Magnesium - Zirconium master alloys are the dominant grain refiner for a range of commercial magnesium alloys that exhibit good elevated temperature properties. However, the grain refining efficiency achieved by the master alloy is relatively low because Zr particles quickly settle to form sludge. In this study ultrasonic treatment (UST) is applied to investigate whether the grain refinement efficiency can be improved by increasing the Zr solute content, producing a more uniform dispersion of Zr particles and minimizing sludge formation. It was determined that, for a range of Zr contents from 0.2 to 1.0 wt% Zr, UST of the melt above the liquidus temperature decreased the grain size due to an increase in the amount of solute Zr and the number of Zr particles. For Zr contents less than 0.5 wt% the grain size decreased further when UST was applied from above to below the liquidus temperature to include the onset of nucleation of alpha - Mg grains. Additionally, the application of UST substantially decreased the amount of sludge which increased the number density of Zr particles and improved their likelihood of successful nucleation. The Interdependence nucleation model was successfully applied to predict the grain size of each casting condition where the parameter (z) relates to the temperature gradient which in turn is affected by the acoustically-induced convection. A lower value of z when UST is applied facilitates nucleation and the survival of grains. The factors affecting grain refinement under each condition are evaluated in terms of solute and particle Zr contents, cavitation and acoustic streaming and their effect on nucleation and as-cast grain size.