Concentrations fall off rapidly at further distances
Studies by USDA indicate the dust stirred up by wind and restless cattle at dairies does contain bacteria, fungi and small bacterial remnants such as endotoxins. But these potentially problematic particles are not found at high levels far beyond the barnyard.
Rob Dungan, microbiologist at USDA’s Northwest Irrigation and Soils Research Lab in Kimberly, Idaho, set up three sampling sites at a 10,000-cow, open-freestall dairy to study airborne endotoxins and culturable microorganisms.
The researchers found that average inhalable airborne endotoxin concentrations were five endotoxin units (EU) per cubic meter of air 655' upwind of the barn—their "background" levels—and 426 and 56 EU per cubic meter of air 165' and 655' downwind of the barn, respectively.
Close to the barn, endotoxin concentrations at night were significantly higher than in the morning and similar to afternoon levels. The scientists attribute the higher levels to increased animal activity and lower wind speeds during these times. But at the other two locations, endotoxin concentrations did not vary significantly over 24 hours.
Samples of bacterial concentrations showed a similar pattern with the highest counts—84,000 colonies per cubic meter of air—measured near the barn. The other two sites had less than 8,000 colonies per cubic meter of air.
As with the daily endotoxin concentrations, bacterial concentrations near the barn increased significantly at night, but concentrations further downwind did not.—USDA Agricultural Research Service