In the United States, animal agriculture accounts for approximately $100 billion annually, or half of all farm sales. The manure produced by dairy and beef cattle, poultry, and swine contains vast amounts of nitrogen and phosphorus (Table 1) that in some regions can be landapplied at agronomic rates on farms where it is produced.1-2-1 Nitrogen (N) is the most limiting nutrient to cereal crop production, so the fertilizer value of manure is usually equated to its ability to provide N to a succeeding crop. Manure N availability for use by crops is highly influenced by its ammonium content (Fig. 1), which depends on the amount of urine N conserved. Organic N in feces and bedding is more slowly available than urine N, and continues to mineralize and be available for crop uptake years after application. If and when manure is incorporated also affects the availability of manure N to crops.
Two approaches are commonly used to estimate the fertilizer N value of manure: 1) apparent manure N recovery by crops (i.e., the difference method, or difference in crop N uptake in plots that received and did not receive manure); and 2) comparison of crop response with approximately equivalent rates of commercial fertilizer (i.e., the fertilizer equivalence approach). A ''decay series'' is developed that predicts the proportion of manure N available the first, second, and third year after application. Beegle et al. summarized the results of approximately 90 trials conducted between 1931 and 2002 across a wide range of soils and environmental conditions and found that first year manure N availabilities were remarkably consistent within animal species and averaged 36% for dairy, 32% for beef, 27% for sheep, 51% for poultry, and 62% for swine. However, averages tend to obscure within-study variability. For example, using the difference method and the fertilizer equivalent approach, Munoz et al. estimated that corn N uptake during the first year after dairy manure application ranged from — 60% of applied N (a negative value obtained when crop N uptake in nonmanure control plots is greater than manure-amended plots) to 148% (values >100% obtained when crop N uptake exceeds N additions). The use of manure labeled with the stable isotope 15N provides a more direct and less variable measure of manure N availability to crops.
Manure applications provide crop yields comparable or superior to yields with inorganic fertilizer. The beneficial effects of manure are due not only to nutrients, but also to improvements in soil organic matter content (SOM), soil structure, and tilth. Improvements in infiltration, soil aggregation, and bulk density due to manure application can reduce runoff and erosion. SOM enhancement due to manure increases soil cation exchange and buffering capacities, which enables manure-amended soils to retain nutrients (and chemicals such as pesticides) for longer periods of time. SOM increases carbon sequestration in soils, which mitigates the effects of rising atmospheric CO2 levels on global climate. However, manure may also provide soluble carbon and nitrates, which can enhance N2O emissions from soil and contribute to global warming.
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