Fonctionnement de la prairie pâturée : structure du couvert et biodiversité

The workings of grazed pastures : sward structure and bio-diversity

The results from studies on the workings of plant populations help to conceive and to assess management rules adapted to the various types of grazing practices met with nowadays. Sward structure influences both the growth and the quality of the available herbage. On the residual leaf index after grazing depend not only the growth rate (interception of the radiation), but also the rate of senescence (amount of senescent matter getting into the litter). Sheath length influences both blade length and blade digestibility. The grazing management consists in finding a compromise between a large height bringing about a maximum ingestion in the short term, and a small height which improves the digestibility but may endanger subsequent growth. The phenotypic plasticity of the forage grasses is both morphogenetical and physiological. It is reversible, but with limits, which may vary according to species. There is a strong genetic variability within natural populations and within cultivars ; the fittest individuals dominate those legs well adapted (who may even disappear altogether) to an extent that depends on the environment. Defoliation interacts with mineral nutrition on the bio-diversity of the natural and semi-naturaI pastures. The diversity of species increases when the availability of mineral nutrients decreases ; it decreases when there is over-grazing (intended for the avoidance or toleration of certain species) or marked undergrazing (encroachment by undesirable species). With a functional approach based on the grouping of species sharing common biological features, it is possible to set up predictive models for the evolution of the sward population.