May 4, 2007

With the recent rainy and windy conditions, many of you may be behind in your pesticide applications. While you may feel an urgency to get in the field as soon as the weather allows, be sure to wait until conditions are optimal.

With pesticide applications, there are two major concerns: pesticide efficacy and drift management.

Pesticide drift is usually a lose-lose situation. The first loss is the damage the drift may cause to an off-target area such as susceptible vegetation, water supplies and wildlife. The second loss is decreased control of the original target, which receives less than the intended amount of pesticide. To avoid problems, always read and follow pesticide label recommendations for drift management. Surveys indicate that about two-thirds of drift complaints derive from applications known to be off-label.

Many factors can affect spray drift. The most important are: 

• wind speed
• boom height
• distance downwind from the sprayer,
• spray particle size and, the often ignored,
• atmospheric stability.

Temperature Inversions

Temperature inversions occur when a layer of cool air near the soil surface is trapped under a layer of warmer air. With temperature inversions, the temperature increases as you move upward. This prevents air from mixing with the air above it and causes small suspended droplets to form a concentrated cloud that can move long distances. If large numbers of small droplets are captured in this warm air or inversion layer, the deposition of pesticide on the original target is decreased. Records indicate that movement of these inversion layers may transport pesticides several miles.

The most common cause of temperature inversions close to ground level is radiant cooling of the ground — the ground cools off quicker than the air above it. Clear skies favor radiant cooling and therefore the formation of surface inversions. These are most likely to occur in the early morning and late afternoon. Conditions not favoring inversions include low heavy cloud cover, strong to moderate winds (greater than 5-6 mph), a temperature increase of 5 degrees, and bright sunshine.

Selecting Nozzles to Reduce Drift

Many new spray nozzle tips are designed to reduce drift. Many of these use a pre-orifice which controls flow rate. The exit orifice controls the pattern formation. The result is larger spray droplets which are less susceptible to drift. Also, some of these nozzles can be used over a wider pressure range, which produces large droplets at low pressure and small droplets at high pressure. The ability of these nozzles to produce good spray patterns over a wide pressure range makes them an excellent choice to use with rate controllers which control the application rate by pressure changes.

Many of the new nozzle tips which use a pre-orifice and include air-induction could only be used at a minimum of 30 psi pressure and performed best at about 50 psi. A new feature of air-induction spray tips is that several are designed to be used at operating pressures as low as 15 psi. Work with the Sympatec laser at the University of Nebraska West Central Research and Extension Center at North Platte compared the spray particle sizes of the new AIXR tips (Air Induction Extended Range) with AI tips (Air Induction) from Spraying Systems.

Figure 1. The volume median diameter (VMD = 1/2 of spray volume is in smaller droplets and 1/2 is in larger droplets) and percent of volume made up of spray particles less than 200 microns with air induction nozzle tips from Spraying Systems for water and a spray solution containing Roundup WeatherMax and ammonium sulfate.

In Figure 1 we compare the volume median diameter (VMD = 1/2 of spray volume is in smaller droplets and 1/2 of spray volume is in larger droplets) and percent of volume made up of spray particles less than 200 microns with air induction nozzle tips from Spraying Systems for water and a spray solution containing Roundup WeatherMax and ammonium sulfate. The VMD and percent of the volume made up of spray particle sizes less than 200 with the Air Induction Extended Range 11004 nozzle tip at 40 psi as compared to an Air Induction 11003 nozzle at 70 psi is very close.

The output of an AIXR1004 at 40 psi is very close to the output of a AI11003 at 70 psi. The objective here was to compare the VMD and the percentage of the spray volume made up of particles less than 200 microns, which are the spray particles susceptible to drift. The higher pressures were used because operators may reach these pressures when using a sprayer with a rate controller.

Our recommendation is to use a Spraying Systems Air Induction nozzle tip at 50 psi or their new air induction extended range nozzle at 28.5 psi or about 30 psi. Many operators will prefer this lower pressure. Many nozzle tip manufacturers have similar nozzle tips and the reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by University of Nebraska-Lincoln Extension is implied.

Robert Klein
Extension Crops Specialist
West Central REC, North Platte