Analysis of a crop’s resilience in an agroclimatic context.

This article follows up on our first brief on the carbon impacts of rapeseed and sunflowers. Read it here.

The agricultural sector contributes significantly to global warming ^[1], but it also suffers the consequences. Record-breaking heat waves every year, warm spells in winter, and late frosts are all unpredictable events that can directly affect a crop’s healthy growth and yield.

It is nothing new for farmers to study the impact of the climate on their crops, since certain climate indicators are key to plant growth. This is particularly true of three parameters, which are monitored annually: radiation, temperature, amount of water. However, climate change is making these indicators less predictable and less favorable for the types of farming practices that have been in use for several decades. A climate risk analysis, such as those conducted by Carbone 4, enables long-term projections to anticipate the negative impacts associated with changes in a crop’s key climate parameters.

In our previous article^[2], we examined the carbon footprint of oilseed crops such as rapeseed and sunflower, highlighting the main factors influencing greenhouse gas emissions during the production of these two seeds. Here, we will focus on the resilience of one of these crops—rapeseed—to the effects of climate change.

Solar radiation during flowering and soil moisture content at the beginning and end of the growing season: two key factors for rapeseed

Assessing the climate risks associated with a crop first requires understand which climate factors are critical to its production. To do this, we define the agro-climatic indicators (ACI) which set thresholds below which crop yield is significantly affected. These IACs are defined by crop and by geographic area^[3]. Examples include water shortages, winter frost, excessive rainfall, etc.

Let's take winter rapeseed as an example.

The diagram below shows the stages of rapeseed development throughout the year.

                                                   Growth Stages of Winter Rapeseed (Source: Yara)

An analysis of IACs for winter rapeseed cultivation reveals the following susceptibilities:

• The soil drought, that is, a lack of water combined with high temperatures, during the planting season (late summer for winter rapeseed):
  • Without sufficient water at the time of planting, rapeseed seedlings will not take root, and the chances of a harvest will be lost. The risk of drought is likely to increase with longer, drier summers, as projected by climate scenarios.
  • For example, in 2020, conditions favorable to rapeseed planting (particularly sufficient rainfall) did not materialize until late in the season, and when the rain finally arrived, it came in excessive amounts, reducing the success rate of planting^[4]. Poor yields in previous years and uncertain conditions for the coming year have only encouraged farmers to abandon rapeseed cultivation. The result: a decline in crop rotation in 2021.
• The lack of visibility during flowering (May):
  • Sunlight triggers photosynthesis and, consequently, the formation of flowers and grains in a crop, a key factor in future harvests.
  • For the same amount of radiation, if the temperature rises, the time it takes for silica to form^[5]is reduced and results in less significant development of these phenomena. In practice, we observe that radiation increases with temperature and, conversely, that a lack of radiation is often correlated with low temperatures^[6]. Therefore, rising temperatures are unlikely to have much of an effect on radiation.
• Thermal and Water Stress at the End of the Cycle during flowering and grain filling (May–June):
  • These stress factors correspond, respectively, to particularly high temperatures and a prolonged period of water shortage. Both of these stress factors can lead to a reduction in the number of seeds per pod in rapeseed.
  • Scientific studies agree that for every day when temperatures exceed 29–30°C, rapeseed yields are significantly reduced^[7].
  • In 2020, yields were low due to a spring drought, as well as late frost and excessive winter moisture (yields varied widely by region). In 2021, yields exceeded even the most pessimistic forecasts thanks to favorable conditions during the grain-filling period in May, highlighting the importance of this period.^[8].
• The Impact of Pests rapeseed throughout the growing season, especially in the fall and spring:
  • Rising temperatures are conducive to microbial and animal activity, promoting their growth while the rapeseed is still in its early stages of development^[9]. This is true of insects such as large flea beetles or terminal bud weevils, or diseases such as verticillium wilt.
• Extreme weather events, such as late frost or excess water :
  • Late Frost: Rapeseed has good resistance to winter frost once it reaches the B8 stage (rosette). However, rising fall and winter temperatures lead to early bolting, which increases the crop’s susceptibility to late frosts in late winter and early spring. According to Terres Inovia, the bolting period has already shortened by 10 to 20 days over the past 10 years (2020 vs. 2010)!
  • Excess water: Excessive rainfall over a short period of time can have dramatic consequences for crops (such as root asphyxiation in winter) and is often linked to a lack of sunlight in the spring.

Rapeseed is a crop that is highly resistant to heat and frost and can tolerate moderate water stress thanks to its physiological adaptability and its ability to seek out nutrients at greater depths.^[10]. However, It requires specific conditions during critical phases, namely the sowing period at the beginning of the growing season (August–September) and the flowering and grain-filling period at the end of the growing season (May–June). We will therefore focus on these phases in the next section, which will highlight the impact of climate change on certain agroclimatic indicators for rapeseed, particularly indicators of water and heat stress^[11].

Rapeseed-producing regions affected by drought

Rapeseed production in France totaled 3,330,000 metric tons in 2020, concentrated primarily in four regions: Grand Est (20%), Centre-Val-de-Loire (18%), Hauts-de-France (14%), and Normandy (11%)^[12].

