 0.03 when only a known Holsteinsire was required. Genetic correlation estimates withyield traits were moderately positive, but a greater flyload was associated with reduced yield after accountingfor genetic merit. Lower fly loads were associated withwhite coat coloration; a significant genomic region onBos taurus autosome 6 was identified that contains thegene KIT, which was the most plausible candidate genefor fly resistance because of its role in coat pattern andcoloration. The magnitude of heritable variation in flyinfestation is similar to other traits included in selectionprograms, suggesting that producers can select forresistance to horn flies.