Cattle Emissions

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). 


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)


Key variables

EFe = GE·Ym /55.65 = (NE/DE)·Ym /55.65

In which,

     EFe = enteric CH4 emission factor, kg CH4 /head/year;

     GE = gross energy intake, MJ/head/year;

     Ym = enteric CH4 conversion factor, percentage of gross energy in feed converted to CH4;

     NE = summed net energy requirements, MJ/head/year;

     DE = digestibility of feed, percentage of GE intake digestible;

     The factor 55.65 MJ/kg CH4 is the energy content of CH4.

  • NE includes net energy required by the animal for maintenance, growth, activity, pregnancy, lactation, work, etc.), and depends on type and weight of the animal;
  • DE can affect gross energy intake; typical DE in U.S. was 66.7% for dairy cows and 82.5% for feedlot cattle in 2012 (EPA, 2014);
  • Ym depends on several interacting feed and animal factors; default values of Ym provided by IPCC (2006) are 3.0±1.0% for feedlot cattle that are fed diets contains 90% or more concentrates, and 6.5±1.0% for dairy cows and cattle that are primarily fed low quality crop residues and byproducts.


EFm = VS·365·Bo·MCF·0.67

In which,

     EFm = manure management CH4 emission factor, kg CH4/head/year;

     VS = daily excreted volatile solid, kg/head/day;

     B0 = maximum methane producing capacity of manure, m3/kg of VS;

     MCF = manure management CH4 conversion factor, percentage of VS actually converted to CH4 compared to B0;

     The factor 0.67 kg/m3 is conversion factor of m3 CH4 to kg CH4.

  • VS can be estimated based on feed intake and digestibility; typical VS in U.S. is 5.4 and 2.4 kg/head/day for dairy cow and other cattle respectively;
  • B0 varies by species and diet; typical B0 in U.S. is 0.24 and 0.19 m3/kg of VS for dairy cow and other cattle respectively;
  • MCF depends on manure management system, temperature, and retention time of the storage unit; MCF for liquid manure systems is much larger than that for dry manure systems (e.g. 50-80% for anaerobic lagoon vs. 2-5% for dry manure, solid storage).


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


Source area

Cattle type

Emission factor

(kg NH3 /head/year)

Todd et al., 2006

Open lot pens

Beef steers and heifers, 275-550 kg


EPA, 2004b

Dry lot

Beef and heifers


NAMES, 2010

Free stall barns



Liu et al., 2012

Simulated tie-stall



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 (NOx). 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 NOx 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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