Viviane Fahrni

The forage legume clover, especially the red clover (Trifolium pratense L.), has green house gas mitigation potential in more than one way. Not only can the use of clover decrease the amount of nitrogen fertilizer applied in managed grasslands without significant yield loss while emitting less N₂O into the atmosphere but the enzyme PPO found in red clover can also improve the nitrogen utilization of plant protein in cattle. A more sustainable industry is possible today. 

When we think of agriculture in the context of climate change and green house gas (GHG) emissions methane emissions from cattle and deforestation easily come to mind. But the industry has many more aspects in which its sustainability can be improved. This blogpost explores the potential and limitations of using clover in grasslands as a way of mitigating GHG emissions in agricultural production systems.

While agriculture accounts for about 10-12% of total global anthropogenic GHG emissions it is estimated to account for up to 84% of global anthropogenic N₂O emissions.² Nitrous oxide (N₂O) has a CO₂ equivalent of almost 300 – which means that releasing one unit of N₂O into the atmosphere therefore equals releasing almost 300 times that much CO₂. When we think about what a sustainable agricultural industry could look like, we should therefore not only think of decarbonization but also denitrification.

The N₂O in question originates mainly from microbial activity in soils and manures.¹ It is produced especially when there is more nitrogen (N) available in the soil than needed by the plants.¹ N₂O is also more prevalent under wet conditions.¹ In many grasslands the easy solution would be to apply less nitrogen fertilizer leading to less available nitrogen in the soil and therefore less N₂O emissions . But what about the yield loss? In comes the clover: Rhizobia associated with the clover plant fix nitrogen from the air and make it available for the surrounding flora. Nitrogen provided through this path is called ‘biologically fixed nitrogen’ (BFN).² While this process is generally known not all aspects are yet very well understood.² The question we face is wether the clover is a sufficient replacement for the fertiliser or not.

In a two year field study on an intensively managed grassland in Switzerland Fuchs et al. (2018) tested the mitigation strategy of sowing clover instead of applying nitrogen fertiliser on two adjacent parcels.²  One parcel was fertilised “business as usual” while the other was oversown with clover.² This means that the clover parcel differed from the control parcel in two ways: 1) No fertiliser application and 2) added clover. The aim of the study was to test the mitigation strategy in regard to N₂O gas exchange and productivity.² The hypothesis was that there would be less nitrous oxide emissions from the clover parcel.²  The study showed that indeed the clover parcel emitted less N₂O compared to the control parcel.² While the yields achieved on the clover parcel were slightly lower than on the control parcel they can still be considered stable since they were within the range of yield diffferences achieved in other years.²

What do we take away from this? First that there can be an economic incentive to sowing clover in grass mixtures: using clover can save on artificially fixed nitrogen fertiliser while still maintaining a stable yield. Farmers face initial expenses when oversowing a production site with clover but in permanent grassland this would not have to be repeated so often. The equivalent fertiliser application would require more working hours and when using clover the farmer does save money for the fertiliser. On a GHG mitigation level saving on industrial fertiliser means that there is no energy used to produce the fertiliser nor to transport it. Industrial fertiliser is generally won with the Haber-Bosch method which is fairly energy intensive. We also take away that the fertiliser alternative clover emits less N₂O on the field and is therefore directly beneficial. In short: This approach saves energy for fertiliser production, energy for fertiliser transportation and direct emissions on the field.

Another source of N₂O in agriculture is the incomplete digestion of plant protein and poor nutrient use efficiency of cattle.^3,4 About 70% of nitrogen ingested by cattle is excremented again – unused, in the form of urea and ammonia⁴  – and therefore lost to the animal.³ Potential for mitigation is found in red clover (Trifolium pratense L.).^3,4 Red clover contains an enzyme called polyphenol oxidase (PPO) which acts as a catalyst.⁴ PPO  catalyses the oxidation of endogenous phenols to quinones.³ When active, the PPO enzyme protects the forage protein from degradation early on in the digestive tract by increasing complexing of protein with said quinones.^3,4 For the efficiency of the nitrogen use it is an advantage if the dietary protein passes the rumen and flows to the small intestine in a non-degraded state.³ In other words: the PPO improves the when and where of protein degradation, which improves the uptake of the nitrogen in the protein. Because of its PPO activity red clover has an advantage over comparable forage crops like alfalfa, which has a similar protein content, both when ensilaged and in a pasture setting.³

What is our take away from this? This approach again has an economic incentive: Less supplemental protein feeds are required to maintain the standard production.³ At the same time one animal can provide more milk or meat from the basic (non-supplemented) forage feed. As with the production of fertiliser in the previous approach monetary and energetic costs of the supplement production would disappear while on the same land a high quality forage with a sufficient nutrient content could be achieved. On the GHG mitigation level there is less wastefully excremented nitrogen that has to be dealt with. Of course different production systems manage manures differently which results in more or less nitrogen leaching and different levels of N₂O pollution and therefore smart management is required. Nevertheless it is beneficial to have less nitrogen in the overall system that has to be managed in the first place. In short: This approach saves energy for supplement production and direct emissions from animal faeces.

We have seen that clover has different properties that can help mitigate nitrous oxide emissions from forage production and husbandry through different approaches and make the industry more sustainable. These approaches are interesting and can be implemented according to local natural and economical conditions so that they do indeed have a positve GHG mitigation effect. We should nevertheless take them with a grain of salt. Leguminous pastures without fertiliser application can still lead to nitrogen leaching and therefore N₂O emissions when n0t managed correctly.⁴ We also have to consider that many production systems already include the use of clover –  in Switzerland for example grass/clover mixtures are the standard – therefore the mitigation effect could end up being fairly small depending on the location. As with most approaches to solving the climate crisis: there is probably no silver bullet but well combined measures from many fields may yet lead us there – and maybe we could indeed use some luck with that.

While a lot remains to be said about the ressource intensive and highly controversial consumption of animal products in the context of climate change, the intentional use of clover in forage can already make a change today –  be it with four leaves or not.