Plant responses to defoliation and relationships with pasture persistence

The effects of defoliation on plant morphology and the structure of perennial ryegrass and white clover populations are reviewed with reference to the persistence of yield in sown pastures. Maintenance of high densities of perennial ryegrass tillers and white clover stolons is fundamental to persistence. Tiller and stolon population densities are subject to within- and between-year variation, and are strongly influenced by defoliation management through processes such as competition for light, and size-density trade-offs mediated by self-thinning processes. Spring is a time when high tiller/stolon appearance and death rates occur simultaneously. Spring is also the time when clover populations undergo rapid structural change as large clover plants fragment to release small, unbranched plants with limited ability to compete for light with grasses. The central role of reproductive stem development in perennial ryegrass for summer survival of tillers and autumn re-establishment of tiller populations also highlights the importance of defoliation management during the spring for population persistence. Examples of the application of tactical defoliation management in spring, and at other times of the year, to manipulate stolon and tiller densities are presented. In some instances, such tactics may improve animal production potential, as well as the persistence of new cultivars. It is notable that the New Zealand literature on this topic is mostly more than 15 years old. Recent changes in farming systems and plant genotypes (e.g. tetraploidy in perennial ryegrass, late flowering perennial ryegrass) may require a reanalysis of established principles and practices related to defoliation x plant interactions in grazed pastures to ensure best possible pasture yield persistence and profitability of grazing systems. Keywords: grazing management, perennial ryegrass, persistence, population dynamics, white clover