Jan 20, 2009
Sprayer Hits Weeds, Avoids Tomatoes

Researchers at the University of California, Davis, are developing new precision weed control technology for processing tomatoes.

The UC Davis researchers have been collaborating with the California Tomato Research Institute on the project since 2004 (both organizations have provided funding). The results of research trials performed last year were “quite good,” and the concept is ready to be developed into a commercial product by a manufacturer, according to David Slaughter, a professor at UC Davis and the project’s principle investigator.

According to a report submitted by Slaughter and others working on the project, the new technology they’ve been developing is based on a “hyperspectral imaging sensor and a rapid-response thermal micro-dosing sprayer.”

“The system is designed to automatically detect and identify the species of weeds in the row of direct-seeded processing tomatoes and to precisely apply a lethal dose of a high-temperature, organic food-grade oil to the weed foliage and not to the surrounding soil or crop foliage.”

In other words, the machinery will detect weeds and kill them with a spray of hot oil that avoids killing the tomato plants.

“In 2008,” according to the report, “significant progress was made toward the development of both the weed species identification system and the thermal weed control system. We successfully conducted an outdoor test where weeds in the row were automatically detected, identified and treated by the system. Test results show that 95.8 percent of black nightshade and 92.7 percent of pigweed plants automatically treated by the heated oil spray were dead 15 days post-application. Only 2.4 percent of the tomato seedlings – accidentally sprayed by the system – were killed. Nearly all (97.6 percent) of the tomato plants were alive and had a dry mass not significantly less than that of the untreated control tomato plants at 15 days post-treatment.”

According to the report, a new “pumpless, thermal weed treatment system was designed, constructed and tested” as part of the project. “In the past, we have used a re-circulating pump to transport and pressurize the heated oil used to control weeds. In a re-circulating flow design, the heated oil is exposed to air when unsprayed oil is returned to the main reservoir.”

The researchers found that re-circulated food-grade oil degraded over time – probably due to oxidation – which resulted in clogging of the micro-jet nozzles and gel formation in the solenoid valves, degrading their performance, according to the report.

In the new design, the re-circulating pump was replaced by a pressurized nitrogen gas reservoir. The nitrogen gas serves two purposes. First, it provides the pressure required to propel the heated oil from the micro-jet nozzles. Second, it provides an oxygen-free atmosphere that prevents oil oxidation, according to the report.

The oil is not re-circulated and heating occurs by conduction in the new design. According to the report, the food-grade oil is placed in an aluminum pressure chamber and the pressure chamber is placed in the center of a heated oil reservoir.

“The interior pressure chamber is sealed and there is no fluid contact between the food-grade oil in the interior chamber and the heating oil located in the outer reservoir. Heat is supplied by an electric immersion heater in the outer oil reservoir. A second electric resistance heater is located in the spray nozzle manifold and is used to boost the temperature of the oil as it passes through the nozzles,” according to the report.

“An outdoor spray test was conducted last August to evaluate the performance of the modified hot-oil spray system,” according to the report. “The spray test was designed to simulate plants growing in a row of a direct-seeded tomato bed. Tomato plants were placed in a row, with the pots approximately 18 inches apart. The tomato plants were then randomly intermingled with black nightshade and pigweed plants so that the final plant spacing along the row was 6 inches.

“The modified hot-oil spray system was mounted on a cultivator sled drawn by tractor driven at a speed of 0.08 mph. The travel speed was limited by the data transfer rate from the hyperspectral camera to the machine vision computer. As the hyperspectral imaging system traveled over the row of plants, the species of the foliage of each plant was classified in real-time to create a spray map for the thermal weed control system. The bank of eight hot-oil spray nozzles was mounted approximately 20.5 inches behind the hyperspectral camera. Each nozzle was energized when a weed plant passed beneath to apply food-grade canola oil heated to 155-165˚ C onto the weed foliage. The hot oil was applied to foliage at an average dose rate of 85 mg/cm2,” according to the report.

“Overall, the system did an excellent job of recognizing and treating weeds, with more than 90 percent of the weed foliage being treated for most plants,” according to the report.

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