Beef cattle

Assessing the Economic Injury Thresholds and Decay of Performance After Onset Due to Horn Fly Abundance in Beef Cattle at the population and Animal Levels

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Abstract

Horn flies (Haematobia irritans) are among the most damaging ectoparasites to beef cattle, with an economic impact ranging between $1 and $1.8 billion dollars annually. Currently, the economic injury threshold (EIT), a measure of animal resistance to horn flies, is heuristically set at 200 flies per animal side. This threshold is assumed to be the same across animals and traits. Data were collected on beef heifers from two university research herds in Georgia (Eatonton and Calhoun) between 2019 and 2024. Image-derived horn fly counts, body weights, and success of first insemination were collected on 337 heifers. Using image-based horn fly counts and longitudinal growth and fertility records on heifers, Hierarchical Bayesian changepoint models were developed to estimate EIT (onset of performance loss) and the decay of performance after onset (DPO), a measure of the animal tolerance of horn flies, at the population level. The dataset included repeated records per animal across the fly season (weights, time-matched fly counts, and reproductive performance), enabling analyses at the population and the individual levels. These population-level parameters revealed large heterogeneity in resistance and tolerance (performance loss per additional fly) across traits. Furthermore, our estimates of EIT were significantly different for the 200 flies currently used by the industry. In fact, estimates of EIT ranged between 261 and 297 for growth traits and 140 for the success of first insemination. The decay of performance after onset varied between -0.58 and -0.64 g per additional fly after the EIT for growth traits and −0.0001 g per additional fly for the success of first insemination. Subsequently, the models were extended to assess the EIT and DPO at the sire family level. All sires with at least 5 daughters were included in the analyses (N=49). EIT and DPO were assumed to be under genetic and environmental factors and were assumed to be genetically related. Preliminary results showed a negative genetic correlation (-0.41) between EIT and DPO and a large variation among sires in their estimated EIT and DPO, indicating that horn fly resistance and tolerance have enough genetic variation and are potentially amenable to selection. Furthermore, the moderate negative correlation between EIT and DPO potentially allows for an efficient selection for high resistance and tolerance animals to horn flies. Collectively, our results show that combining image-based phenotyping with changepoint methodologies at the population and animal levels can yield biologically interpretable and genetically variable traits that directly quantify the impact of horn flies on beef cattle. Using animal biological advantages to reduce horn fly attraction presents an efficient, cost-effective, and environmentally sustainable solution.

Keywords: 2026

How to Cite:

Aggrey, S., Das, K., Rekaya, R., Warner, A. & Zare, M., (2026) “Assessing the Economic Injury Thresholds and Decay of Performance After Onset Due to Horn Fly Abundance in Beef Cattle at the population and Animal Levels”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2286717. doi: https://doi.org/10.31274/wcgalp.24103

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