Proper herbicide application is very important to ensure that products achieve their full potential for weed control. “For postemergence herbicides, this includes application to appropriately sized weeds and equipping the sprayer to achieve uniform coverage of target weeds,” says Bob Hartzler, an Iowa State University Extension weed management specialist. Hartzler, along with ISU Extension ag engineer Mark Hanna, have been discussing this topic with farmers and custom applicators at ISU Extension agronomy meetings around the state this winter.

GET GOOD COVERAGE: The volume your sprayer delivers has a greater effect on spray coverage than type of nozzle. Using spray volumes at the high end of the recommended range should improve the consistency of weed control. Nozzle selection should be based on herbicide label specifications.

Hartzler says weed scientists at Purdue University recently reported on the influence of nozzle type and spray volume on target coverage within the soybean canopy. A traditional flat fan (XR) nozzle and three drift reduction nozzles were included in the study. The XR nozzle produced more driftable droplets than the other nozzles, and the two types of air induction nozzles had much less volume in small droplets than the XR and TTJ nozzles (see accompanying table).

Effect of nozzle type and size on production of driftable droplets

Source: Research by T.R. Legleiter and W.G. Johnson, 2016. “Herbicide coverage in narrow row soybean as influenced by spray nozzle design and carrier volume,” Crop Protection Journal, 83:1-8.

Coverage of targets within the canopy was evaluated by placing water sensitive paper at different heights within 12-inch tall soybeans planted in 15-inch row width. Averaged over nozzle types and spray volume, the coverage declined by approximately 50% from the top of the canopy to targets placed 4 inches above the soil surface.

The TTI nozzle was less consistent than the other nozzles on coverage at the top and middle of the canopy. The TTI nozzle produces the largest droplet size of the nozzles evaluated, and thus would be expected to provide less uniform coverage than the others. However, nozzle type did not have a significant effect on coverage of targets at the bottom of the canopy. The researchers speculated that large droplets were more efficient at penetrating the canopy than small droplets, therefore resulting in similar coverage by the different nozzles of targets placed at the base of the canopy.

Better coverage was achieved with larger nozzle size

Regardless of nozzle type or target placement, better coverage was achieved with the larger nozzle size that provided greater gallons per acre (GPA) spray volume. Averaged over nozzle types and the two years of the study, 15 GPA provided 26% coverage compared to 13% at 10 GPA.

“Thorough coverage of weeds is required to achieve consistent weed control, especially when relying on herbicides other than glyphosate,” says Hartzler. The concern over off-target movement of pesticides has led to the movement towards the use of nozzle types that produce fewer small droplets. This research demonstrates that spray volume has a greater effect on spray coverage than the type of nozzle. “Using spray volumes at the high end of the recommended range should improve the consistency of weed control,” he adds. “Nozzle selection should be based on herbicide label specifications.”