Reactive N emissions from beef cattle feedlots

Abstract Text:

Large amounts of nitrogen (N) are fed to meet the nutritional needs of beef cattle in feedlots. However, only from 10 to 15% of fed N is retained in animals. Most N is excreted. Chemical and biological processes transform manure N into ammonia (NH₃), nitrous oxide (N₂O) and nitrate (NO₃^-). These reactive forms of N (N_r) are those most readily lost into the environment. Our objectives are to outline the forms and impacts of N_r lost from beef feedlots, present patterns and magnitudes of emissions, and examine ways to mitigate emissions. We will focus on NH₃, the major form of N_r emitted from beef feedlots. Fugitive NH₃ is a precursor to particulates in the atmosphere that cause air quality problems or overburden N-sensitive terrestrial ecosystems and initiate species changes and loss of diversity. Reactive N contributes to the eutrophication of surface waters and the creation of hypoxic zones in the Gulf of Mexico. Nitrous oxide is a greenhouse gas with almost 300 times the global warming potential of carbon dioxide (CO₂). Stringent regulations to control runoff have virtually eliminated NO₃^- as a source of N_r from beef feedlots. Direct N₂O emissions from beef cattle production are only about 0.1% of the national greenhouse gas inventory of CO₂-equivalent emissions. However, animal agriculture is the major source of U.S. NH₃ emissions (81%), and beef cattle production contributes about 15% to the total national NH₃ emissions. Research on NH₃ emissions has matured to where we have a good understanding of the pattern and magnitude of emissions. Ammonia volatilization depends on temperature, reflected in the daily and annual patterns of emission, with peak emissions during the warmest time periods. The magnitude of NH₃-N emissions is consistent across the cattle-feeding region from Texas to Nebraska. Reported winter emissions range from 25 to 35% of fed N, while summer emissions range from 50 to 75% of fed N. Research on multiple scales shows that crude protein content of diets is a critical driver of emissions. Diets that meet NRC guidelines for crude protein lose about 50% of fed N as NH₃-N. Diets with byproducts like distillers grains often exceed recommendations, with increases in NH₃ emissions from 25 to 50%. Several technologies offer promises of NH₃ emission mitigation, but most are expensive and hard to apply. Carefully managed cattle diets remain the most effective and practical way to limit the loss of NH₃ from beef feedlots.

Keywords: Nitrogen