Double cropping
Double cropping is based on the principle of planting 2 cash crops on the same plot, in the same year. Unlike a cover crop, double cropping has the advantage of being valued for sale or self-consumption.
This practice thus appears as a way for farmers to increase their income by optimizing their area and time.
Note that the second crop, which follows the main crop, can also be called a catch crop. But be careful, catch cropping in general can be used outside the framework of double cropping. It is a crop intercalated between two main annual crops to optimize the use of available agricultural land.
Principle
The farmer will first sow a first crop, the main crop, and manage it until maturity in the usual way. It is only after harvest that interventions will change, to implement double cropping. The operator must act quickly by performing either:
- A superficial soil preparation through two cross harrowings.
- A localized soil work on the row, for example with strip-till.
- A no-till seeding with an adapted seeder[1].
The sowing of the second crop must, whatever the farmer's choice, be done as soon as possible, 4 to 6 days after harvest, to promote emergence. Gaining 1 day at sowing allows gaining 4 days at harvest[1].
Depending on conditions, crop type, and farmer practices, fertilizations, treatments, and weedings are carried out. Weeding remains highly recommended to ensure yields and the health of the second crop. Likewise, irrigation is necessary to practice double cropping in Southern France, otherwise the second crop will not emerge and good results cannot be obtained.
The harvest of the catch crop takes place between October and November, but the later it is, the higher the drying costs and the risk of yield losses.
The earliness of the varieties chosen for double cropping is crucial. Preferably, the main crop should be harvested early, before June 30, to avoid sowing the catch crop too late. The catch crop must also be chosen among early varieties to be able to reach maturity.
Double cropping or relay cropping?
Unlike double cropping, a crop conducted in relay cropping is planted before the first crop is harvested.
Relay cropping presents many constraints for its implementation.
The technique requires adapted tools, notably to sow without damaging the first crop but also to harvest the first crop without affecting the second. Crop management routes can also be restrictive. Sometimes there is a need to irrigate the sowing of the second crop while preparing to harvest the first.
However, due to its cropping calendar, relay cropping is adapted to a wider area than double cropping.
Which crop to choose?
For double cropping, short-cycle spring crops are recommended[2]:
Among these crops, early varieties should be favored.
Terres Inovia published guides in 2022 for soybean and sunflower catch crops to advise farmers.
Soybean in double cropping
Often used, soybean is a good candidate for double cropping. It can, for example, be inserted into a two-year rotation: Barley / soybean (catch crop) / maize.
Choosing an early soybean variety is important to successfully carry out this practice. Yield and protein content (especially for human consumption) are also variables to consider. Later varieties sometimes prove more productive but drying costs and difficulties can be encountered during harvest if sowing is not early enough. It is a choice that must be well thought out and adapted, as well as the sowing conditions (date and density), to the region.
Here is the comparative table proposed in 2022 by Terres Inovia:
During the crop, monitoring of slugs may be necessary. Irrigation is essential. Watering is necessary after sowing to promote emergence and must be maintained afterward. A watering of 30 to 40 mm every 8 to 10 days until the end of September (to be adapted depending on rainfall) is recommended.
If the crop is well managed, the price per ton of soybean seeds ranges between 300 and 600 €/t[1].
Favorable conditions for harvest
- Irrigation > 65 - 70 mm.
- If no irrigation or < 65 mm: sowing date between June 24 and July 13[3].
Favorable conditions for highest yields
- Later and more productive varieties.
- Precipitation + irrigation around flowering > 60 - 65 mm.
- Water essential around emergence.
- Sowing date between mid-June and early July[3].
Sunflower in double cropping
For sunflower, which is more delicate than soybean, attention must be paid to certain points[4]:
- To harvest seeds early enough and avoid drying problems, catch crop sunflower is not recommended in all regions, only some in the south of France: Charentes, southern Deux-Sèvres, Southwest, Mediterranean region, and Rhône valley.
- The cultivation of sunflower for oleic production is not suitable for double cropping. Night temperatures imposed by double cropping (shift in calendar) reduce the oleic acid content of the oils produced.
- Varieties resistant to head sclerotinia and phomopsis are recommended.
- Sowing sunflower after localized soil work, with strip till, at a density of 70,000 seeds/ha seems interesting for this catch crop.
Rapid sowing and good sowing density allow earlier harvest with faster head maturation. Early varieties are therefore to be favored and sowing dates are recommended to farmers:
A watering of 15 to 20 mm is necessary after sowing to secure emergence if it does not rain. Irrigation can then be conducted as for sunflower in main crop.
Fertilization is important for sunflower catch crops. Boron, between 300 and 500 grams per hectare, can be applied during growth to prevent increased deficiency problems in double cropping. Nitrogen application should be reasoned according to the previous crop. For a barley predecessor, an application of 40 units before watering or rain will be favorable to the crop.
Finally, harvest before mid-October is essential. After this date, moisture content will be too high for the seeds.
Favorable conditions for harvest
- Sowing early before mid-July.
- Total rainfall + irrigation > 125-130 mm.
- In many cases, irrigation needed above 60 mm: 1 to 2 passes at sowing + 1 to 2 passes at bud/flowering[3].
Favorable conditions for highest yields
- Irrigation > 55-60 mm combined with precipitation around emergence ≥ 10 mm.
- Or irrigation < 55 -60 mm with total water over the cycle > 130-135 mm and soil not too dry before sowing[3].
