Nutritional and environmental effects on ammonia emissions from dairy cattle housing: A meta-analysis

Nitrogen (N) excreted in urine by dairy cows can be potentially transformed to ammonia (NH3) and emitted to the atmosphere. Dairy production contributes to NH3 emission, which can create human respiratory problems and odor issues, reduces manure quality, and is an indirect source of nitrous oxide (N2O). The objective of this study was to (i) investigate environmental factors and measurement method that influence NH3 from dairy housing, and (ii) identify key explanatory variables in the prediction of NH3 emissions from dairy barns using a meta-analytical approach. Data from 25 studies were used for the preliminary analysis and data from 10 studies reporting 87 treatment means were used for the meta-analysis. Season, flooring type, manure handling and housing type and system significantly affected NH3 emission rates as well as the measurement method used to quantify the NH3 emission. Ammonia emissions rates from open-lot and scrape systems were considerably greater and those from deep pit systems lower compared to U.S. Environmental Protection Agency (USEPA) estimates used in national inventory calculations. For nutritional effect analysis, the between-study variability (heterogeneity) of the mean emission was estimated using random-effect models and had a significant effect (P < 0.01). Therefore, random-effect models were extended to mixed-effect models to explain heterogeneity. Available dietary and animal variables were included as fixed effects in the mixed-effect models. The final mixed-effect model included dietary crude protein, milk yield and dry matter intake, explaining 45.5% of the heterogeneity in NH3 emissions. A unit increase in milk yield (kg/d) resulted in 4.9 g cow/d reduction in NH3 emissions, and a unit increase in diet crude protein content (%) and dry matter intake (kg/d) resulted in 10.2 and 16.3 g cow/d increase in NH3 emissions, respectively. Ammonia emissions from dairy barns are driven by several factors including housing system, season and diet. Crude protein content of the diet, dry matter intake and milk production are important animal related factors that significantly affect ammonia emission from dairy facilities