To assess the potential impact of climate change on rapeseed, we studied the two periods of high sensitivity for rapeseed—namely, the sowing period at the beginning of the growing season (August–September) and the flowering and grain-filling period at the end of the growing season (May–June)—in light of three key climate parameters for this crop:

• Changes in Average Temperature, expressed in °C.
• Thermal Stress, as reflected in the trend in the number of abnormally hot days^[13].
• Soil Drought, as reflected in the trend in the number of days with dry soil^[14].

The following analysis highlights changes in these parameters between the present and the medium term (~2050)^[15]in two climate projection scenarios (RCP 4.5 and RCP 8.5)^[16].

Climate projections show a significant increase in temperature during the planting season (August–September): this increase ranges from 2 to 3°C depending on the region and the scenario, with a peak of 3.1°C in the Grand Est region under the worst-case scenario. These results show that climate change affects all rapeseed-growing regions, with an impact that appears to be more pronounced in the Grand Est and Centre-Val-de-Loire regions.

The likelihood of periods of heat stress is also increasing. The maps below show more frequent episodes of extreme temperatures, with the most severe warming scenario resulting in:

• An average increase of 2.5 days per month in the number of days with extreme heat in May and June (representing an increase of more than 50% compared to today), with an average peak of 3.1 days in May in the Grand-Est region;
• An average increase of 5 days per month during August and September (roughly 2.5 times the current level), with a peak of 5.9 days on average in August in the Grand-Est and Centre-Val de Loire regions.

These results point to lower yields and reduced planting success for rapeseed, particularly in the Grand Est and Centre-Val de Loire regions.

As for soil dryness, the scenarios show a trend toward an increase. Unfortunately, the uncertainty in the precipitation models makes it impossible to draw conclusions about regional trends.^[17].

Nevertheless, it is reasonable to conclude that The increase in water stress episodes, combined with extreme temperatures, does not bode well for soil moisture conditions and, by extension, for rapeseed cultivation., and that these fluctuations will likely be more pronounced in the Grand Est and Centre-Val de Loire regions.

Ways to Adapt That Are Within Our Reach

To address the climate risks threatening rapeseed production, there are three types of adaptation measures:

• Adapting agricultural production methods ;
• Developing genetics seeds that are resistant to future climate conditions;
• Relocate production facilities in areas more suitable for rapeseed.

The decision to shift the production areas for a crop such as rapeseed will be a long-term one and will only be made if farmers are guided by stakeholders in the sector—particularly technical institutes—so as to objectively identify production areas suitable for rapeseed cultivation. Furthermore, this decision is not the most desirable option because if rapeseed were to become less viable for cultivation in its regions of origin, it would directly affect rapeseed processing plants.^[18]. In fact, there are two major risks involved:

• Risk of a failed asset : A decrease in the volume of seeds entering the crushing process could lead to a partial or even complete shutdown of the plant;
• Logistical Risk : While rapeseed is no longer grown in the region where the industrial site is located, it may be grown in other regions; however, this would require a reorganization of logistics (longer distances traveled and, consequently, higher costs, as well as associated environmental impacts and risks).

As for the development of suitable genetic material—the second proposed strategy—this is an issue currently being addressed by agricultural research institutes and will require several years of experimentation and feedback.

It is therefore worth taking a closer look at the first lever, the only lever that can be activated in the short term and is directly within the control of rapeseed farmers.

Let's take the example of the associated rapeseed plantings :

Rapeseed is known as a companion crop when planting another cropat the same time and on the same plot of land than him. This culture is chosen so that it will degrade naturally and let the rapeseed grow. This is the case with the cold-sensitive legumes (e.g., vetch, field fava bean) that will grow more slowly than rapeseed (favoring rapeseed) and are highly susceptible to frost.

There are two benefits to planting rapeseed in combination with legumes: in addition to the improvements in soil fertility through the return of nitrogen from companion legumes to the soil, The ground cover provided by this plant reduces the risk of frost and the proliferation of fall insects and weeds. This combination appears to be advantageous in terms of both yield and associated costs^[19],^[20]. This combination seems all the more interesting given that milder fall and winter weather leads to increased activity among insects such as large flea beetles and terminal bud weevils, which are pests of rapeseed^[21].

However, this technique requires a early planting which will give the legume time to grow and fulfill its role. However, planting earlier increases the risk of drought during the summer months (August in particular). Therefore, compromises must be made when choosing technical agricultural routes : Is it better to minimize the risks associated with pests and frost in the fall and early winter, or to facilitate establishment through late planting?

The best practices to follow depend on the specific fields and the desired crop rotations! For a field with a high residual nitrogen level in the fall, growing rapeseed as a companion crop will be less beneficial. It is also possible to plant perennial cover crops that will benefit the crops following the rapeseed.

Adaptation strategies must therefore be examined in light of crop production, the needs of individual farmers and their local areas, as well as climate risks.

Conclusion

Rapeseed production is not immune to the risks associated with climate change and the increasing prevalence of conditions unfavorable to its cultivation. In particular, increased drought in rapeseed-producing regions could significantly affect the yield and sustainability of this crop. Adaptation strategies exist to mitigate these risks, but they are still in the exploratory phase or limited in number. It is important to expand experimentation and share lessons learned in order to increase the chances of finding a variety of sustainable solutions.

And what about you, farmers and all those working on the ground—have you already noticed any changes in agro-climatic indicators? Have you adapted your farming practices, and if so, what lessons have you learned from this?