In Europe, agriculture is responsible for 26% of greenhouse gas emissions and one third of these emissions come from livestock.
In livestock farms, we can distinguish four primary sources of greenhouse gas emissions:
Direct or indirect emissions from the combustion of fossil fuels
Methane emissions during storage/spreading of effluent
Methane emitted by the ruminants during their process of rumination, this emission contributing to the partial loss of energy from food (Lassey, 20071))
Emissions of Nitrous oxide (N2O) generated by spreading mineral or organic nitrate fertilisers. Large differences in N2O emissions are observed depending on climate and grasslands management (Flechard et al, 20072)).
The feeding system has little impact on methane emissions by ruminants. However we can notice that the presence of condensed tannins and saponins in food could reduce methane emissions during rumination.
We can also underline that an increase in energy concentration of the diet of ruminants leads to a reduction of methane emissions.
Managing grasslands, and in general managing livestock farming, can significantly modify the energy consumption and thus reduce the CO2 emissions from the consumption of fossil fuels.
In agricultural production, the main consumption of fossil fuels is nitrogen fertilisation. In fact, all mineral nitrogen fertiliser produced globally use the same industrial process. This is the Haber-Bosch process invented in 1909 and its inventors won a Nobel Prize.
This process uses natural gas as the energy source for this reaction and also as a source of hydrogen atoms for the synthesis of ammonium nitrate (NH4NO3). After various optimisations, this process has an energy efficiency of about 0.95.
In spite of this very high efficiency, one needs the equivalent of 1.8 kg of oil to fix a unit of nitrogen in the form of ammonium nitrate.
There is another possibility to provide nitrogen in agriculture and livestock: it is a symbiotic fixation by legumes, this fixation occurs at the root level, in nodules present in all species of legumes.
We can use the graph above to underline the consequences of the nitrogen sources on the energy cost of various crops. This graph compares the crops of wheat, beetroot and lucerne, harvested for hay in the Champagne-Ardennes.
These three crops produce roughly the same amount of energy exported by the aboveground biomass.
This study conducted by Gosse shows that lucerne, because it does not use nitrate fertiliser, has an energy consumption per area unit that is much less than wheat or beetroot.
After the assessment at the scale of the plot, it is important to make an assessment at the scale of the farm. This work is possible on the basis of observations, such as for example that led by Solagro with the use of “Planet” software (Bochu et al, 20063)).
The analysis of three types of farms (conventional milk, crop/livestock, or organic farming) shows that four variables of energy consumption at the level of these farms are animal feed, fuel, fertilisers and electricity.
For these variables, choices in terms of grassland and forage crops will be essential and in particular will reduce the costs of animal feed related to protein supplementation in particular, costs associated with fuel for harvest, and those related to fertilisers for nitrogen fertilisation of the grasslands.
The overall analysis of all farms that participated in this energy balance assessment demonstrates a certain relationship between total energy consumption (expressed in litres equivalent fuel/ha) and energy efficiency of the farm.
This graph shows that average fuel efficiency of dairy farms is close to 1, which means that energy produced in food form is close to the fossil energy used for the production process.
However, despite this severe situation, it is possible to notice significant variations between farms, with poorest energy efficiencies being close to 0.4, while the best being at almost 2.
To understand the basis for these differences, it is necessary to further analyse the production process.
An example of a detailed analysis of the production processes and the variables of energy consumption was proposed by Haas et al. in 20014) on the basis of dairy farming in Northern Germany.
The study by Haas et al. (2001) compared three types of farming in the same region (intensive, extensive, organic production) that differ in their level of loading (in UGV/ha) and the production in L/cows.
The energy consumption (expressed here in GJ/ha) analysis shows that consumption per hectare or per tonne of milk is much higher in the context of an intensive system than in the extensive or a organic system.
Energy efficiency (energy produced / energy consumed) is slightly greater than 1 for intensive farming while it is greater than 2 for two others, extensive and organic farming. It should be noted that, in the study of Haas et al. in 2001, energy consumption related to fixed assets (equipment and buildings) is not taken into account.
Differences between the intensive system and the two other systems come from the method of harvesting and forage conservation, since the intensive system makes extensive use of drying grass to create an inventory. There is also a significant use of fertilisers.
However, when comparing these three farming systems in terms of greenhouse gas emissions (expressed either per hectare or per ton of milk), the differences are much smaller, 33% ( expressed in units of area) or only 24% (expressed in tonnes of milk).
The main reasons are 1) a significant share of greenhouse gas emissions are related to emissions of methane by animals, and 2) the extensive and organic systems, having smaller productions by animal, use more animals to obtain same amount of production.
The comparison of energy efficiency with the supplementation of animals shows a significant negative correlation. An increase in the amount of dietary supplement given to animals and dairy cows during the year leads to a decrease in energy efficiency.
This is related to the fact that the production of these supplements rich in protein (soybean oil cakes) or energy (cereals), consumes energy in the production process.
Thus the conversion of crops into permanent grasslands accumulates, at least in the early years, some carbon at the rate of about 0.5 tonnes/ha (Dupouey et al, 20055)).
However, it is also necessary to take into account the fact that reversal of grassland and its setting in crop culture leads to destocking of the carbon immobilised in the superficial soil areas zones, and this destocking is done twice as fast as the stocking.