Improving the Production and Persistence of Temperate Pasture Species in Subtropical Dairy Regions of Australia

Abstract

In subtropical dairy regions of Australia, temperate pasture species are sown to overcome a shortage of forage in the cooler months (late autumn to mid-spring), due to a decline in growth and quality of tropical grasses. Ryegrass ('Lolium') species are the most widely sown temperate grasses, and with appropriate management, are capable of high yields of quality forage under the subtropical climate. Perennial ryegrass ('Lolium perenne' L.)/white clover ('Trifolium repens' L.) pastures are less costly, and provide a higher quality forage with more even dry matter (DM) production throughout the year, than annual ('L. temulentum' L. or 'L. rigidum' Gaudin) or biennial (L. multiflorum L.) ryegrass pastures. However, under current management, perennial ryegrass pastures have not persisted beyond 2 years in the subtropics, and this lack of persistence is associated with severe stress conditions over summer. Previous studies have indicated that perennial ryegrass survival over simmer could be substantially improved, and incursion of tropical grasses minimised, by appropriate defoliation management. The studies reported in this thesis aimed to confirm the importance of defoliation interval under grazing, and to determine the mechanism by which defoliation affects the survival of perennial ryegrass and white clover. The initial strategy was to determine the critical time of defoliation on subsequent plant survival over summer. The observed association between ryegrass survival and tiller and root growth, and water soluble carbohydrate (WSC) reserves, was then studied further in the glasshouse, in order to determine the mechanism of action. Inducing plants to become dormant over summer, or to regenerate from seed the following autumn, was then evaluated as an alternative to managing plants to survive summer. Lastly, the effect of defoliation on the white clover component of mixed perennial ryegrass/white clover pasture was studied.