Woody resources in grazing

Beef cattle farming Farming Dairy cattle farming

Woody plants are useful for grazing, livestock farming, and the ecological balance of territories.

Long victims of agricultural and landscape policies that "fought" against land abandonment and "improved" the lands, woody plants are experiencing a resurgence among farmers and their herds. Moreover, an increasing number of scientific and technical references now demonstrate their forage and environmental value.

The term "woody plants" in the pastoral sense includes trees, shrubs, bushes, etc., that is, woody plants of varying heights. They thus form one or more pastoral layers complementary to the herbaceous layer that livestock practices can enhance.

Woody plants in grazing have various functions. They can provide shade, windbreak, etc., but also forage resources for herds.

Today, it is no longer about fighting woody plants but about taking advantage of the food resource they offer. Woody plants notably help to strengthen the food autonomy of farms and secure the livestock system against climatic hazards. They find a place in all animal productions.

Even though the nature of woody species varies according to the pedoclimatic context, the principles and technical levers to manage their use on farms are common. They are not so different from those reserved for grass.

Effects of the presence of woody plants on the food availability of vegetation

The uses of woody plants on farms are varied: grazing but also cutting for fresh or dry distribution. Woody plants diversify the food availability of an environment throughout the year by:

Direct effects

Woody plants produce forage (all or part of the plant). They can be distinguished according to several criteria:

Height, which determines the accessibility of foliage and the impact on grass;
Leaf persistence (deciduous, evergreen), which changes the seasons of leaf presence and thus the light exposure and protection of the lower layer;
Palatability of leaves but also stems;
Toxicity, linked to the presence of compounds that affect the animal's metabolism (tannins, alkaloids, etc.);
Regrowth capacity after harvesting, which is good for continuously growing species like bramble but lower for spring species;
Development of defenses against grazing (thorns, hairs, toxins, lignin) and mechanical, chemical, or fire actions.

Unlike grass, woody plants' growth is more late in spring and continuous for a good part of the summer. Moreover, they have good summer residual standing biomass and also excellent winter residual standing biomass for woody plants with persistent leaves or needles.

Availability and residual biomass curves: Schematic representation of the evolution during the year of growth curves and residual standing biomass for each layer of vegetation with low and tall woody plants. The color bands give an idea of the very complementary food availability of these three layers.

Indirect effects

Woody plants structure the environment and modify the functioning of grass: "under the trees it starts earlier, matures later, grows a bit less densely, regrows at the end of summer."

Using woody plants in herd feeding

The practical value of vegetation including woody plants depends both on their role in the forage system and the practices implemented to guide animal harvesting.

Valorization practices

Direct grazing of edible parts: buds, flowers, leaves, fruits, young shoots, and older more or less lignified stems
Direct grazing of leaves and fruits on the ground
Pollarding / Pruning: tree branches are cut at the end of summer or autumn and given for fresh grazing, or stored dry for distribution several months later.

Seasonal functions

Although also appreciated by herds in spring, woody plants offer a sought-after forage in summer because it complements grass (animals seek the leaves of woody plants which are less fibrous than dry grass at this season) or in winter (animals benefit from persistent foliage or fruits fallen on the ground).

Types of nutritional needs

Some woody species have a higher nutritional value (ash, willow, raspberry, etc.) than others (oak, hazel, etc.). But the common interest of all woody plants lies mainly in the beneficial effects they provide on the overall palatability of the vegetation. They allow the animal to make a variety of bite sizes, which stimulates intake and thus contributes to achieving satisfactory zootechnical performances (including on plants of lower nutritional value).

Moreover, the notion of relative palatability over time must be considered. If bush leaves are not very attractive in spring when grass is young, they become much more so when grass is mature. Under these conditions, the leaves are then less fibrous than grass.

Management of toxins by the animal

Increasing exposure of animals (one to two hours per day to start), even from a young age, gives them time to activate kidney metabolic pathways (elimination of toxins in urine) and liver (modification or destruction of toxins). As for acorns or chestnuts rich in starch, the presence of other fibrous and/or nitrogenous resources will facilitate their digestion. Also, certain plant associations cancel out the effect of toxins. This is part of animal learning.

Long-term renewal of woody resources

If one wishes to valorize woody plants, it is essential to control their density and/or accessibility in order to maintain the resource in the long term.

Whether the goal is to promote woody plants or to make them regress, it is necessary to understand the biological mechanisms involved to avoid unwanted evolution of woody cover by implementing practices not adapted to these objectives.

Biological mechanisms involved in the above diagram

Energy reserve storage (figure below): reserves allow the growth of leaves and stems, they are essential for the survival of woody plants. They are located in the roots and wood. Failure to replenish reserves over the years due to inappropriate and repeated harvesting of leaves and stems (grazing or cutting) leads to dwarfing, even death of woody plants.
Avoidance strategies: after grazing, woody plants tend to escape upwards to avoid animal bites. After shredding, woody plants defend themselves by reducing their palatability (rapid lignification of stems, increased thorn production, bitterness of leaves, increased toxins).
Reproduction mode: after grazing, woody plants mostly reproduce sexually (seeds, seedlings, saplings). Some management practices can favor or penalize the survival of young saplings (complete or incomplete consumption of grass). After shredding, some woody plants activate vegetative reproduction (stump and/or root shoots).

Autres fiches Pâtur’Ajuste

Sources

SCOPELA, with the contribution of farmers. Technical sheet from the Pâtur’Ajuste network: Woody resources. November 2017. Available at: https://www.paturajuste.fr/parlons-technique/ressource/ressources-generiques/les-ressources-ligneuses