The Relationship Between Coverage and Efficacy with Selected Low-Drift Sprays

Tom Wolf, Eric Johnson, and Brian Caldwell

Agriculture and Agri-Food Canada, Saskatoon, SK

Introduction:

Low-drift nozzles can produce very coarse sprays that could result in reduced target coverage under some conditions. The fear of reduced coverage, and subsequently reduced pest control, contributes to reluctance by some applicators to use low-drift sprays. Research results have shown that low-drift sprays usually produce similar herbicide efficacy compared to conventional sprays despite much larger droplet sizes and potentially reduced coverage. This apparent discrepancy lead to a study investigating the interactive effects of spray quality, spray pressure, and application timing on deposit coverage and weed control with Liberty. The goal was to induce poor weed control through improper use patterns in an effort to determine whether early spray application with a coarse spray would lead to reduced weed control, and if so, to identify the reasons for this reduction and define operational parameters for optimum nozzle operation.

Methods:

Trials were conducted in 1999 and 2000 at the Saskatoon Research Centre in Scott, SK. Tame buckwheat, oriental mustard, and tame oats were seeded in plots, and Liberty was applied at two rates (1.35 L/acre and 0.67 L/acre) in 40 L/acre using the following nozzles: Air Bubble Jet (ABJ), Greenleaf TurboDrop XL (TD), and SprayMaster Ultra (SM) (Table 1). Each nozzle had a nominal flow rate of 0.15 US gal/minute at 40 psi and was operated at three spray pressures (20, 40, and 75 psi). A conventional TeeJet Extended Range flat fan nozzle (XR) at 40 psi was included as a standard. All nozzles were calibrated prior to application and travel speed was adjusted to maintain a constant application volume. Boom height was set to 50 cm above target height for all treatments. Applications were made at an early weed stage (1 week after emergence with weeds in the 1 to 2-leaf stage) and a late stage (2 weeks after emergence with weeds at the 3-4 leaf stage). Weed control was assessed through visual ratings and biomass samples. Spray swath uniformity and deposited droplet size were evaluated under laboratory conditions.

Results and Discussion:

Results showed that tame buckwheat was least sensitive, and tame oats was most sensitive to the tested variables (Figures 1 and 2). Late application reduced weed control for oats, but increased it for buckwheat. This could be attributed to uneven emergence of the buckwheat, resulting in some late-emerging plants to remain unsprayed.

For tame oats, higher pressures significantly increased weed control for the TD and SM sprays, but had only a small effect on the ABJ (Figure 1). Coarser sprays, either through nozzle or pressure changes, decreased mustard and oat control, but usually did not affect buckwheat control (Figure 2). Early timing and reduced rates increased the sensitivity to nozzle and pressure selection for oats and mustard. Overall, similar oat control to a conventional flat fan nozzle could be achieved with the ABJ at 40 psi or greater, with the TD at 75 psi, but not with the SM which had lower weed control even at the highest pressure. For buckwheat, similar control to the standard could be achieved with the ABJ and TD at 40 psi, and with the SM at 75 psi.

Figure 2: Interactive effects of application timing, nozzle type, and spray pressure
on tame buckwheat control with Liberty.

Swath deposit uniformity, as measured by the dynamic Coefficient of Variation (CV) was best with the TD (10%), intermediate for the ABJ (23%) and worst for the SM (28%) (Table 2). Increased pressure increased uniformity, except for the SM, where the intermediate pressure had the best uniformity (CV=21%). Spray deposits on water sensitive paper are presented in Figure 3. Coverage was similar for all nozzles (about 19%), and did not change with nozzle choice or pressure (Table 2). Deposited Volume Median Diameter (VMD) was lowest for the ABJ, and highest for the SM, and tended to decrease with pressure, although VMD was always higher than a conventional flat fan nozzle at 275 kPa. Deposit density was highest for the XR nozzle and increased with pressure for all nozzles, reflecting the greater production of fine droplets. A fairly high droplet density was achieved with the coarsest spray, the SM, at the highest pressure, an observation, which was not expected.

Table 2: Deposit characteristics of sprays on water sensitive paper.

Deposit CV was not correlated to any of the other deposit characteristics (Figure 4). In other words, pattern uniformity did not depend on spray quality. Deposit density was not correlated to coverage, but was negatively related to VMD. This is an important distinction, as coverage (% of total surface covered) can obviously be similar for fairly fine and fairly coarse sprays, but the number of droplets per cm² may vary considerably. It is droplet density that may therefore be most important for weed control.

Weed control was related to swath deposit uniformity, deposit density, and VMD, but not coverage (Figure 5). Since it was already established that deposit CV and deposit density were not related to each other, but each on its own affected weed control, these two variables are likely to interact in their effect on weed control. Multiple regression demonstrated that effects of deposit CV and droplet density together could predict between 62 and 80 % of weed control variation for Liberty. Higher CVs reduced weed control more for coarser sprays than for finer sprays. It can be concluded from this observation that a coarse spray can provide good product efficacy provided it is applied uniformly. A fine spray is not as dependent on uniformity for good performance.

It should be noted that a spray pattern CV may not be the most appropriate way to characterize uniformity because it poor uniformity manifests itself differently depending on the spray. For a coarse spray, a non-uniform deposit is usually in the form of striping due to poor patterns or overlaps. For a finer spray, low uniformity is usually the result of random variation in deposit due to wind- or travel-induced displacement of the spray. The former is more detrimental to weed control.

Figure 3: Spray deposits on water sensitive cards from four nozzles at three pressures each.

Figure 4: Correlations between various spray deposit characteristics of three nozzles at three pressures plus a standard nozzle at 40 psi.

Figure 5: Correlations between various spray deposit characteristics of three nozzles at three pressures and weed control biomass for Liberty at the low herbicide rate.

Conclusions:

• Earlier sprays tended to have improved weed control compared to late applications, even with coarser sprays.
• Coarser sprays tended to result in lower Liberty efficacy, especially on tame oats, even though they did not reduce coverage on water-sensitive paper.
• Higher pressures improved weed control, particularly with tame oats. Tame buckwheat was relatively insensitive to spray pressure. Higher pressures reduced droplet size and increased deposit uniformity.
• Buckwheat control equivalent to the conventional spray could be achieved with all nozzles provided spray pressures were at least 40 psi for the ABJ and TD, and 75 psi for the SM.
• Oat control equivalent to the conventional spray could be achieved provided spray pressures were at least 40 psi for the ABJ and 75 psi for the TD. The SM could not achieve equivalent weed control even at the highest tested pressure.
• Weed control was related to both deposit CV and droplet density. High CVs (poor uniformity) resulted in lower effectiveness, especially for coarser sprays. High deposit densities improved weed control. A combination of good uniformity and high droplet density resulted in the best weed control. High densities were achievable with coarser sprays at higher pressures.
• Very coarse sprays were able to provide good weed control provided they were applied uniformly. Adjustment of spray pressures or boom height to correct for narrow spray angles may be required to optimize nozzle performance on difficult-to-control weeds.

Acknowledgements:

The assistance of Jody Pederson, Rachel Buhler, Murray Nelson, and Trevor Linford (at Saskatoon) and Herb Schell, Murray Nielsen, Natasha Kripps, and Curtis Sieben (at Scott) is much appreciated. Financial and in-kind support for this study was provided by the Saskatchewan Agricultural Development Fund, the Pest Management Regulatory Agency, and Agriculture & Agri-Food Canada.