Field olfactometry assessment of dairy manure land application methods

Because odor potential is poorly correlated with measured concentrations of component gases, human sensory assessment (olfactometry) remains the ultimate means for quantifying agricultural odors. Field olfactometry measurements vary with wind velocity and source distance. To minimize this variability, dairy manure slurry was applied in a 10-ft swath to grassland in 200-foot diameter circles. Nasal Ranger® Field Olfactometer (NRO) instruments were used to collect dilution-to-threshold (D/T) observations from the center of each circle using four odor assessors taking four readings each over a 10-min period. The Best Estimate Threshold D/T (BET₁₀) was calculated for five manure application methods and an untreated control. Field odor panel observations were performed before application and at <1 h, 2-4 h, and ∼24 h after manure spreading. Whole air samples were simultaneously collected for laboratory dynamic olfactometer evaluation using the triangular forced-choice (TFC) method. The BET₁₀ of NRO data composited for all measurement times showed D/T levels decreased in the following order (α = 0.05): surface broadcast > aeration infiltration > surface + chisel incorporation > direct ground injection ≈ shallow disk injection > control, which closely followed laboratory TFC odor panel results (r = 0.83). We conclude field olfactometry can be useful for quantifying agricultural odor emissions but multiple assessors and observations, strict compliance with established protocols, and careful data analysis are essential.