Management of grass cover through permanent associated cover crops

Technical articles - Centre National d'Agroécologie - Conserwa - Weeds, Soil regeneration

Field crops Field crops Cover crops Permanent covers Crop association Relay covers Intercropping

Issue du GIEE Magellan

Associated covers are plants of agronomic interest whose development cycle takes place partly or entirely during the period when the crop is in place. These covers are supposed to remain in place for several years (2 or 3) and ensure relay covers during intercrops but also provide services to cash crops.

Advantages

Competition with weeds

The ability of a permanent cover to suppress weeds depends on its ability to occupy and cover the space before the weeds establish.

The competitive effect on the weed population is observed only from a soil coverage greater than 50% by the legumes of the permanent cover.  

Beyond this threshold, the greater the coverage, the better the reduction in the proportion of weeds.

It is when 80/85% colonization by legumes is reached that weeds are effectively controlled (less than 10%).

This graph is taken from the specific case of legumes in sunflower.

Among the three species tested (alfalfa 10 kg, bird's-foot trefoil 10 kg, white clover 3 kg), only alfalfa provided sufficient soil coverage to achieve weed control effectiveness comparable to the chemical weeding treatment.

Here too, an absence of weeds is observed when the cover is very developed.

The reduction of weeds is noticeable from 50% coverage but is significant only from 90%.

However, the less competitive the cover, the greater the diversity of weed species. Therefore, there is not necessarily a quantitative impact on weeds when coverage is below 50%, but there is a qualitative change.

A correlation between the specific diversity of weeds within a plot and the yield losses associated with them has been established.

The greater the diversity, the less the effects on the harvest are felt as shown in the graph. The more developed the permanent cover, the less species diversity in the plot (cover + weeds) and the greater the competition and impact on crop yield.

Disadvantages

Impact on crop yield

Still on the same trial, when there is a cover, the performance of sunflowers is impacted as shown by all the characteristics evaluated in the table.

The species with the greatest impact on the crop is alfalfa, on all points. This is because it provided the best coverage, 75% of the surface compared to 57% for clover and only 32% for bird's-foot trefoil.

There is therefore a strong yield decrease when there is a cover or spontaneous grassing compared to a chemically weeded sunflower.

Overall, yield losses (15 q/ha in maize under permanent white clover cover) are caused by no-till under permanent cover practices for spring crops especially if soils are shallow and/or non-irrigated.

For winter crops, it is easier to control permanent covers chemically or by mowing but if there is no intervention, yield losses are almost systematic.

Conclusion

When the cover is sufficiently developed (beyond the 80% vegetation coverage threshold) to allow effective and significant weed management, it causes significant yield losses. To avoid this, covers are controlled by mowing or chemically but their competition with weeds is limited.

The major advantage of the permanent cover is during the intercrop period where it takes over from the crop immediately after harvest. The limitation of weed infestation during the intercrop is directly related to the biomass production of the permanent cover and the speed of development.

Sources

This article was written by Jasmin Razongles, agronomy engineering student in work-study at the Centre National d'Agroécologie.