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Genetic parameters of horn fly resistance in pastured Holstein cattle.

B. L. Basiel



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Genetic parameters of horn fly resistance in pastured Holstein cattle.
B. L. Basiel*1, L. C. Hardie1, B. J. Heins2, C. D. Dechow1. 1Pennsylvania State University University Park, PA, 2University of Minnesota St. Paul, MN.

Horn flies (Haematobia irritans irritans) contribute to major economic losses of pastured cattle operations. Infestation can cause reduced milk yield and spread disease-causing pathogens which further reduce yield and incur health expenses. Most fly control methods require synthetic chemicals that cannot be used on organic operations. The objective of this research was to determine if natural resistance to horn flies is a heritable trait that can be incorporated into a traditional breeding program. Observations of fly load were recorded from pastured Holstein cows on 13 organic dairies across the United States. Fly load was determined using a 0 to 4 scale based on fly coverage from chine to loin on one side of the body; a score of 0 indicated few to no flies and a score of 4 indicated high infestation. There were 1,823 records of fly load from 1,265 Holstein sired cows included in the analysis. The scoring system was validated by counting flies from photographs taken at the time of scoring on a subset of observations (n = 290). Contemporaries that left the herd before scoring were accounted for by merging 160,740 observations of stayability with fly score records. The rank correlation between the subjective fly score and the objective count was 0.80 (P < 0.0001). Genetic parameters were determined using mixed models in ASReml 4.1 that included fixed effects of percent pedigree completeness (50% to 100%) to account for variation contributed by unknown breeds in the pedigree, herd-year-season of calving, days in milk, scorer, date, and age at calving with parity group (1, 2, ≥ 3) for fly score and count; stayability included fixed effects of parity (1 to 5), herd-year-season of birth, and pedigree completeness. Random effects included animal and permanent environment for all traits. The heritability of fly score was 0.09 � 0.05 with a repeatability of 0.21 � 0.04. The genetic correlation between fly load and stayability was not significant. The magnitude of heritable variation in fly infestation is similar to other traits included in selection programs, suggesting that we can select for resistance to horn flies.

Keywords: fly, organic, heritability.