Assessing the economic injury threshold and decay of performance after onset due to horn fly abundance in beef cattle

Horn flies ( Haematobia irritans ) are one of the most damaging ectoparasites for the cattle industry due to their negative impact on animal productivity, reproductive efficiency, and overall welfare. Although it is well known that the economic impact is trait-specific, decision points are rarely quantified. This study targeted two metrics that impact management decisions: the economic injury threshold (EIT), the burden at which losses begin, and the decay of performance after onset (DPO), the rate of decline beyond EIT. Image-based horn fly counts and their association with growth and fertility traits were analyzed using data collected on 337 Angus beef heifers (2022–2024). Hierarchical Bayesian changepoint models (one- and two-changepoints) were fitted for three average daily gain (ADG) intervals (ADG 1 –ADG 3 ) and the success of first insemination (SFI). The two-changepoint model also estimated an insensitivity threshold (IST; a high-burden range where extra flies do not further reduce performance). In the one-changepoint model, growth EITs were 261 to 297 flies, and DPO ranged between −0.64 and −0.58 g per additional fly above EIT (trait-specific). For SFI, the EIT was 140 flies, and the DPO was −0.0001 per fly (on the liability scale), indicating lower EIT for SFI compared to growth traits. In the two-changepoint model for ADG, growth EITs were slightly higher (293–322 flies), and IST thresholds ranged between 783 and 1167 flies. The DPO rates varied between -0.78 and −0.58 g per additional fly, a slightly larger range than in the one-changepoint model. Overall, fertility decayed at lower horn-fly levels than growth traits, and differences in DPO between traits were clear. These trait-specific thresholds provide a practical, data-driven basis for time and target of horn-fly control, and the modeling framework can be adapted to optimize management for both growth and reproductive objectives. These findings emphasize the importance of data-driven, trait-specific thresholds for fly control. The changepoint modeling framework offers a novel, biologically meaningful tool for optimizing horn fly management strategies in beef cattle production.