Nitrogen in U.S. Agriculture

Further substantial reductions of nitrogen applications—and in this case in both absolute and relative terms—would be possible if the only objective of U.S. agriculture would be to produce a healthy diet. About 70% of the country s cereal and legume harvest is fed to animals in order to make available almost 75 g of animal protein a day per capita. Production of high-quality animal protein entails inevitable losses of plant energy and protein metabolized by animals reared for meat, eggs. 

Feed conversions vary, but even for very efficiently produced broilers the rate is more than 4 kg of feed for 1 kg of meat, implying the transformation of at least 400 g of plant proteins into 100 g of meat protein. Protein conversion efficiencies for other domestic animals are substantially lower 10-15% for pork, and just 5-8% for beef (fig. 8.4). An equal mix of chicken, pork, and beef protein would have thus required about seven times as much plant protein to produce. Reducing America s per capita consumption of animal foods by Vs would result in cutting its nitrogen applications used to produce domestically consumed food by about The total use of fertilizer nitrogen could then decline from 7-7.5 Mt N to 5.3-5.6 Mt N. 

Protein conversion efficiencies in animal food production. 

cropping is clearly a prime example of a land-rich agroecosystem that has the luxury of being able to produce mostly animal feed rather than food and still provide enormous nutritional reserves. Its applications of synthetic nitrogen fertilizers—not very high per average harvested hectare, but substantial for certain crops— have made it possible to provide diets unusually high in animal foods supply and to expand food exports to the extent unmatched by any other country. 

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