Greenhouse Gas (GHG) emissions in Kenya in 2013 stood at 60.2 Million metric tons of carbon dioxide equivalent (MtCO2e)i which represented 0.13% of the world total (WRI CAIT 2.0, 2017). Agriculture was the leading source of emissions contributing 62.8% of total emissionsii. Within the agricultural sector itself, 54.8% of emissions were due to enteric fermentation from livestock and 40.7% due to manureiii (FAOSTAT, 2017). According to the above sources emissions due to enteric fermentation were:
60.2 MtCO2e × 62. 8% × 54.8% = 20.71 MtCO2e
Similarly emissions due to manure management were:
60.2 MtCO2e × 62. 8% ×40.7% = 15.38 MtCO2e
Dairy and beef cows have been identified as an important source of GHG emissions. Sources of GHG emissions from dairy farms include methane (CH4) and nitrous oxide (N2O) from enteric fermentation, manure storage and handling, and nitrification/denitrification processes in soil that is used to produce feed crops and pasture. Anthropogenic carbon dioxide (CO2) emissions from fossil fuel combustion and the decomposition of lime applied to crop and pasture land may also contribute. N2O emissions include both direct emissions from the farm and indirect emissions from ammonia (NH3) and nitrates (NO3) emanating from the farm that may ultimately transform into N2O elsewhere. According to the Inter Governmental Panel on Climate Change (IPCC) -Fifth Assessment Report (5AR) (IPCC, 2014), the Global Warming Potential (GWP) from the above emissions are as follows:
GWP from (5AR)
A quantity of GHG can be expressed as CO2e by multiplying the amount of the GHG by its GWP. For instance 1kg of methane emitted can be expressed as 28kg of CO2e (1kg CH4 × 28 = 28kg CO2e). Livestock emit 37% of anthropogenic CH4 and 40% of N2O on a global basis (FAO, 2006). Globally in 2014, cattle (a subset of livestock) are estimated to have produced 11% of all anthropogenic GHG emissions which is about 5,335 MtCO2e (Smith et al., 2014). On the local front, the livestock population figures as released by the Kenya Bureau of Statistics from the 2009 Population Census state that, exotic cattle numbered 3,355,407 while the indigenous breed (zebu) numbered 14,112,367 (KNBS 2017).
The vast majority of the indigenous cattle are kept by pastoralists in Arid and Semi Arid Lands, ASAL regions. Small scale dairy farming activity is mostly found in the former central and rift valley provinces including parts of the coastal lowlands. Added onto this are a limited number of large scale dairy farms that are owned by both private and public firms.
So how exactly do we arrive at the GHG emission figures quoted above? Without getting too technical and academic, I will attempt to answer that question.
The main factors affecting CH4 emissions in the livestock sector are:
i. the amount of manure produced by the various livestock types.
ii. the portion of the manure that decomposes both aerobically and anaerobically.
The former depends on the number of animals and the rate of waste production per animal, while the latter depends on how the manure is managed. When manure is left unattended in our ‘bomas’ (cattle sheds), or when it is deposited on pastures and rangelands, or when it is handled as a solid (e.g. in stacks or piles), it tends to decompose under more aerobic conditions and comparatively less CH4 is produced.
According to the IPCC, there are three tiers used to estimate GHG emissions from livestock and livestock manure (IPCC, 2006). Tier 1 is a simplified method that only requires livestock population data by animal species (category) and climate region or temperature in combination with IPCC default emission factors to estimate GHG emissions. Tier 2 and Tier 3 are more complex methods that require detailed information on animal characteristics and manure management practices, which are then used to develop emission factors specific to the conditions of the country.
When using Tier 1, the simple equation below is used to calculate CH4 emissions from manure management:
CH4, Manure =∑(𝐸𝐹(𝑇) ×𝑁(𝑇)) (𝑇) / 106
CH4Manure = CH4 emissions from manure management, for a defined population, (GgCH4 yr-1)
EF(T) = Emission Factor for the defined livestock population, (kg CH4 head-1 yr-1)
N(T) = Number of head of livestock species/category T in the country.
T = species/category of livestock.
