Spray deposits and losses in different sized apple trees from an axial fan orchard sprayer: 3. Effects of air volumetric flow rate

As part of a wider study to optimise the adjustment of axial fan orchard sprayers to suit differing modern crop structures, three field experiments compared the effects of reductions in air volumetric flow rate (from 11.3 to 7.5 to 4.1m3s−1) on spray deposits and losses to young dwarf, dwarf and semi-dwarf apple trees. Reducing the air volumetric flow (achieved by reducing the speed of rotation of the fan without changing the air ducting or orifice size) from the high to the medium rate did not significantly affect mean deposits on the target trees but the lowest air flow rate increased mean deposits on leaves, flowers and fruits significantly in two experiments. In the other experiment, done in the early season in higher wind speed conditions, mean deposits were decreased by the lowest air flow rate treatment which made the spray plume vulnerable to cross-winds. Decreasing the air flow rate generally increased mean deposits in tree zones close to the sprayer and decreased those in zones furthest from the sprayer. The leaf-to-leaf distributions of deposits in the centre of the trees were not affected significantly by the reductions in air flow rate. Decreasing the air flow rate substantially increased the percentage spray cover on downward-facing water sensitive papers fixed horizontally in the centre of the trees but decreased cover on upward facing papers. The medium air flow rate gave at best only small (