UNDERSTANDING NET GREENHOUSE GAS (GHG) EMISSIONS BY CATTLE

Methane (CH₄) is the primary GHG of cattle, of which 80 to 90% is from enteric fermentation. It is this enteric fermentation that drives the narrative that cattle are detrimental to the climate and the environment in general, and thus the suggestion that globally cattle should be reduced and society increasingly rely more on alternative foods, such as plant-based. Unfortunately, the narrative is often driven by own interest and does not take into account or does not understand how nature deals with the released CH₄. Partially also the problem lies with the way conventional carbon accounting has been done according to what is called a Life Cycle Analysis (LCA) based on, among others, ISO 14040:2006. The LCA approach is linear that largely focuses on emissions, while little emphasis is put on mitigation and sequestration. The more recent standard of carbon accounting (ISO 14067:2018), however, does include mitigation and sequestration by following a biogenic approach. A biogenic approach/cycle is a systems-based approach whereby the enteric fermentation of cattle is weighed relative to their interaction with the pasture or veld, i.e. the local context within which they graze.

The biogenic cycle can be described as follows:

1. As cattle graze and exhale CH₄, they provide the food of methanotrophs, which are soil-based bacteria that use CH₄ as energy and which converts methane into soil-based sugars, thus reducing the CH₄ load that is emitted into the atmosphere.
2. The remaining CH₄ travels to the top of the troposphere (the lowest layer of the atmosphere, i.e. where we live). This journey takes about 90 days and there they encounter the hydroxyl (HO) radicals.
3. The HO radicals are a group of very short-lived molecules that act as nature’s scrubbers. They convert CH₄ and carbon monoxide (CO), among others, into carbon dioxide (CO₂) and H₂O (rain/water).
4. Hydroxyl reacts faster with CO than with CH₄. The more CO is emitted due to industrial processes and fire, the more it outcompetes the CH₄, which then results in more CH₄ being released from the troposphere into the stratosphere, the next atmospheric layer. It is in the stratosphere where CH₄ acts as a GHG. The CH₄ molecule, however, has a very short lifespan of 7 - 12 years, before it is broken down and returned to the troposphere as CO₂ and H₂O.
5. The returning CO₂ and H₂O, in combinations with sunlight, stimulate plant growth through photosynthesis.
6. It is the plant that is grazed, and notably the carbon within that plant, that is used for growth, development, milk production etc., and deposited into the soil in the form of manure and urine. Only a fraction, between 3% and 5%, of the carbon is released back into the troposphere through enteric fermentation, and the cycle starts at number 1 again.

Not only is it just a portion of the CH₄ released that ends in the stratosphere, but its stay is short-lived as indicated above; that while the returned CO₂ and water are instrumental in plant and animal growth. These insights, among others, led to the development of an alternative global warming potential measure (GWP*) to that of the conventional GWP. According to the conventional GWP measure, CH₄ has a radioactive forcing 27 times that of CO₂, but according to GWP* it is much lower and fluctuates at about 8. The Inter-governmental Panel on Climate Change (IPCC), in their 6th Assessment Report (2023), furthermore distinguishes between GWP, which is an energy-based metric, and global temperature change potential (GTP), a temperature-based metric. GTP is much lower as GWP, namely just 4.7 for non-fossil fuel CH₄, such as from enteric fermentation.

When considering the carbon sequestration capability of plants and the contribution that responsible herd management can make to accelerate such sequestration, a farm housing cattle can function as a potential net sink of carbon, cooling the atmosphere. This can be done by applying regenerative practices such as multiple rotations. This expands the annual carbon drawdown area significantly. In addition, such management systems can promote improved water infiltration, biodiversity and enhanced nutrient cycling, among others. If not grazed, the life cycle of grass follows one of three possible pathways. Firstly, it can be burned releasing particulate matter and GHG into the atmosphere. It also mostly releases CO which reduce the HO’s ability to remove the CH₄, while depleting the soil bacteria. Secondly, it can be ploughed or mowed using fossil fuels. This, however, is akin to mining the resource since it removes the nutrients contained therein without replacing it. Thirdly, grass can also become moribund and dry – inert – becoming a sterile system. Often the only way to regenerate such as system is by means of burning or mowing.

In summary, photosynthesis stimulates grass growth and increases carbon drawdown and the deposit thereof in either biomass or the soil – and this entire process is stimulated and accelerated through grazing. This systemic and mutually beneficial co-existence of ruminants and grass maintains the functioning of grass-dominant ecosystems. It has done so since pre-historic times. The enteric fermentation further stimulates the methanotrophs while the enzymes in the saliva kick-start the re-growth of plants. In addition, the hoof movement loosens the soil and the nitrogen in the urine and manure stimulates plant growth and soil carbon development. This activates sugars that leads to further root and plant development, resulting in a process whereby cattle not only can, but do, offset their released emissions. They do so while upscaling low-value and inaccessible carbohydrates into high-value, nutritious and accessible protein used by man.