Rumen at Work: How to Make a Climate-Friendly Cow
With fertile soils, a temperate climate and abundant rain water, Ireland is ideal for farming. Our farming output is much higher than our population’s needs, so we are able to export large quantities of agri-food products (worth ~€13.6 billion annually). Farming is critical for the economy. However, farming also contributes one third of our total greenhouse gas (GHG) emissions. UCD researchers are currently working to reduce a cow’s carbon footprint by up to 50%.
Beef and milk production account for 66% of total agricultural output and roughly 50% of agricultural GHG emissions. Cows have a plant-based diet but do not possess the enzymes necessary for breaking down plant components, and so they rely on microbes in their rumen, the largest compartment of their stomach, to carry out this function. Microbes break down ingested feed to short chain fatty acids (SCFA), predominantly acetate, butyrate and propionate. The animal can absorb SCFAs and use them for energy. Carbon dioxide (CO2) and hydrogen (H2) are produced as by-products of microbial degradation.
If hydrogen is allowed to accumulate in the rumen then it will cause the pH to drop and lead to acidosis, which can cause death. To avoid this, a group of microbes called ‘methanogens’ use hydrogen to reduce CO2 to methane (CH4). Methane can be emitted from the animal by belching. This is good for the animal but detrimental to the environment. Methane is a potent GHG, with a global warming potential 27 times higher than CO2, and its increased abundance in the atmosphere is contributing to the increasing rate of climate change. The question is, how do we preserve our farming outputs while reducing our GHG emissions?
One way to reduce methane emissions is to manipulate rumen microbes through diet and dietary supplements. There are two broad approaches; eliminate methanogens and find a different way to remove hydrogen or manipulate microbial activity so less hydrogen is produced. Production of acetate and butyrate create hydrogen as a by-product, whereas pathways that produce propionate consume hydrogen. Therefore, driving microbial activity toward propionate production could result in less hydrogen produced and therefore less methane.
Feed management, high-sugar forages, plant derived supplements (e.g. tannins, flavonoids, seaweeds) and oils (e.g. garlic, clove, linseed, soya) have all been investigated. They can work in different ways such as altering the substrates available and therefore microbial activity, inhibiting methanogens or acting as alternative hydrogen sinks.
The two problems with any dietary strategy are feasibility and impact. Dietary intervention only works short-term and there are often inconsistent results between herds. For this reason, we have been looking at targeted intervention. This means choosing a time period during the animal production cycle where short-term intervention could lead to the largest reduction in overall methane emissions.
One such time period is late lactation in dairy cows where we found methane emissions are always increased. Another time period is early life when dietary intervention could help us shape a favourable rumen microbiome that leads to reduced methane production and persists for the lifetime of the animal. When designing any strategy, it is important to consider how it could affect animal health, productivity or, in the case of early-life interventions, development of the immune system.
Tamsin Lyons – Science Writer