Grazing and Browsing: Reduction of Ground Cover

Description: The ground is protected by plants from the aggressive effects of wind, rain and runoff as well as from direct exposure to the sun, heating and drying, all of which are harmful to soil biological activity. By grazing and browsing, herbivores remove a certain amount of biomass, thus modifying the plant cover and the degree of soil protection, which is determined by the ratio of plant cover compared to bare soil. | Description Positive effects Negative effects

The impact of grazing on plant cover varies depending on the type of plant:

- When severely grazed, many perennial grasses develop tillers close to the ground, which increases soil cover. The best example of this is that of a lawn maintained by grazing.

- When annual grasses are heavily grazed during the growing season they often fail to develop properly and to complete their reproductive cycle. This affects the plant cover that they provide, which can disappear altogether in the long term.

- Tall perennial grasses that grow in tufts or tussocks are not directly damaged by the action of animals' teeth when heavily grazed. However, soil between the tufts becomes exposed to light and sun, which encourages weed germination and growth (particularly young ligneous plants). Grazing also affects the level of competition between grasses and broad-leaved plants. In general, from the farmer's point of view, any changes in pasture composition are undesirable.

- In semiarid climates with steppe vegetation, animals simply defoliate the shrubs and short ligneous plants. Their density can be affected by overgrazing. There is usually bare soil between plants if annual species have not had the chance to grow because of erosion.

- When the vegetation consists of shrubs and trees, animals browse the leaves and branches up to a maximum height (often between 1.50 - 2 m for cattle and goats). When animals eat young shoots and seedlings, they can threaten the plant's capacity to regenerate naturally. Ruminants are sometimes used to clear woody vegetation and to maintain an open landscape.

Grazing and browsing can have positive and negative effects on ground cover depending on:

- the type and structure of vegetation,

- the intensity of grazing,

- grazing frequency (cf. continuous grazing and rotational grazing) and seasonality.

Positive effects: heavy grazing in wet temperate or tropical conditions can make the grass cover lower and make it spread to form a carpet or continuous protective layer. Grazing removes some of the biomass and prevents the accumulation of litter, which can impede grass growth and germination. The reproductive cycle of certain grass species is delayed if the young culms are grazed before flowering. This has the effect of maintaining pasture quality because young plants are much more nutritious than mature plants.

Negative effects: when annual plants are grazed in the growing season, their capacity to produce seeds is limited. Heavy selective grazing makes the most palatable species less competitive and favours the less useful species. Excessive grazing and browsing can weaken and even kill palatable plants. Trampling and soil compaction often damage the ground cover.

When there is inadequate ground cover, the soil is exposed to erosion. When soils are very dry, strong winds can pick up fine particles and dust, which charge the atmosphere. These particles affect air purity and light transmission in the atmosphere.

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