A principal strategy to facilitate manure management has been to improve manure handling and storage. Most dairy and swine manure is now flushed from housing, and manure is stored in outside lagoons. The widespread expansion of flush and lagoon systems was premised on labor efficiency and the notion that storage would facilitate calculation of manure nutrients available and allow for land application during favorable weather conditions and close to crop nutrient demands. However, anaerobic manure lagoons accounted for approximately 7% of the global methane emissions in 1991, a figure that has likely increased substantially. Covered lagoons, complete mix digester systems, and plug-flow digester systems can capture methane that can be converted into energy and used for electricity production, heating, and cooling. The cost effectiveness of methane recovery and energy conversion is becoming increasingly attractive in areas where livestock concentrations, and therefore the supply of manure, is high enough to produce sufficient energy competitive with classical energy sources.
The use of manure as a substitute for fertilizer N may become more attractive as energy costs increase. Natural gas is used to produce a large fraction of fertilizer N, and natural gas accounts for 75 90% of the cost of making anhydrous ammonia. Conserving manure N may be of much greater importance as energy costs continue to escalate. Furthermore, it will reduce carbon dioxide (greenhouse gas) generation during the manufacture of N fertilizer.
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