For average annual temperatures of between 23C and 28C, the default emission factors for Africa are 1kg CH4 head-1 yr-1 for both the exotic (dairy) and indigenous (zebu) breeds (IPCC, 2006). These default values may have a large uncertainty (+30 %). for an individual country because they may not reflect the specific manure management conditions present within the country. These uncertainties can be reduced by developing and using factors that reflect country/region specific conditions. Some of these factors include, Bo (the maximum methane-producing capacity of the manure), MCFs (Methane Conversion Factors which are provided for different manure management systems and annual average temperatures) and VS (the organic material in livestock manure). These factors are then used in the Tier 2 methodology.
Using the figures from the KNBS, we can sum up the totals for the emissions from the exotic dairy cattle and the zebu cattle separately and thereafter obtain the sum total of the two categories:
CH4, Manure =∑ (1(𝐷𝑎𝑖𝑟𝑦) × 3,355,407(𝐷𝑎𝑖𝑟𝑦)) +(T)(1(𝑍𝑒𝑏𝑢) × 14,112,367(𝑍𝑒𝑏𝑢)) (𝑇) 106 106
approx 18 Gg CH4 yr-1
According to the IPCC 5AR, 1kg of CH4 has a GWP of 28. This then allows us to express GgCH4 in terms of MtCO2e.
In a similar manner, CH4 emissions from the other categories of livestock can be computed and added to the above figure. These include emissions from sheep, goats, donkeys, swine, horses, and poultry.
Methane is produced in herbivores as a by-product of enteric fermentation. The Tier 1 method for estimating methane emission from enteric fermentation is a simplified approach that relies on default emission factors drawn from the literature. According to
the IPCC, exotic cattle and zebu cattle have emission factors of 46 kg CH4 head-1 yr-1 and 31 kg CH4 head-1 yr-1 respectively (IPCC, 2006). To estimate total emission, the selected emission factors are multiplied by the associated animal population in the respective category:
𝐸𝑚𝑖𝑠𝑠𝑖𝑜𝑛𝑠 = 𝐸𝐹 × (𝑁(𝑇) )
Emissions = Methane emissions from Enteric Fermentation, Gg CH4 yr-1
EF(T) = Emission factor for the defined livestock population, kg CH4 head-1 yr-1
N(T) = Number of head of livestock species / category T in the country
T = species/category of livestock
The emissions from each category are then summed up:
𝑇𝑜𝑡𝑎𝑙𝐶𝐻4𝐸𝑛𝑡𝑒𝑟𝑖𝑐 = ∑𝐸𝑖 𝑖
Total CH4Enteric= Total methane emissions from Enteric Fermentation, Gg CH4 yr-1
Ei = Emissions for the ith livestock categories and subcategories.
Using the figures from the KNBS, we can obtain the total emissions from the exotic dairy cattle and the zebu cattle separately and thereafter obtain the sum of the two categories:
𝐶𝑎𝑡𝑡𝑙𝑒𝐸𝑚𝑖𝑠𝑠𝑖𝑜𝑛𝑠 = [46(𝐷𝑎𝑖𝑟𝑦) × (3,355,407(𝐷𝑎𝑖𝑟𝑦) )] + [31(𝑍𝑒𝑏𝑢) × (14,112,367(𝑍𝑒𝑏𝑢) )] 106 106
=154.34 + 437.48 Gg CH4 yr-1
= 591.8 Gg CH4 yr-1
In a similar manner, enteric fermentation CH4 emissions from the other categories of livestock can be computed and added to the above figure. These include emissions from sheep, goats, donkeys, swine, horses, and poultry.
N2O emissions generated by manure in our bomas and shambas occur directly and indirectly from the soil.
Direct N2O emissions occur via combined nitrification and denitrification processes. Nitrification is the oxidation of ammonia nitrogen (literally nitrogen as part of the ammonia molecule) to nitrate nitrogen (nitrogen as part of nitrate compounds).The products of this nitrification process are then transformed into N2O via a denitrification process under anaerobic conditions. Indirect emissions result from volatile nitrogen losses that occur primarily in the forms of ammonia and Nox. Simple forms of organic nitrogen such as urea from mammals and uric acid from poultry are rapidly mineralized to ammonia nitrogen, which is highly volatile and easily diffused into the surrounding air.
