Variations of ammonia emissions from cattle operations: Effects of air temperature and dietary crude protein content

The objective of this study was to conduct a systematic review of the published literature on ammonia (NH₃) emissions from beef feedlots and dairy barns using a meta-analytical approach in order to improve the estimation of emission rates, to understand the sources of variation, and to specifically quantify the effects of air temperature and dietary crude protein (CP) on NH₃ emissions. Emissions from lagoons or other manure storage facilities were not included. The review compiled results from 91 studies (published from 1997 to 2016) and from the five dairy sites in the National Air Emissions Monitoring Study (NAEMS). Based on the results, the average NH₃ emission rate was 105.9 g d^-1 animal^-1 for beef feedlots and 125.6 g d^-1 animal^-1 for open-lot dairy barns, which were much higher than the emission factors proposed by the U.S. Environmental Protection Agency. Expressing NH₃ emissions as the NH₃-N loss as a percentage of nitrogen (N) intake can address the effect of N feed intake, can reduce uncertainties in estimation of emissions, and can be used as a key parameter in establishing emission inventories and evaluating mitigation strategies. The average NH₃-N loss as a percentage of N intake was 53.1%, 15.6%, 8.7%, and 5.1% for beef feedlots, open-lot dairy barns, free-stall dairy barns, and tiestall dairy barns, respectively. Results showed that the measured NH₃ emission rates could be affected by measurement method. The chamber method tends to underestimate, while the N balance method tends to overestimate. Proper adjustment and validation may be needed when using the tracer ratio method in free-stall/tie-stall barns in order to avoid potential overestimation. Regression models were developed to quantify the effects of air temperature and dietary CP on emissions from various types of cattle operations. The sensitivity of NH₃ emission rate to air temperature obtained for various operations was consistent (2.33, 2.39, and 2.66 g d^-1 animal^-1 per °C, respectively, for beef feedlots, open-lot dairy barns, and free-stall/tie-stall dairy barns with natural ventilation). Higher dietary CP resulted in significantly higher NH₃ emission rates and also corresponded with higher NH₃-N loss as a percentage of N intake for beef feedlots and free-stall/tie-stall dairy barns with natural ventilation. The sensitivity of emission rates to dietary CP obtained for various types of dairy cattle operations was consistent (9.91, 12.68, and 10.40 g d^-1 animal^-1 per 1% CP, respectively, for beef feedlots, open-lot dairy barns, and free-stall/tie-stall dairy barns with natural ventilation). The average NH₃ emission rate for barns with mechanical ventilation was less than half that for barns with natural ventilation. When developing mitigating strategies, managing N intake and improving feed efficiency seem to represent the greatest opportunities.