Published online by Cambridge University Press: 27 March 2009
The influence of rest interval and grazing duration on herbage production of dryland lucerne (Medicago sativa L., cv. Hunter River) pastures was studied in a 6-year experiment on a black earth in subtropical southern Queensland. Rest intervals (R) of from 32 to 60 days were combined with grazing durations (G) of 4, 8 or 16 days, or with mowing (M), in the following treatments: (1) R32-4, (2) R32-G16, (3) R44-G4, (4) R44-G16, (5) R56-4, (6) R40-G8, (7) R36-M, (8) R48-M and (9) R60-M. Grazing was with sheep at a rate equivalent to 20/ha on a year-round basis. Amounts of dry matter of lucerne and of volunteer species on offer were measured prior to grazing or mowing, and some measurements of the residue remaining after grazing were made. Chemical composition of representative lucerne samples was also determined.
Mean total amount of lucerne on offer declined from 10·8 to 7·2 t/ha/year between the first 2 years and the last 2 years, whereas total amount of volunteer species, principally grasses, increased from 3·0 to 18·2 t/ha/year over the same period. This difference was partly due to lower utilization of volunteer species. The decrease in amount of lucerne was due to loss of plants, and treatment effects arose through differences in the degree to which amounts from individual surviving plants increased to compensate for stand thinning. Concentrations of mineral elements in lucerne were always above critical levels for plant and animal growth.
Treatment R40-G8 maintained the largest amounts of lucerne on offer to the end of the experiment, with treatments R32-G4 and R32-G16 then having least. Large amounts of grass were associated with 4-day grazing, and to a lesser extent with short rest periods. The results confirm the need for a minimum rest period between grazings in a subtropical environment and also show the need for grazing to control excessive grass growth as well as lucerne if lucerne productivity is to be ensured. Some requirements which would lead to more successful management of lucerne in the subtropics are discussed.