Airborne dust and microorganisms from hay produced on intensively and extensively managed grasslands in Switzerland

Hay is a key component of equine nutrition, supplying fibre essential for gastrointestinal function and natural chewing behavior. However, it is also a major source of respirable dust and airborne microorganisms in stables and thus a relevant risk factor for equine asthma. This study quantified PM₂.₅ concentrations and airborne microbial loads from hay derived from intensively managed (HI; n = 11) and extensively managed (HE; n = 7) grasslands and evaluated the effects of hay type and drying method. Eighteen samples were characterized for dry matter, functional, structural composition, and sensory quality. Respirable dust was measured using a standardized vibration system with a laser dust sensor, and airborne microorganisms were sampled and quantified by qPCR. Fifteen variables were analyzed using principal component analysis followed by hierarchical clustering, identifying four clusters. The cluster comprising HI, barn-dried hay showed the lowest airborne contamination (PM₂.₅: 115 µg/m³; total aerobic, mesophilic bacteria: 10,288 CFU/m³; molds: 2238 CFU/m³; yeasts: 1163 CFU/m³; actinomycetes: 500 CFU/m³) and the highest sensory scores. In contrast, clusters dominated by HE hay, particularly field-dried samples, exhibited markedly higher PM₂.₅ (up to 310 µg/m³) and microbial load (up to 54,122 CFU/m³). While drying method alone had limited influence on PM₂.₅, intensive grassland management combined with barn drying consistently reduced dust and microbial exposure. Selecting such hay may reduce airborne contaminants in stables and should be considered in preventive and therapeutic strategies for equine respiratory health.