Most of you reading this article have likely never seen a wind tunnel. I want to describe what a wind tunnel looks like, how it operates, and, more importantly, how we use it to help soybean growers apply pesticides more efficiently and effectively by matching sprayer technology to soybean canopy characteristics. More specifically, we hope the results from this research will assist soybean growers in selecting the best nozzles and soybean varieties to increase the likelihood of pesticide droplets depositing on hard-to-reach areas of the soybean canopy where insects and diseases may be present. We aim to address two questions that soybean producers frequently pose:
- Are row spacing and plant population essential factors for effective spray deposition and coverage in the soybean canopy, and
- What spray technology is available and recommended to maximize the efficacy expected from the pesticides applied?
Studies have highlighted the complexity of spray penetration in dense crop canopies. Over the years, we have conducted groundbreaking field studies to determine how spraying equipment and methods influence the deposition of pesticides in soybean canopies to combat insects such as aphids and diseases like Asian Soybean Rust and Sclerotinia Stem Rot (white mold), which take hold in the mid-to-lower parts of the canopy. However, weather conditions—including temperature, relative humidity, wind speed, and direction—can be unpredictable and may change rapidly during outdoor experiments. Additionally, during field studies, the density of soybean canopies can vary significantly across randomly assigned plots. We found through our field studies that despite using multiple replications, data obtained from field experiments are subject to wide variations.
To eliminate the undesirable influence of weather and canopy conditions on the test results, we conducted experiments under controlled environmental conditions by placing soybeans in a wind tunnel to study the effects of nozzles and droplet sizes on spray deposition at the top, middle, and bottom sections of the soybean canopy. The wind tunnel is located in the Agricultural Engineering building on the Wooster campus of The Ohio State University. Only two other agricultural colleges have wind tunnels; one features a very short test section with a small cross-section, which does not allow for a study like this one.
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The primary components of the wind tunnel, illustrated from left to right in the schematic below, include a centrifugal fan that discharges air into the tunnel, a fan housing assembly with a variable-speed motor to drive the fan, a deflector plate, and a set of screens to ensure the forced air is uniform as it enters the tunnel. In addition, there is a wide-angle diffusion and contraction chamber, a honeycomb settling chamber that further enhances airflow uniformity within the wind tunnel, and a detachable test section. The total length of the wind tunnel is 56 feet, which encompasses a 24-foot-long test section with a cross-sectional area measuring 6 feet wide by 6 feet high. A small, moving spray boom, traveling at a speed of 4.5 mph, is installed on the ceiling of the wind tunnel test section to simulate the movement of a sprayer boom in the field. The boom is equipped with three nozzle bodies spaced 20 inches apart.
Soybean plants were grown in 3-gallon plastic pots (1-foot diameter) and maintained under outdoor conditions. When the canopy reached growth stages between R5 and R6, pots of soybeans were placed in the wind tunnel to conduct tests to: 1) understand the influence of canopy characteristics and airflow patterns in soybean canopies, 2) determine the most effective spray equipment and operating conditions for applying appropriate chemicals for soybean protection, and 3) assess the drift potential of various spray nozzles. Tests were conducted using the following variables:
- Wind Speed: No wind, 2.5 mph, 4.5 mph, 9 mph.
- Various types of nozzles that produce different spray patterns and droplet sizes.
- Soybean Row spacing (center to center of each pot): 15 inches and 30 inches.
Eight 4-foot-tall stakes were positioned at eight collection points in the center of the soybean pot to support the placement of water-sensitive paper targets used for measuring spray deposition and coverage. Blue stains left on the water-sensitive card after spraying are scanned and analyzed to determine the percentage of the card covered by the spray droplets. To capture potential airborne spray drift from different nozzles under various wind speed conditions, a 5-ft-tall laboratory stand was secured at the exit of the wind tunnel. Six collection points were employed to measure particle drift with water-sensitive papers and airborne drift using 60-mesh small round stainless-steel discs.
The results of this study indicated that nozzle selection played a significant role in reducing spray drift and ensuring adequate pesticide deposition and coverage to protect soybeans from diseases and insects, particularly when targeting the lower parts of the canopy. Noticeable differences in spray coverage and penetration were observed within the soybean canopy, regardless of the nozzle used. The specific conclusions from this research will be detailed in an upcoming article after we complete this summer’s experiments. The effect of row spacing (15” vs. 30”) on the deposition of pesticides in the middle and bottom parts of the soybean canopy, along with some new nozzles we have not tested before, as well as how applying pesticides on soybeans at different growth stages will enhance the efficacy of the pesticides applied, will be the subjects of the experiments we intend to conduct this summer.
A study like this one, which utilizes a wind tunnel to explore how to enhance the effectiveness of pesticides applied to soybeans, has never been conducted anywhere in the world. This is primarily due to the lack of trained personnel and the necessary facilities, such as a suitably sized wind tunnel.
Acknowledgement:
This research is a collaboration between the OSU and the USDA-ARS Application Technology Research Unit. The Ohio Soybean Council has partially funded it.