Management practices and other factors contributing to the decline in persistence of grazed lucerne in temperate Australia: a review

The literature relevant to the grazing management of lucerne in temperate Australia is reviewed with emphasis on the factors likely to affect its persistence. Knowledge of lucerne physiology is used to question the validity of the traditional methods of managing grazed stands, which rely mainly on using 10% flowering as a guide to root carbohydrate levels. From these data several alternative management guidelines are proposed that may lead to increased persistence; however, for long-term persistence, there is little doubt that lucerne needs to be grazed leniently and at a late stage of maturity. Several grazing experiments indicate that grazing periods of 16-20 days should have no effect on persistence, provided that the rest period between successive grazings is 35 days or longer. Data from other countries and Australian data from a limited number of experiments also indicate that grazing in either autumn or winter may substantially reduce production and could affect persistence. Three grazing studies in New South Wales were used to highlight critical differences in experimental design which make comparisons among experiments difficult. Standardised sowing rates and grazing management, and statistical procedures which account for the genotype x management x environment interaction, are suggested to improve the extrapolation of results from experiments to other environments. Persistence of different lucerne types under grazing, particularly those recently imported from the U.S.A. or bred in Australia, is considered. While it has been proposed that grazing effects may be related to crown structure, interactions with other factors which affect persistence may also occur. If grazing can be considered to be stressful to a lucerne plant then it could interact with other stresses, caused by moisture deficit, excessive moisture, insect pests and disease, to reduce persistence. Additionally, considerable variation in varietal resistance to some pests and diseases has been recorded in haycut stands, and so there may also be cultivar x grazing effects. All of these factors could combine to affect the persistence of a particular cultivar under grazing. Patterns of lucerne decline were either continuous or step-like. Continuous decline was associated with prolonged grazing, grazing and moisture stress, grazing under waterlogged conditions, or grazing in situations where the incidence of disease was likely to be high. To understand the reasons why plants fail to persist, measurements need to be made frequently and a1 regular intervals, and the moisture and disease status of the site needs to be accurately monitored. The adequacy of different methods of measuring stand persistence is also questioned. The implications for graziers, researchers and lucerne breeders are discussed.