To compute direct N2O emissions from manure management using the Tier 1 method entails multiplying the total amount of N excretion (from all livestock species/categories) in each type of manure management system by an emission factor for that type of manure management system. Emissions are then summed over all manure management systems. The computation of direct N2O emissions from manure management is based on the following (slightly intimidating but simple none the less) equation:
𝑁2O𝐷(𝑚𝑚) = [∑[∑( 𝑁 (𝑇) × 𝑁𝑒𝑥 (𝑇) × 𝑀𝑆 (𝑇,𝑆))] × 𝐸𝐹 3(𝑆)] × 44 /28
N2OD(mm) = direct N2O emissions from Manure Management in the country, kg N2O yr-1
N(T) = number of head of livestock species/category T in the country
Nex(T) = annual average N excretion per head of species/category T in the country, kg N animal-1 yr-1
MS(T,S) = fraction of total annual nitrogen excretion for each livestock species/category T that is managed in manure management system S in
the country, dimensionless
EF3(S) = emission factor for direct N2O emissions from manure management
system S in the country, kg N2O-N/kg N in manure management system S
44/28 = conversion of (N2O-N)(mm) emissions to N2O(mm) emissions
Typical manure management systems (MMS) for exotic and zebu cattle are “solid storage” and “pasture/range/paddock” respectively (IPCC, 2006). Default Nitrogen excretion rates associated with these MMS are 0.6 kg N animal-1 yr-1and 0.63 kg N animal-1 yr-1 for exotic and zebu cattle respectively. Default emission factors (EF3) for direct N2O emissions from manure management “solid storage” for the dairy cattle is 0.005 kg N2O-N/kg N. Default values for total nitrogen loss MS(T,S) from manure management is 40% (IPCC, 2006). This however has a wide margin of uncertainty.
Using the figures from the KNBS, we can obtain the total N2O emissions from the exotic dairy cattle under MMS categorized as “solid storage (ss)”as an example:
𝑁2𝑂𝐷(𝑚𝑚) = [∑[ ∑ ( 3,355,407(𝐷𝑎𝑖𝑟𝑦) × 0.6(𝐷𝑎𝑖𝑟𝑦) × 0.4(𝑆𝑆,,𝐷𝑎𝑖𝑟𝑦))] × 0.005(𝑆)] × 44
Multiplying the above by the N2O GWP of 265,
= 1,676,744.8 kgCO2e
= 0.0167 MtCO2e
= 6,327.3 kg N2O yr-1
The computation above does not include the indirect N2O emissions from the diary category and the N2O direct and indirect emissions from the larger indigenous (zebu) category.
In each of the foregoing three instances (emissions from manure management, enteric fermentation, and nitrous oxide respectively), I have used what is ostensibly the simplest of the IPCC methodologies, i.e. the Tier 1 approach. Both Tier 2 and Tier 3 are much more elaborate. Ultimately however, these latter two need to be applied for more accurate estimations of the GHG emissions from the livestock sector.
Each of us impacts the world we live in, either negatively or positively. This is especially true for those of us who own cattle and other livestock. Properly managing livestock manure is our responsibility. The impact from livestock manure can be negative if we disregard our responsibilities to the earth. Alternatively, the impact can be positive and financially rewarding if we view manure as a resource and an opportunity. Composting has the potential to drastically reduce emissions of Volatile Organic Compounds (VOCs), Nox, and NH3 respectively thereby making it the most effective and profitable means to properly manage livestock manure and to support sustainable agriculture. While small scale agricultural producers and livestock keepers, especially subsistence farmers, are relatively small contributors to greenhouse gas (GHG) emissions, they have a key role to play in promoting and sustaining a low-carbon rural path through proper agricultural technology and management systems.
About the author.
James is a research fellow with the Africa Climate Leadership Program. James has a BSc (Hons) in Chemistry, a MSc in Renewable Energy and is currently pursuing a PhD in Climate Change and Adaptation. This work has been made possible by a grant from the IDRC.