Modeling ammonia emissions from manure in conventional, organic, and grazing dairy systems and practices to mitigate emissions.

Almost 60% of all ammonia (NH) emissions are from livestock manure. Understanding the sources and magnitude of NH emissions from manure systems is critical to implement mitigation strategies. This study models 13 archetypical conventional (5 farms), organic (5 farms), and grazing (3 farms) dairy farms to estimate NH emissions from manure at the barn, storage, and after land application. Mitigation practices related to management of the herd, crop production, and manure are subsequently modeled to quantify the change in NH emissions from manure by comparing archetypical practices with these alternative practices. A mass balance of nutrients is also conducted. Emissions per tonne of excreted manure for the manure system (barn, storage, and land application) range from 3.0 to 4.4 g of NH for conventional farms, 3.5 to 4.4 g of NH for organic farms, and 3.4 to 3.9 g of NH for grazing farms. For all farm types, storage and land application are the main sources of NH emissions from manure. In general, solid manures have higher emission intensities due to higher pH during storage (pH = 7.4 for liquid, 7.8 for slurry, and 8.5 for solid manure) and lower infiltration rates after land application when compared with slurry and liquid manures. The most effective management practices to reduce NH emissions from manure systems are combining solid-liquid separation with manure injection (up to 49% reduction in NH emissions), followed by injection alone, and reducing crude protein in the dairy ration, especially in organic and grazing farms that have grazing and forages as the main component of the dairy ration. This study also shows that the intensity of NH emissions from manure depends significantly on the functional unit and presents results per manure excreted, total solids in excreted manure, animal units, and fat- and protein-corrected milk.