A methodology is developed to analyse the contribution to sward persistence of the processes tiller birth and tiller death, and variation in their rate over an annual cycle. The methodology is tested using previously published data for timothy grass (Phleum pratense). The analysis shows that high death rates of timothy tillers in summer present a problem for persistence in that tiller appearance rates required to maintain the tiller population in these conditions require tiller bud site usage statistics that are biologically unlikely in field swards. While it is not suggested that ryegrass has a naturally high tiller death rate in summer as demonstrated here for timothy, where abiotic stresses reduce ryegrass tiller survival, the same principles are likely to apply. Keywords: Leslie matrix, Phleum pratense, population stability, tiller birth rate, tiller survival rate, timothy grass
