To evaluate the relationship between the external plant attributes (EPA) and animal preference (AP), plant composition in the study area and in the diet of sheep and goats, as well as the selection index of species were determined. Then, the most important EPA was selected and evaluated using literature review. Since there were a lot of EPA, plants were ranked based on these attributes using the principle component analysis (PCA). The eigenvalue and eigenvectors of the plant species and EPA were graphed, and the correlation between the species selection index by sheep and goats and the eigenvalue of the PCA axis for each plant species were calculated. Results indicated that while there was no significant difference between grazing time of sheep and rangeland plant composition, a significant relation was found between grazing time of goat and rangeland plant composition. Hence, it can be concluded that goat is not selective as they grazed plants in proportion to their forage yield and canopy cover in the field. Moreover, there is a significant relationship between the species’ selection by sheep and goats and EPA. This study showed that the optimum grazing can be achieved by common grazing of goat and sheep as a result of different grazing time of plant life-forms and plant species.
