Establishing Cover Crops Between Crops to Manage Weeds
The cover crops between main crops serve as a link between two crops so that the soil is not bare. The goal is to achieve the most complete and rapid canopy closure possible so that the effect of smothering weeds is maximized (allelopathic effects may also play a role but are difficult to quantify). The competition against weed infestation strongly depends on the amount of biomass produced by the cover crop as well as the speed of its development.
Theory
In general, weed biomass is low when the biomass of the crop or cover crop is high; this is the effect of competition (for water, minerals, and light). Some allelopathic interactions can also favor or hinder weeds.
The graph below shows that when the cover crop reaches its maximum biomass, the weeds have practically disappeared.
Trial feedback
Trial presentation
The trial took place in Vinon-sur-Verdon (83) on a sandy loamy-clay soil (clay 18.9%, silt 30.7%, sand 17.6%; organic matter = 2.9%; pH = 8.4). The experiment included 3 treatments with 3 replicates each (6 m x 100 m), arranged in 3 blocks, each treatment differing by its intercrop:
- T1 CC barley + oat: Presence of a cover crop (CC) of barley (Orostar 75%) and oat (Fringante 25%).
- T3 control bare soil: Absence of CC during the intercrop period. The plot was planted with soybean (Sphera), in strict No-till.
What impact on weeds?
Strong reduction of total weed biomass in the crop for treatments with a preceding cover crop. This observation holds throughout the soybean cycle and weed plant density follows the same trend as biomass (weed biomass higher in the control and no significant difference between the two cover crops).
What impact on yield?
A decrease in soybean plant density is observed for treatments with a preceding cover crop. The high biomass produced by the cover crop that was destroyed with a Cambridge roller may have hindered the sowing carried out at the end of May in No-till under the cover crop, thus affecting germination and/or initial growth of soybean. Regrowth of cover crops (mechanically destroyed only) may also have caused competition.
Despite this, there is no impact on yield. There is even a tendency for increase after a cover crop, even though crop density is lower. This compensation is due to an increase in the number of pods for treatments with cover crops (thousand seed weight being comparable otherwise).
Which mixture to choose?
Classic mixtures are known with species that often recur such as mustard, radish, phacelia, faba bean, etc. However, each farmer must adapt their selection based on:
- Their objectives, which can be multiple (weed management, fertility increase, erosion limitation, avoiding leaching losses, protecting soil biology…).
- The pedoclimatic context (soil type, pH, rainfall, temperatures).
- The period and duration of cover crop establishment.
- The previous and future crops.
- The sowing and destruction methods.
The ACACIA decision support tool, created by GIEE Magellan, takes into account all parameters specific to each situation to find the suitable combination.
Do different cover crop species have the same ability to regulate weeds?
The potential of species to suppress weeds is evaluated according to two variables:
- The biomass production potential
- The speed of soil coverage
These two parameters allow comparison (scoring) of species depending on sowing dates.
Sources
This article was written by Jasmin Razongles, agronomy engineering student in apprenticeship at the National Center for Agroecology.
