The aim of this study was to determine the population dynamics of tillers of perennial ryegrass (BP; Lolium perenne L.), as a monoculture and in mixtures with orchard grass (PO; Dactylis glomerata L.) and white clover (TB; Trifolium repens L.) under grazing conditions. Sheep belonging to Suffolk with Dorset breed were used as defoliators. The treatments were the combinations: 100:00:00, 40:20:40, 50:00:50, 20:40:40, 70:20:10, 20:70:10 and 40:40:20% of BP, PO, and TB, respectively. A complete randomized block design with three replications was used. The variables evaluated were: plant density (DP), density of population of tillers (DPT), rate of appearance (TAT), death rate (TMT) and survival rate of tillers (TST). The highest average of DP in BP was found in autumn-winter with the mixture 50:00:50, reaching 13 plants m2, respectively (p < 0.05). In DPT there were differences (p < 0.05) among BP mixtures; however, a highly variable performance was observed, being in winter when the highest DP values were found (422 tillers m2) in the mixture 50:00:50 (p < 0.05). In BP, the mixture 20:40:40 had the highest TAT with 7.0 tillers 100-1 tillers, same mixture presenting the highest TMT in March; as with the mixture 20:70:10, with 4 tillers 100 tillers-1 in April, respectively (p < 0.05). Tiller population dynamic of ballico perenne (DPT, TAT, TMT, TST) is not determined by the proportion of species in the associations; however, it is defined by the season regardless of the associated proportions.