Understanding the Sources of GHG Emissions from Oilseed Production

Oilseeds are essential components of the diet, primarily because of their high protein and unsaturated fatty acid content. Through the crushing process, oilseeds yield oil and a byproduct—oilseed meal—which is used as animal feed. These crops accounted for 16% of France’s cultivated land in 2021. ^[1]. Among these, rapeseed is the main oilseed produced in France, accounting for an average of 73% of oilseed production, making France Europe's leading producer of rapeseed ^[2]. Sunflowers rank second, accounting for 20% of oilseed production. France is a net exporter of these seeds and meets the needs of its European neighbors ^[2].

Given such a large area of cultivated land, it is important to understand the key factors shaping the carbon footprint of oilseed production and thus gain a better understanding of its specific characteristics and similarities with other agricultural crops. We will focus solely on field emissions—that is, from planting to harvest—excluding processing stages (crushing, distribution, etc.).

Various databases allow us to compare greenhouse gas (GHG) emissions generated by seed production. These emissions are calculated based on average national production practices. For crops in France, we specifically use the Agribalyse database ^[3], produced by ADEME.

The first factor to consider: efficiency

When comparing two crops, the first point to consider is the unit of comparison. In Life Cycle Assessment, this is the functional unit. When considering food, it makes sense to analyze the carbon impact per kilogram of food. If we look at it from the perspective of a region or a farm, we’ll use hectares as the unit. The difference? Yield!

Taking so-called conventional rapeseed and sunflowers as examples, we observe that the carbon footprint per hectare is more than twice as high for rapeseed. When calculated per kilogram, this difference narrows, as rapeseed’s carbon footprint is only 1.5 times higher. ^[4]. This is due to a higher yield (kg/ha) for rapeseed (approximately 3,800 kg of rapeseed per hectare, compared with 2,700 kg/ha for sunflowers) ^[5].

Yield is therefore the primary factor that distinguishes crops from one another.

For this comparison exercise, we’ll put ourselves in the shoes of a buyer or consumer who wants to compare the carbon footprints of two products. We will therefore continue to use the metric of the carbon footprint per kilogram of product.

Fertilizers, the main source of emissions

When we break down the carbon footprint of the two crops, we see that the largest source of emissions is the same: Fertilizer use and associated nitrogen losses. Fertilizers are, in fact, composed of nitrogen, one of the nutrients necessary for plant growth. When there is an excess of nitrogen or when chemical conditions are not conducive to its uptake by plants (soil structure, moisture conditions, etc.), three processes that generate nitrous oxide (N₂O) emissions - a powerful greenhouse gas - the following may occur: denitrification (direct N2O emissions), volatilization, and leaching (indirect N2O emissions)^[6]. The chemical reactions underlying these three processes depend heavily on the nature of the soil and the types of fertilizers used.

For every kilogram of rapeseed, an average of 2.2 times more nitrogen is required compared to a kilogram of sunflower seeds, which is one factor explaining the difference in carbon footprint observed between rapeseed and sunflower seeds (as a reminder, +54% for rapeseed). Rapeseed is, in fact, more sensitive and demanding in terms of nitrogen, phosphorus, and sulfur inputs. Sufficient inputs enable continuous growth and suppress the emergence of certain weeds ^[7]. The amount of mineral nitrogen applied depends, of course, on the residual nitrogen present in the soil before the crop is planted and is lower when the crop is grown in combination with legumes, for example.

The difference in the distribution of the carbon footprint between sunflowers and rapeseed is most pronounced in the following categories:

• Nitrogen losses, i.e., emissions related to fertilizer use: 60% vs. 66% of the carbon footprint for sunflowers and rapeseed, respectively.
• Fertilizer Production, i.e., emissions associated with fertilizer manufacturing at fertilizer production plants: 17% vs. 22%.
• Mechanization, i.e., fuel use for farm machinery: 22% vs. 9%.

By examining in detail the types of fertilizers used and the hours of operation of agricultural machinery recorded in the Agribalyse database, we can gain a better understanding of this breakdown:

• The fertilizer The ones used are primarily minerals (i.e., from petrochemical-based materials), there is little input of organic matter (i.e., livestock manure, compost, etc.). Of the total nitrogen input, 18% comes from organic sources for sunflowers, compared to 9% for rapeseed. Organic fertilizers result in fewer nitrogen losses than mineral fertilizers, it becomes clear why the use of fertilizers makes a greater contribution to rapeseed cultivation ^[8].
• Emissions from fertilizer production come mainly from nitrogen fertilizers. This is the case for rapeseed and sunflowers, where the emission factors associated with fertilizers are similar and high (between 2 and 4 kgCO2e/kg of nitrogen): fertilizer production (such as urea or ammonium nitrate, for example) require the use of high-carbon inputs, such as ammonia, produced from fossil fuels and gases (notably natural gas), and like nitric acid, whose formation reaction is responsible for fugitive N2O emissions. The difference in contribution between the two crops stems more from the amount of nitrogen applied than from the type of fertilizer used.
• Finally, the farm machinery used for rapeseed consumes 25% less fuel than that used for sunflowers. This explains why mechanization plays a greater role in sunflower cultivation. The difference lies mainly in the soil preparation phase. In fact, sunflowers are a crop that is prone to many weeds ^[9] and whose tillage techniques are effective in controlling these weeds. For example, sunflowers offer more false-seeding windows ^[10] In early spring, a technique that limits weed growth during the final emergence of sunflowers and requires an additional mechanical pass to destroy the weeds.

In conclusion

From a carbon perspective alone, and when considering the national level, sunflower seeds produce fewer greenhouse gas emissions during production than rapeseed. That said, there are several points to consider:

• This reasoning, which applies at the national level, does not apply at the level of a specific region, where climatic and agronomic conditions may favor one of the two species. It is therefore essential for an industrial player or a cooperative to shift from a national perspective to a “farm-level” perspective in order to improve the accuracy of emissions estimates.
• Other environmental criteriacan and should be taken into account such as the water requirements of these two crops, the amount of pesticides used, the carbon sequestration potential of each crop, etc.
• The The resilience of crops to pathogens and climate hazards must also be analyzed because this affects yields first and foremost (and thus the carbon intensity of crops), but also supply capacity.
• If we broaden our perspective beyond France to include all oilseeds, Rapeseed and sunflowers often compete with other seeds such as soybeans for soybean meal used in animal feed, or palm oil for food or biofuels. However, soybeans and palm oil are primarily produced in countries where the risk of deforestation is high. The challenge for a buyer, therefore, is to know the origin of their raw materials and to prioritize seeds that have a lower environmental impact.

Ultimately, the goal is not so much to compare rapeseed and sunflowers as it is to understand how conventional farming practices affect GHG emissions from the agricultural sector. This step then makes it possible to understand and identify the measures needed to reduce emissions.

This “reduction” section will be the subject of another article in the coming weeks.