In earlier times, animal manure was usually mixed with bedding material such as straw and then applied to cropland. This "dry-pack" was of a solid nature and applied frequently to fields in small amounts. It was easily absorbed into the soil and not likely to become "run-off." Although dry-pack could have an unpleasant odor, the odor lasted only until the manure aerated and dried in the field. Because this usually happened quickly, the few folks who lived in farm country easily tolerated the odor. Of course farming operations were relatively small. There were seldom enough animals to produce the manure needed to fertilize all a farm's fields. There were few environmental or social issues.

Today, however, changes in herd sizes, manure management practices, and population densities have transformed manure from an occasional inconvenience to a significant environmental challenge and serious social concern.

Economies of scale have created herd sizes that produce more manure (and therefore, more nutrients) per farm than can be absorbed by available cropland.

To handle larger manure volume, manure management practices have evolved from dry-pack to liquid manure, known as "slurry." Modern free-stall dairy barns scrape or flush manure from the barn, store the liquefied manure in large tanks or pits called lagoons, then tank or pump the slurry to fields for application. The very real advantages of slurry are offset by odor and concentration.

Because manure slurry is a liquid, soil and weather conditions require careful application to protect groundwater, aquifers and watersheds from nutrient-rich run-off.

The odor is caused by anaerobic microbes that thrive in lagoon environments. These anaerobes also produce gasses such as ammonia, hydrogen sulfide and methane. (Methane is a greenhouse gas 20 times more destructive than carbon dioxide.) Applying this rich mixture to fields releases these noxious gasses, resulting in significant odor events.

Couple these factors with urban encroachment upon farms, and the need for more manure management technology becomes clear.