ABSTRACT
Since the earliest development of agriculture, animal manures have been applied to cropland to supply essential nutrients and increase crop yield. Modernization of agriculture has been typified by greater yields of both crop and animal products from smaller land areas, and a greater emphasis on external inputs such as chemical fertilizers and concentrated animal feeds. Intensification and confinement of animal production have now led to a situation in some areas where there are more nutrients available in animal manure than what the local crops require. These excesses of manure have raised concerns about the fate of the nutrients they contain, as losses of nutrients such as nitrogen (N) and phosphorus (P) from agricultural land can cause problems such as eutrophication in rivers and lakes. To address these regional excesses of P, research efforts have recently been focused on reducing the P concentrations in diets fed to animals, as well as on improving the availability thereof, both of which result in a reduction in the P concentration in manure generated. These dietary alterations affect not only the concentrations of P in manures produced, but also the forms of P, their fertilizer value, and environmental impact.
