Improving aerial topdressing in New Zealand through particle ballistics modelling and accuracy trials

Fixed wing aircraft are utilised in New Zealand to apply dry bulk fertiliser on hill country farms. The fertiliser is most often applied manually as a blanket rate over the entire farm. Previous study indicates that this yields a field coefficient of variation (CV), which is the standard deviation over the mean application rate, of 63 – 70%. The CV decreased to 44% when the hopper door was automatically controlled using aircraft installed global positioning system (GPS) in lieu of manual intervention by the pilot. This is comparable to fertiliser application by fully GPS enabled truck spreaders. Spreadmark® specifies that the transverse overlap CV should be 15% for nitrogen-based fertilisers and 25% for all other products; however transverse overlap tested CV is considerably different to field CV. Variation in aerial topdressing is a barrier to achieving these CV. These variables include wind conditions, topography, aircraft speed and fertiliser properties. Ravensdown Limited is upgrading their topdressing aircraft fleet with differential rate application technology (DRAT), which uses the automated hopper door and GPS to apply various application rates over specified target areas within a farm. The advantage of this system is that fertiliser can be applied to these areas with the largest potential benefit in terms of increase pasture productivity and reduced environmental impact. Two trials utilising cone shaped collectors were carried out at coastal sheep and beef farms to determine the DRAT system’s accuracy when applying two application rates. Proof of release maps, which is deduced from aircraft recorded data, showed the system was able to vary rate. The CV ranged between 34% and 56%. The CV can be further improved by using a granular fertiliser ballistics model that predicts the transverse and longitudinal spread patterns based on wind conditions, fertiliser properties and aircraft operation. Validation data for this model was collected in validation trials for superphosphate, urea and di-ammonium phosphate. A validated model can provide guidelines on the optimum conditions and settings for aerial topdressing.fals