Grassland and the role of ruminants in carbon management

Grasslands are a vital ecosystem, covering over 52 million kilometres, or 40.5%, of the total terrestrial area globally (excluding Greenland and Antarctica). Historically, huge numbers of wild grazing animals roamed across these naturally occurring landscapes and they, in conjunction with climatic conditions, kept the trees at bay.

In modern times, the majority of these large herds have been replaced by domesticated grazing herds, and they, and increasingly human activities, continue to maintain the open grazing areas familiar today.

A significant proportion of these grasslands worldwide are not suitable for cultivation, due to a combination of altitude, topography, geology, hydrology, geography: in other words, it may be too steep, too wet, too dry, too cold, too rocky or too remote to be viable for crop production and the raising of livestock and production of meat proteins is often the only way to exploit such areas to meet human needs.

These ‘permanent’ grasslands are an important ecosystem and carbon sink, as well as playing a vital role in the human food chain. However, many grasslands are more temporary in nature, and have been used as part of a crop rotation, to build soil fertility, for hundreds of years.

Typically, an area of land is sown with grasses, clovers and possibly many other species of forage plant and would be grazed over a three- to-five-year period. During this time, ruminants (typically cows and sheep in the UK) would repeatedly graze the forage plants during the growing season, converting tough cellulose to dung and urine and this, in conjunction with trampled leaves, root exudates and fungal and microbial soil organisms, would increase the fertility of the soil.

The farmer would then terminate the forage crop, either chemically or mechanically, and would grow combinable cash crops for several years until the soil fertility had been depleted. He or she would then return the land to grass and the cycle would start again.

In the 20th and early part of the 21st century, the increasing availability of artificial nitrogen, manufactured using the very energy-intensive Haber-Bosch process, and the application of other agrochemicals, meant that farms could reduce and even eliminate livestock from their rotations and still grow acceptable yields of grains and plant proteins.

This allowed specialisation to occur and resulted in the investment in fewer, but much larger and heavier machines, and a corresponding reduction in the rural workforce. This, in turn, meant output per unit of labour increased dramatically over the last 60 or 70 years.

The combination of the loss of livestock in the rotation, the increased weight of machinery and the application of large amounts of artificial chemicals and fertilisers did not come without a cost, though. Artificial nitrogen oxidises and removes vast quantities of humus and other organic matter from the soil, making the soil more prone to slumping, compaction and erosion.

The ever-heavier machinery being used compounds this effect. Arable-only rotations have long periods when the soil is either bare and exposed to the elements (especially in a tillage-based system) or has senescing and dead monoculture plants awaiting harvest.

Both of these situations kill one of the most important soil organisms, arbuscular mycorrhizal fungi, and leads to long term damage of the soil. They also mean that, for a significant part of the year, little or no sunlight is being captured. Given that farmers effectively sell sunlight (in the form of meat, milk, grains, eggs, etc) and that soil biology depends on the energy from sunlight for its very survival, this is doubly detrimental to the farm business.

The reintroduction and use of livestock within arable rotations can reverse many of these problems, something that has been recognised for hundreds of years. Recently, researchers in the USA and elsewhere have developed five ‘golden rules’ for building and maintaining a healthy soil. These are:

1. Always keep soil covered to avoid exposure to extremes of sunlight, temperature, rainfall and wind

2. Always keep a living root in the soil

3. Avoid disturbing the soil by either physical or chemical

   means

4. Maintain a diversity of plants

5. Include livestock in the system.

Very few all-arable systems manage to achieve even two or three of these, even when introducing practices such as direct drilling, under-sowing crops and using cover and break crops.

The introduction of livestock can help farmers to achieve all five
of the golden rules which will lead to a dramatic improvement in soil-sequestered carbon and a significant increase in soil health. However, it is essential that the grazing animals are managed correctly to maximise the beneficial impact they have on the land.

The first aim is to graze plants at the correct stage in their lifecycle. They should be at, or nearing, maturity which means they are starting to enter the reproductive phase. At this point, root exudates, sugars and other carbon compounds arising from the photosynthetic process in the plants are at their peak, feeding the microorganisms in the soil.

This leads on to the second aim when grazing, which is to group animals together in a tight ‘mob’ on a patch of land and to move them regularly to fresh grazing. The regular moves ensure the animals only have to eat the best bits of the near-mature plants; they are not being forced to eat the older, more lignified parts.

‘Herbal leys’ are very fashionable at the moment, for good reason. If the 20th century was the age of chemical farming, the 21st century has to be the age of biological farming. A polyculture, or mixture of plant species, can speed this move away from a reliance on chemicals and increase the biological activities of our soils.

There are a myriad of benefits:

• Legumes in the mix can fix nitrogen from the atmosphere, for free, without recourse to fossil fuels;

• Different plant varieties root at different depths meaning
  the access to nutrients, minerals and water from the whole soil profile is maximised;

• Diseases find it very difficult to spread from plant to plant within a diverse herbal ley as they are almost always species- specific;

• Livestock performance is much improved and trials have shown animals are capable of self-selecting the most nutritious diet when offered a varied platter;

Additionally, some of the plants grown in a herbal ley – chicory or sainfoin for example – have anthelmintic properties which reduces the internal parasite burden in the grazing animal.

Scientists are split on how much carbon may be sequestered using grazing animals, with some research showing as much as 8 tonnes of carbon per hectare per year may be captured under adaptive multi-paddock, or ‘mob’ grazing systems. Whilst this may be at the upper end of expectations, it is not inconceivable that, under good management practices, between 1 and 3 tonnes of carbon per hectare per year should be achievable.

The ‘elephant in the room’, when discussing ruminant livestock and carbon sequestration, is the effect of their emissions, particularly methane, on the climate. The issue came to the forefront of the debate when the Intergovernmental Panel on Climate Change (IPCC) published methodology showing the heating effect – known as the ‘Global Warming Potential’ (GWP) – for various ‘greenhouse gases’. The initial calculations attempted to show the impact over a one-hundred-year period of each gas (the figure being referred to as GWP100).

Recently, serious questions have been raised over the original calculations behind these figures as they take no account of, for example, the degradation of methane to carbon dioxide in just over a decade. Making the assumption that the gas persists for 100 years seriously over-estimates its effect on the climate.

Scientists are now working on a new metric, referred to as GWP* which adjusts for this degradation. However, even GWP* appears to have a major flaw in that it still only focuses on the emissions side of the equation.

Ruminants are part of the carbon cycle: for every atom of carbon they emit, whether it is as methane from enteric fermentation, or as carbon dioxide from respiration, an atom of carbon has been removed from the atmosphere by the photosynthesising plant, prior to being eaten. None of the GWP calculations account for the global cooling effect of removing this carbon from the atmosphere.

Until this issue is addressed and embraced by policy makers, figures on emissions from ruminants should be taken with a pinch of salt and should not be allowed to mask the invaluable work grazing animals can do in sequestering carbon into our soils.

Extract taken from the RASE Farm of the Future: Journey to Net Zero Report, written by Tom Chapman, Farm Manager, St. Pauls Weldon Bury Estate and Mob Grazing Specialist.