A field study was made of the tiller dynamics of the New Zealand perennial ryegrass (Lolium perenne L.) cultivars Grasslands Ruanui, selected from a Hawke's Bay ecotype, and Ellett, selected from the Mangere ecotype. The work tested whether nitrogen (N) fertiliser or irrigation (Irr) could manipulate the persistence strategies of these morphologically distinct perennial ryegrass ecotypes, represented by these cultivars. Tiller births and deaths, and tiller survival, were monitored over 2 years, under rotational grazing by dairy cows. Patterns of cultivar tiller birth and death were similar. Much of the temporal change in tiller population density was shown to be a response to increasing or decreasing herbage mass associated with seasonal feed surplus or deficit. The probability of tiller survival between observation dates varied markedly with date, and was lowest in autumn. The cultivar effect on tiller survival was small but a significant (P<0.05) cultivar x date interaction was detected, with survival probability for Ellett tending to be higher than for Grasslands Ruanui in March-April 1997 but lower from July 1997 until March 1998. A cohort effect on tiller survival was also identified, in that tillers formed in January, July, and August 1997 had lower survival probability than those formed at other times. N-treated plots produced 34% more tillers than did untreated plots, and irrigation had no effect on overall tiller density. N and Irr treatments had variable effects on tiller survival. Size density compensation theory and analysis of survival probability are introduced to aid the interpretation of tiller dynamics data.