The influence of spring grazing management on yield and water use of rainfed lucerne

Abstract

Flexible spring grazing management can increase the use of lucerne on farms. However, the negative impacts of set stocking on the survival of plants usually preclude its use. In this experiment, crops of ‘Stamina 5’ lucerne at Ashley Dene, Lincoln University were rotationally grazed, semi-set stocked or set stocked from September to December 2011. Dry matter (DM) yield to December was 6.3 t DM/ha, and consistent among crops. However, the larger leaf area of the semi-set- and set-stocked crops intercepted 45% more solar radiation and transpired 25% more water than the rotationally grazed crop. The set-stocked regime had a lower efficiency of resource use. This was probably due to greater respiration and/or partitioning of DM to the root mass. Set-stock regimes required continual initiation of new shoots from basal buds, which remobilised and, therefore depleted, stored assimilates. To minimise this impact, farmers should maintain a canopy cover of 2 to 2.5 t DM/ha (~20 cm tall) on any set-stocked lucerne. This will maximise radiation interception and compensate for reserve losses. In the study year, grazing management did not influence feed supply through the manipulation of water use because soil evaporation losses were inversely related to crop water use. Consequently, total water use was consistent among crops, with higher soil evaporation from frequent rewetting of the soil for rotationally grazed crops. For farmers, these results suggest continuous grazing of lucerne is possible, but it should be managed to maintain full canopy cover and minimise reserve depletion that will reduce DM yields and stand persistence.