The beef and dairy industries emit gases to the atmosphere from animal feed digestion (enteric fermentation) and manure decomposition.In 2012, beef and dairy cattle accounted for 71% and 25% of the total emissions from enteric fermentation respectively, and 15% and 46.7% of the total emissions from manure management, respectively (EPA, 2014). The GHG from enteric fermentation is mainly methane (CH₄), while the GHG from manure management include CH₄ and nitrous oxide (N₂O). 

Methane

The on-farm CH₄ emissions are mainly from enteric fermentation and manure management, with enteric CH₄ accounting for about 75% (EPA, 2014). The International Panel on Climate Change (IPCC) presented guidelines for estimating CH₄ emissions from both sources (IPCC, 2006). The enteric CH₄ emission factors can be estimated based on the gross energy intake by individual animals and the CH₄ conversion factor (an estimate of CH₄ loss per unit of feed). The manure management CH₄ emission factor depends on the quality and quantity of the excreted volatile solid (VS) and how it is managed.

Table 1. Equations for estimating emission factors for enteric fermentation and manure management (adapted from IPCC, 2006)

Ammonia

Ammonia is produced as a by-product of the microbial decomposition of the organic N compounds in manure. For dairy cows, 25-35% of the N they consume is secreted in milk (Liu et al., 2012). For beef cattle, only 14% of the N they consume is retained by the animal (Cole and Todd, 2009). Almost all the remaining N is excreted in urine and feces. Estimated N excretion rates in U.S. are 0.44 and 0.31 kg N per day per 1,000 kg animal mass for dairy cattle and other cattle respectively (IPCC, 2006). The N in the urine is mainly in the form of urea, which can rapidly be converted to NH₃ when in contact with the urease enzyme in feces. Higher pH and temperature favor this enzymatic conversion and thus increase NH₃ emissions. Ammonia emissions are sensitive to urinary urea levels, which increase as diet protein content increases beyond cattle requirements. The total N volatilization from manure (primarily in the form of NH₃) ranges from 5% to 80% of manure N, depending on different manure management systems (IPCC, 2006). It was estimated that annual NH₃ loss from open lot cattle feed yard was about 50% of fed N. Summer emissions are about twice as great as in the winter (Todd et al., 2006). Results of some recent studies on NH₃ emission factors for beef and dairy cattle are presented in Table 2.

Table 2. Estimated NH₃ emission factors for beef and dairy cattle in recent studies

Nitrous oxide

Though most of the N loss from manure is in the form of NH₃, a small part of the N loss is in the form of N₂O and mono-nitrogen oxides (NO_x). Direct N₂O emissions occur via combined nitrification and denitrification of N contained in manure. Oxidized forms of N are first formed through nitrification with a sufficient supply of oxygen, and then they are transformed to N₂O through denitrification in an anaerobic environment. For uncovered anaerobic lagoon or liquid/slurry, direct N₂O emissions are negligible, while N loss as direct N₂O is estimated to be 2%, 0.5% and 0.5% of manure N for dry lots (including feedlots), solid storage, and liquid/slurry with crust cover, respectively (EPA, 2009). Indirect N₂O emissions result from other forms of N loss from manure. About 1% of N loss in the forms of NH₃ and NO_x can be accounted as indirect N₂O emissions. Total (direct and indirect) N₂O emissions in 2012 were estimated to be 1.4 and 0.3 kg N₂O /head/year for dairy and beef cattle respectively (EPA, 2014).

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