Dairy farms are thought to emit large amounts of ammonia and therefore contribute to nitrogen (N) fertilization of natural ecosystems and provide precursors for particulates that adversely affect air quality and human health. The 2003 NRC report “Air Emissions from Animal Agriculture” (NRC, 2003) made an urgent call for processed-based research that assists livestock producers and regulatory agencies in developing strategies that reduce the emissions of ammonia and other gasses that impair air quality.

Only approximately 20 to 35% of the N (protein) fed to dairy cows is converted into milk (Figure 1). The remaining N is excreted in urine and feces. Feeding N to dairy cows in excess of their requirements is excreted in urine. About three-fourths of the N in urine is in the form of urea. Urease enzymes, which are present in feces and soil, rapidly convert urea to ammonium. Ammonium can be transformed quickly into ammonia gas. Feces contain little or no urea. For this reason urinary N is much more vulnerable to ammonia volatilization than is fecal N.