Submitted to: Professional Animal Scientist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/16/2015
Publication Date: 10/1/2015
Citation: Waldrip, H., Cole, N.A., Todd, R.W. 2015. Nitrogen sustainability and beef cattle feedyards: II. Ammonia emissions. Professional Animal Scientist. 31(5):395-411.
Interpretive Summary: It is important that beef in the United States be produced in a way that is profitable while minimizing the negative effects on society and the environment. The manure (a mixture of urine and feces) from beef cattle contains high concentrations of nitrogen, which must be managed carefully. Manure nitrogen can be lost to air as ammonia and nitrous oxide, or to water as nitrate and other compounds. Nitrogen loss also means that the manure has lower value as crop fertilizer. Sustainable beef production requires consideration of the long-term effects of management practices. It is also important to have a good understanding of how different environmental and management factors affect the fate of feedyard nitrogen. Nitrogen enters a feedyard in the form of feed and purchased animals. This nitrogen input must be balanced with output from the feedyard as manure and beef. A literature review was conducted to determine the potential impact of beef cattle feedyards on air, soil, and water quality. This work resulted in a comprehensive analysis of the state-of-the-science regarding feedyard nitrogen. Methods to reduce feedyard nitrogen losses were evaluated. We also identified knowledge gaps and areas where more study is needed. Two key factors were feedyard manure management and cattle diet. Nitrogen balance studies showed that only about 15% of the nitrogen input from cattle feed remained in animal tissue. About 44% of nitrogen output was lost to air or water and 41% of output was nitrogen in manure removed from the feedyard. The concentration and digestibility of protein in cattle feed generally controlled how much nitrogen that cattle eliminated in urine and feces. Up to 90% of feedyard ammonia originates from urine deposited in animal pens. The amount of this loss depends on both weather and management practices. Feedyard ammonia emissions were higher in summer than winter due to increased temperature. Both nitrogen excretion and ammonia emission increased with the amount of protein fed to animals. Ammonia emission rates for feedyard cattle ranged from 50 to 280 grams per day. This is equal to 28 to 72% of the nitrogen fed. It was estimated that feedyard cattle produce from 90 to 120 grams of ammonia per day. Constant emission factors and most statistical models to predict feedyard ammonia losses are based on limited research. Process-based mechanistic models offer more precise estimates for open-lot systems because they include management and environmental factors that are related to ammonia production. Managing cattle diets so that they do not exceed cattle protein requirements is probably the most practical way to reduce ammonia losses. Other ways are: 1) dietary manipulation to decrease nitrogen excretion, 2) inhibition of urea breakdown, and 3) capture of ammonium in manure with pen surface amendments. More study is needed to improve understanding on the effects of diet and manure management on the fate of feedyard nitrogen.
Technical Abstract: Sustainable management of beef cattle feedyard nitrogen (N) helps avoid environmental degradation and loss of manure fertilizer value due to ammonia volatilization. In this review we report the state-of-the-science concerning feedyard ammonia and evaluate methods to mitigate N losses. Up to 90% of feedyard ammonia originates from urine deposited in animal pens, but the magnitude of this loss depends on both weather and management practices. Feedyard ammonia emissions were higher in summer than winter, largely due to increased temperature. Both urea excretion and subsequent ammonia emission increased with dietary crude protein concentration. Per capita ammonia emissions rates (PCER) for feedyard cattle ranged from 50 to 280 g/d: equivalent to 28 to 72% of fed N. Annualized PCER for feedyard cattle ranged from 90 to 120 g/d. Constant emission factors and most current empirical models to predict feedyard ammonia produce relatively coarse estimates based on limited research. Process-based mechanistic models offer more precise emission estimates for open-lot systems due to inclusion of management and environmental factors related to ammonia production. Managing cattle diets to meet, but not exceed, metabolic crude protein requirements is the most practical way to reduce N losses; however, diets must be changed carefully to avoid unintended negative consequences on animal production. Other possible mitigation approaches are: 1) dietary manipulation to decrease N excretion, 2) inhibition of urea hydrolysis, and 3) capture of ionic ammonium in manure with pen surface amendments (e.g., urease inhibitors, alum, zeolites).