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 4.1 m(3)s(-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 (<50%) reductions in spray drift but the lowest air flow rate treatment significantly reduced drift by 55-93%. Deposits on ground sampling lines increased correspondingly. Drift amounts were greatest and the reductions in drift due to the reduced air flow rates were least in stronger winds conditions early in the season when the canopy was least dense. Taken together, these results indicate that large reductions in air volumetric flow rate can substantially reduce spray drift from axial fan sprayers without adversely affecting the variability of deposits on either leaf surface though the relative mean amounts on the upper versus the lower leaf surface can change substantially. The reductions make the spray plume more vulnerable to crosswinds which can reduce spray penetration. The extent that reductions can be made depends on wind conditions and tree density. However, the provision for air flow adjustment on many commercial sprayers is generally inadequate to maintain an optimal spray distribution pattern across the broad range of orchard structures that exist on many commercial farms in the UK. Other practical difficulties of greatly reducing the air flow rate are discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.