Equipping and operating sprayers to control insects and diseases in soybeans

Taking the time to equip and operate your sprayer properly will improve insect and disease control in large and dense soybean canopies.

Applying insecticides and fungicides to large soybean plants is much different than applying systemic post-emergence herbicides to weeds that are 2 to 4 inches tall. In fact, entomologists and pathologists don’t recommend tank-mixing herbicides with insecticides or fungicides due to the timing and application incompatibilities. Producers need to be aware of the key differences and make the necessary equipment and operating adjustments to maximize insect and disease control in soybeans. Leaf coverage is much more important with insecticide and fungicide applications than it is with systemic herbicides. Another key difference is that insecticide and fungicide droplets need to penetrate large and dense soybean canopies. The following recommendations will help you achieve the canopy penetration and leaf coverage required for good insect and disease control in tall and dense soybeans.

Spray volume has the greatest impact on canopy penetration and leaf coverage. Increasing spray volume increases penetration and coverage. Spray volumes of 15 gallons per acre are required when applying insecticide and fungicides to soybean through growth stage R3 (pod development). After R3, 20 gallons per acre are necessary.

Droplet size is the second most important factor affecting canopy penetration and leaf coverage. Small droplets provide the best leaf coverage, but lack the momentum to penetrate the canopy. Large droplets easily penetrate the canopy, but don’t provide adequate leaf coverage. Research has shown that fine to medium droplets having volume median diameters (VMDs) ranging from 200 to 350 microns will provide the optimum canopy penetration and leaf coverage. All nozzle manufacturers use a common spray quality classification system that divides droplets into six droplet size categories. Please refer to Table 1. The colors listed in table 1 should not be confused with the color of the nozzle itself. The color of the nozzle describes the flow rate of the nozzle and the colors in the table describe the nozzle’s droplet size range.

Table 1. ASAE standard S-572 (spray quality categories)

Droplet Category Symbol Color VMD (microns)
Very Fine VF < 150
Fine F
150 - 250
Medium M
250 - 350
Coarse C
350 - 450
Very Coarse VC
450 - 550
Extremely Coarse XC
> 550

Source: Spray-droplet size measurement and classification, Scott

Ground speed is an important consideration as it affects spray volume and vertical droplet velocity. As ground speed increases, spray volume per acre and vertical velocity of the droplets decrease, reducing canopy penetration. Ground speeds of less than 10 miles per hour are recommended.

Pay attention to nozzle pressure also as it affects droplet size, spray volume and droplet velocity. In general, higher pressures will provide better canopy penetration and leaf coverage as long as droplets remain in the fine to medium category and not too many fine droplets are produced.

Select nozzles that produce a flat fan spray pattern as these perform better than cone nozzles. Research conducted at Ohio State University showed that nozzles producing a single, flat fan pattern provided better canopy penetration than nozzles or combinations of nozzles producing a twin fan pattern when used in large and dense soybean canopies. However, twin fan patterns improve coverage on smaller plants. Venturi or air-induction nozzles should not be used for insecticide and fungicide applications as they require very high pressures to produce 200 to 350 micron droplets.

Consider spray volume, droplet size, ground speed and operating pressure when selecting spray nozzles. When using droplet size classification charts, select nozzles that produce droplets near the fine end of the medium (yellow) category and deliver 15 gallons per acre at your desired ground speed and operating pressure. Using the information in Table 2, we can determine that a sprayer traveling at 10 miles per hour, equipped with XR11005 nozzles and operated at 40 psi, will deliver 14.9 gallons per acre while producing fine to medium droplets. Increasing the nozzle pressure to 50 psi and keeping all other conditions static increases the spray volume to 16.6 gallons per acre and still produces fine to medium droplets. If the ground speed was less than 10 miles per hour, a nozzle having a lower flow rate would be required to produce the optimum droplet size. All nozzle manufacturers provide information similar to that contained in Table 2 for each of their nozzles. Note that the color of the XR11005 nozzle is brown and it produces medium (yellow) to fine (orange) droplets, depending on the operating pressure.

Table 2. Relationship between spray volume, ground speed, pressure and
droplet size for Teejet XR8005 and XR11005 nozzles

*Droplet Size MPH
7 8 10
Tip PSI 80o 110o GPA

15 C M 15.3 11.5 9.2
20 C M 17.3 13.0 10.4
30 C M 21 16.0 12.8
40 M M 25 18.6 14.9
50 M M 28 21 16.6
60 M F 30 23 18.1

Source: Spraying Systems Co. * C = coarse, M = medium and F = fine

Robert Grisso, agricultural engineer at Virginia Tech, has developed an Excel spreadsheet to help applicators select nozzles.

Operating the spray boom at the correct height is essential. Boom height controls spray pattern uniformity and droplet velocity. Erdal Ozkan, agricultural engineer at the Ohio State University, recommends setting the target area midway between the lowest leaves on the plant and the top of the canopy. Use the manufacturer’s recommendations for your nozzle spacing and nozzle spray angle to determine how high to set your boom above the target area. For example, a boom equipped with 110 degree nozzles spaced 20 inches apart should be operated 16 to 18 inches above the target area.

Recommended boom height

Recommended boom height for 110 degree flat fan nozzles spaced 20 inches apart when applying insecticides and fungicides in 30-inch tall soybeans. Source: Diane Brown-Rytlewski, MSUE.

Taking the time to equip and operate your sprayer properly will improve insect and disease control in large and dense soybean canopies.

This article was produced by the SMaRT project (Soybean Management and Research Technology). The SMaRT project was developed to help Michigan producers increase soybean yields and farm profitability. Funding for the SMaRT project is provided by MSU Extension and the Michigan Soybean Checkoff program.


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