2.2.4.b Why consider planting a grass cover crop?

It limits the risks of soil erosion and leaching

If the plot is on a gradient, there is a risk of erosion. If the slope is less than 5%, this risk is low, but if the slope is greater than 20%, the risk of soil lost due to gravity and or leaching/runoff is high. In such cases grading and levelling operations can help limit any soil loss. Erosion risks, whatever the gradient, are accentuated on bare soil because there are no obstacles to retain water and solid aggregates carried by the water.
Erosion can cause a significant loss of the vine's agronomic potential (destruction of vines, de-hilled vines, exposed root systems, loss of soil towards the bottom of the slope, increased difficulty for agricultural machinery to pass through, etc.). When grass cover is put in place, it structures the soil and improves its water-bearing capacity. The plants that grow there constitute a first barrier against any water runoff. It also limits the movement of soil and pesticides towards the bottom of the slope when rainfall goes into groundwater.

Erosion is a major problem as it can cause up to 12t/ha/year of soil loss for a tolerance threshold of 1t/ha/year. When erosion phenomena are significant, up to 30cm of eroded soil from the top of the slope can accumulate at the bottom. Grass cover is therefore essential.

It competes with a vigorous or a excessively vigorous vine

When a plot is vigorous, a grass cover crop can reduce this vigor by adding an element of competition. A cover crop will retain water but also nutirents that are essential for its development. Grass cover can also limit the growth of some of the most invasive weeds, which, depending on the species and variety, can develop easily on a vigorous plot. In addition, the root penetration of the grass improves the soil's structure, bearing capacity and drainage.
If the plot is not vigorous and/or regularly suffers severe hydric constraints, the cover crop must be suited to the plot's situation so the decision of the exact type and species must not be taken lightly. For example, Tall Fescue and Orchardgrass, are some of the most competitive species and, as such, are good at controlling excessive vine vigor.
The competitive effect increases during the first four years and this can seen in the corresponding decline in cane weight. After five years the plant's vigor stabilizes.

It facilitates the movements of agricultural machinery and limits their impact on soil compaction and asphyxiation.

A plot made up of 45% or more clay is characterized as a plot with heavy and compacted clays. The main problem with this soil type is the slow drying rate. Clays are better at retaining water than sandy-textured soils, and when there is a very high proportion of clay, water does not drain very easily. This usually makes it harder for tractors to get through the rows between November and March.
There is a risk that plough pan will form, limiting water drainage and increasing the risks of runoff. However, grass cover can, in most instances, mitigate this rsk. Some clay types "inflate" more than others, depending on their mineral composition (smectite clays inflate more than kaolinite clays).
Before using any heavy machinery on a plot, study the soil's make-up to anticipate any sinking or compaction risks. The development of the grass cover's root system will help water drain more easily on these plots and limit crust formation, soil asphyxia and water runoff.

It reinforces biodiversity and improves nutrient fixation

Grass cover can be considered a "service" crop. Whilst the winemaker is primarily interested in the advantages a cover crop confers from an agricultural perspective, grass cover is very eco-friendly and its many environmental benefits are also felt by the vine.
For example, grass cover encourages biodiversity by providing shelter and food for auxillary fauna, the species of which will vary depending on the type of cover crop planted. Permanent grass cover in the vineyard creates an ecological niche for auxiliaries such as typhlodrome mites, predators of phytophagous mites. If the soil is not bare, the plant cover protects microfauna and also provides favorable conditions for earthworms.

Grass cover also helps the vine to bind nutrients, of which nitrogen is just one example. Some leguminous plants can fix atmospheric nitrogen and make it available to the vine once they have been uprooted and/or buried.