Manure Management and Environmental Issues in the United States
In one year, a dairy cow generates liquid and solid manure that contains 58 lb phosphorus, 168 lb ammonia (a form of nitrogen), and 336 lb total nitrogen (ASAE 2005). Dairy manure is expensive to transport, so it is generally applied to nearby fields, which sometimes leads to excess applications of nutrients. The ongoing trend of increased numbers of dairy cows per farm in the U.S. (USDA-NASS, 2010) results in greater concentrations of manure, bedding, and urine being produced by the dairy operation. This increases the transport distances (and costs) required for appropriate land application of manure. In 2000, only 1% of large dairies (those with more than 1000 animal units) were applying phosphorus at agronomic rates, while only 23% were applying nitrogen at agronomic rates (Ribaudo et al. 2003). More recent data indicate that larger operations apply manure to cropland at rates that are more than three times higher than smaller farms, suggesting that excess nutrient applications are still an issue, particularly for large operations (MacDonald and McBride 2009). This observation is also supported by a recent study of manure application to field corn (the receiving crop for more than half of all applied manure), which found that the vast majority of dairies applied manure to fewer acres than would be needed to meet best management practices for nutrient management (USDA ERS 2011).
The loss of phosphorus and nitrogen to the environment during manure management can contribute to a number of significant water and air quality concerns:
- Phosphorus and Nitrogen Eutrophication. Both phosphorus and nitrogen can be lost through runoff or infiltration and leaching at manure storage locations and field application sites, as well as through soil erosion. Losses increase substantially as nutrient application exceeds the plant needs (Bock and Hergert 1991; Schlegel et al. 1996). Once lost from agricultural
systems, phosphorus and nitrogen can migrate to lakes, rivers, estuaries, and coastal oceans. Overabundant nutrients can then lead to excessive growth of algae and aquatic weeds and subsequent oxygen shortages (Carpenter et al. 1998), fish toxicity (Ward et al. 2005), habitat loss (NRC 1993; Jeppesen et al. 1998) and decreased species diversity (Sutton et al. 1993).
- Nitrate Pollution of Water Sources. Infants under six months of age who ingest high levels of nitrates in the water supply can acquire blue baby syndrome. Symptoms include bluish skin, stupor, brain damage and in severe cases, death (US-EPA 1991).
- Ammonia Volatilization. An estimated 70% of total manure nitrogen is lost as ammonia during manure management and application on U.S. dairies and feedlots (CAST 2002). Ammonia is highly reactive and contributes to the development of ultra-fine particulate matter (PM 2.5) in the atmosphere. PM 2.5 has detrimental effects on overall air quality and human and animal health (Erisman and Schaap 2004; McCubbin et al. 2002; Archibeque et al. 2007).
Greenhouse gas emissions are also a concern of current manure management practices. Dairy cattle create direct and indirect emissions of greenhouse gases throughout the production process, with over half of direct emissions generated by manure management (US-EPA 2013a). There is significant variation in emissions depending on the types of manure management systems; with higher methane emissions coming from liquid manure management systems. These liquid manure systems are increasingly used in dairy operations (US-EPA 2013a), leading to recent increases in greenhouse gases associated with manure management. In total, manure management for dairy cattle in the U.S. contributed an estimated 46% of the greenhouse gas emissions associated with manure management for all live stock and poultry in 2011; or 0.48% of gross greenhouse gas emissions in the United States (this was an estimated 32.4 million metric tonnes, MT; US-EPA 2013a).
Factors Contributing to Nutrient Overloading
Because phosphorus and nitrogen losses increase rapidly when they are applied in excess of plant needs, one strategy for minimizing losses to the environment is to ensure that manure applications do not provide more nutrients than can be taken up by the crops being grown on the land. However, there are