Particulate exposure during spermatogenesis is associated with adverse bull reproductive fitness in a wildfire impacted area.

Wildfire-associated smoke has been increasing in frequency and severity in recent years in the western United States, posing complex health risks. In this study, we investigated the effects of smoke and fine particulate matter (PM2.5) on sperm quality in a Colorado breeding facility bull population (n = 100), leveraging the facility's sample quality assessment records as a longitudinal data source. We focused on sperm viability as an outcome, using reproductive health records (February 2021-October 2023) to identify sperm sample dates and discard status, with 11,217 samples meeting inclusion criteria. For each preceding spermatogenesis period (61 d), we calculated median air pollution exposures (PM2.5, PM10, carbon monoxide, sulfur dioxide, and ozone) using proximate EPA monitors and a derived "smoke day" index integrating NOAA Hazard Mapping System smoke plumes with PM2.5 monitor data. We used three pairs (one each with and without interaction terms) of generalized linear mixed-effects models to predict sample discard probability and assess trend stability over increasingly complex air quality characterizations. We adjusted for bull age, two aggregate breed groups (specific to this study population and based on project partner guidance), and heat index, as well as individual bulls and collection date as random effects. Our best-performing model (per Akaike Information Criterion) found each additional smoke day increased discard odds by ∼4% (odds ratio 1.04, 95% CI 1.02-1.05), with a nonsignificant primary PM2.5 effect but a significant breed interaction for ∼31% higher odds per additional 1-μg/m3 median PM2.5 (OR 1.31, 95% CI 1.15, 1.49) in Angus and Red Angus bulls. A model using only PM2.5 for air quality explained the most variance (R2) with a 1-μg/m3 increase raising sample discard odds by ∼7% (OR 1.07, 95% CI 1.01, 1.14) and a similar breed interaction effect (OR 1.31, 95% CI 1.15, 1.47). These findings help establish a baseline correlation between smoke and fine particulate matter exposure and reproductive fitness in bulls, with an apparent difference in vulnerability to PM2.5 across groups of related breeds. These results establish a baseline relationship that can begin to inform communication and risk mitigation strategies for cattlemen and veterinarians, and act as groundwork for future research into biological mechanisms and mitigation strategies.