Lucerne survival in relation to grass management on a brigalow land in south-east Queensland

Abstract

Lucerne, Medicago sativa cv. Hunter River, was planted on a 90 cm x 90 cm grid on a fertile brigalow clay soil in south-east Queensland in November 1971. The lucerne was either cut regularly or cut annually. Volunteer species, principally grasses, were either mown regularly, 2-3 times as often as the lucerne during periods of active growth, or were also cut annually in late summer. Changes in lucerne vigour and survival were recorded about four times a year through to 1977. A few lucerne plants died between most samplings in each treatment. More substantial losses due to individual treatments were superimposed on the continuing small losses in successive summers. The first two summers following establishment were wetter than average, and as many as 41% of the original population died when volunteer species were cut annually. Where volunteer species were mown regularly from between lucerne plants, survival was good until the very dry 1974-75 summer when 35% of the original plants died. Our results thus suggest that lucerne plants were lost through their incompatibility with vigorous tropical grass growth, through drought effects, and also through the continuous slow erosion of the population : they emphasise the importance of managing vigorous grass growth if lucerne survival is to be prolonged in fertile sub-tropical environments. Vigour declined markedly only where4wzerne was regularly cut from within the annually cut volunteer species, but there were large seasonal fluctuations. The relative size of plants decreased over some 4 to 9 months before death, although there was an indication that those dying in summer were more representative of the population mean size. This suggests that summer deaths rapidly follow the occurrence of predisposing factors. In summer, management should aim mainly to prevent plant deaths, whereas at other times there may be additional scope for alleviating losses in productivity.