The utilization ability of spontaneous vegetation by domestic ruminants suggests an ideal complementarity between the extensive grazing systems development and the environment maintenance. The connection between livestock impact and the vegetation response imposes a maximum limit to animal production that should not be overpassed. The livestock impact on each plant species appears conditioned by the utilization ability each animal exhibits on it, the efficiency of the plant tolerance mechanisms against herbivory as well as the competitive interaction developed between plants. As a result, the knowledge of animal/plant interaction and, in particular, the herbivores' grazing habits are essential for an efficient management of natural resources.
This article reviews some of the main findings in this area, mainly in relation with the mechanisms involved in feed selection and the bases holding the competitive inter­ actions between them.