Advantages and limits
Advantages
- Increased income: This technique allows economic valuation of covers and a significant increase in profitability.
- Increased production: Double cropping allows harvesting two cash crops in the same year with fairly good yields (depending on conditions and cultural practices).
- Soil coverage: Environmentally, it helps preserve biodiversity and soil structure. Inputs of organic matter and improvement of soil structure can also be provided agronomically.
- Carbon sequestration: A key lever to promote carbon storage on plots is to leave the soil bare as little as possible. Double cropping allows soil coverage thus favoring carbon storage.
- No equipment investment: One advantage for farmers is the possibility to carry out this cropping method without investing in specific equipment. Double cropping can be done with any type of seeder and combine harvester.
Limits
- Need for irrigation: Irrigation is essential to obtain emergence of the second crop and favor yields. There is notably an increased risk of soil drying, the second crop may reduce its useful reserves by drawing on them during severe drought years.
There is thus an increase in water quantities used, which must be anticipated financially. The quotas implemented in recent years seem to be a limit for this technique.
- Risk of plot contamination: Since the second crop must be sown very quickly after harvest, soil work is reduced. However, some farmers report the help provided by soil cover in controlling certain weeds.
Also, the risk of persistence of phytosanitary products, i.e., their presence in the soil following the previous crop and their impact on the current crop, may be increased.
- Increased production costs: Drying costs may be added if the second crop is harvested too late.
- Increased workload in June: Additional work is added in June, a period already busy with ongoing harvests.
Key points of the method
- Early maturity: Choosing early varieties is important. The first crop must be harvested before June 30 to allow planting the second crop, which must also have a short cycle to be harvested.
- Irrigation of the second crop after sowing: To avoid drought and allow good emergence as well as interesting yields.
- Concentration of several operations in June: Sowing the second crop adds to an already busy calendar for farmers at this time. In addition to finishing harvests, grain sorting must be done and forage collected.
- Adaptability and management capacity: To implement this practice, it is necessary to be able to adapt to the pedoclimatic context and to master the crops implemented.
- Adaptation of weed control programs: Weeding must be well reasoned to avoid the presence of annual weeds.
Comparison of different soil covers
Impact on soil
- Main crop then bare soil: Endangers soil life and destroys its structure due to climatic hazards.
- Main crop then cover: Preserves soil biodiversity and structure, reduces weeds.
- Double cropping: Preserves soil biodiversity and structure, reduces weeds.
Carbon storage
- Main crop then bare soil: No carbon storage in the soil.
- Main crop then cover: Significant biomass input favoring carbon storage.
- Double cropping: Carbon storage but less than a classic cover (less biomass).
Soil fertility
- Main crop then bare soil: No input of organic matter into the soil, fertilization inputs must be made to compensate.
- Main crop then cover: Interesting input for the soil and following crops, reduces fertilizer use.
- Double cropping: Nitrogen return to soil if legume in second crop and input of harvest residues.
Additional workload
- Main crop then bare soil: Time allocated for fertilization and soil work.
- Main crop then cover: Reduced fertilization and soil work time, but time used to establish and destroy the cover. Concentration of several operations in June.
- Double cropping: Additional time needed for establishment, management, and harvest of the second crop, less time for soil work. Concentration of several operations in June.
Production costs
- Main crop then bare soil: Fertilization and mechanization costs.
- Main crop then cover: Reduced fertilization and mechanization costs but additional seed purchase.
- Double cropping: Fertilization and seed purchase costs, reduced mechanization costs related to soil work after harvest, possible drying costs.
Income
- Main crop then bare soil: Linked only to the main crop.
- Main crop then cover: Linked only to the main crop.
- Double cropping: Two crops sold.
Other environmental benefits
Double cropping can present other environmental benefits than those previously mentioned. Like classic intermediate cover crops, double cropping helps reduce nitrogen losses from soils. It maintains a low nitrogen residue in the soil which, before the start of the drainage period, reduces the risk of nitrogen leaching.
But double cropping also helps reduce the quantities of drained water. Drained water is not available to roots. This reduction is therefore good for plants. However, water that is not drained is also not percolated, meaning it will not go into groundwater. Compared to a classic cover, the soil can thus be dried out[3].
Conclusion
Double cropping is a growing practice in field crops. It appears as an opportunity for farmers to increase their performance while obtaining agronomic and environmental benefits.
Simple to implement and economically advantageous, double cropping seems to truly satisfy the farmers practicing it.
In a context of climate change and increasingly hot summers, double cropping could expand into areas where the current temperature sum does not allow reaching maturity of a second crop within the year. This practice could thus extend northwards. However, the water need is likely to increase in the regions of the south, potentially becoming limiting in the long term.
Research has been conducted in recent years to characterize this practice. The IPHARD project led by INRAe studied soybean and sunflower varieties adapted to double cropping.
The Chambers of Agriculture of Occitanie and Nouvelle Aquitaine also worked on the subject within the 3C2A project.
Sources
- This article was written by a group of students from the National Superior Agronomic School of Toulouse in January 2024, after reading the results of the IPHARD and 3C2A projects and notably after meeting Gilles Tison, team leader within the IPHARD project, and Matthieu Abella, Development Engineer at Terres Inovia in Baziège, who participated in the 3C2A project, and several farmers practicing double cropping.
La version initiale de cet article a été rédigée par Jérémy Caumel, Océane Legendre, Amelie Leon, Clara Perissé, Nicolas Piette et Simon Pruchon.
