Review: Nitrogen sustainability and beef cattle feedyards: II. Ammonia emissions

Abstract

Sustainable management of nitrogen of beef cattle feedyards helps avoid environmental degradation and loss of manure fertilizer value because of ammonia volatilization. In this review we report the state of the science concerning feedyard NH3 and evaluate methods to mitigate NH3 losses. Up to 90% of feedyard NH3 originates from urine deposited in animal pens, but the magnitude of this loss depends on both weather and management practices. Feedyard NH3 emissions were higher in summer than winter, largely because of increased temperature. Both urea excretion and subsequent NH3 emission increased with dietary CP concentration. Per capita NH3 emissions rates for feedyard cattle ranged from 50 to 280 g/d, equivalent to 28 to 72% of fed N. Annualized per capita NH3 emissions rates for feedyard cattle ranged from 90 to 120 g/d. Constant emission factors and most current empirical models to predict feedyard NH3 produce relatively coarse estimates based on limited research. Process-based mechanistic models offer more precise emission estimates for open-lot systems because of inclusion of management and environmental factors related to NH3 production. Managing cattle diets to meet, but not exceed, metabolic CP 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, and zeolites).