Spray chamber evaluation of air-assisted spraying on broccoli

Conventional over-the-row sprayers achieve very little deposit on leaves near the ground and on the underside surfaces of leaves throughout the canopy. Air assistance has the potential to improve deposition of droplets on these leaf surfaces. To gain some insight on the effect of air assistance, the effects of airspeed, airflow rate, and air jet orientation were isolated. The study was carried out in a spray chamber with a standard spray boom over micro-plots of greenhouse grown broccoli (Brassica oleracea var, botrytis L.) plants. Air was delivered slightly behind the nozzles from a variable width slot producing a uniform two-dimensional air jet. The orientation of the air jet with respect to the vertical could be adjusted from -10 to 40 degrees The ranges of the independent variables were airspeed, 0 to 36 m s(-1); airflow rate, 0 to 1.3 m(3) s(-1) m(-1), and air jet angle, -10.2 to 40.2 degrees. Two sets of flat fan nozzles (Volume Median Diameter = 230 and 400 mu m, both delivering 250 L ha(-1) at 6 km h(-1)) were used to tarry out two full sets of experiments. Results showed that airspeed had the larger impact on leaf coverage. Higher airspeeds (>25 m s(-1)) and airflow coupled with finer sprays increased the coverage of the underside of the leaves at all levels within the canopy and of the top side of the leaves in the lower third of the canopy. However, lower airspeeds (<20 m s(-1)) are desirable for a better coverage of the upper side of the leaves in the higher two-thirds of the canopy. In all cases, angling the air jet forward at 20 to 25 degrees is recommended.