Sward structural resistance and biting effortin grazing ruminants

Grazing ruminants face complex decisions in searching for and harvesting adequate forage to meet their requirements in the face of heterogeneity in the abundance, nutritive value and distribution of resources. Some of the major decisions which affect the bite mass and therefore, forage intake adjustments in bite depth, bite area and exerted bite force, are made in relation to heterogeneity in sward structure. A number of relationships have been established linking the adjustment of bite mechanics in response to variations in sward height and bulk density on temperate forages for animals of a range of body sizes. However, there is less consistency in the response of the bite mechanics to the greater variations in structural resistance that arise from the vertical and horizontal arrangements of the plant morphological organs of leaf, pseudostem and stem. Furthermore, only limited progress has been made in quantifying the biting forces involved in grazing, the linkages between the plant morphological organs and their effect on bite force and, the interaction with body mass. In this paper, we discuss the different hypotheses that have been proposed to explain the regulation of bite depth and provide evidence for their acceptance or rejection. We comment on the knowledge gap in understanding biting force mechanisms across animal species of contrasting body mass, and stress the need for differentiation between the concepts of biting force and biting effort. bite / effort / force / ruminants / structure Rsum -Rsistance de structure du couvert vgtal et effort de prlvement des bouches chez les ruminants au pturage. Face l'htrognit de la ressource en abondance, en valeur nutritive et en rpartition, les ruminants au pturage doivent prendre des dcisions complexes pour rechercher et rcolter les fourrages ncessaires leurs besoins. Certaines de ces dcisions affectent la masse de la bouche et donc, l'ajustement de l'ingestion en terme de profondeur et de surface de la bouche, ainsi que de force de prlvement. Ces dcisions sont prises en relation avec l'htrognit de la structure du couvert vgtal. Nombre de relations ont t mises en vidence liant l'adaptation de la mcanique de la bouche aux variations de la hauteur et de la densit du couvert vgtal de fourrages temprs pour des animaux de diffrentes tailles corporelles. En revanche, les grandes variations 145