Organic Broadcaster

Research shows ‘sandblasting’ works to control weeds

By Kelli Boylen

The unlikely hobby of growing apricots in Minnesota may have led a research agronomist to a new way to control in-row weeds on organic farms: blasting them away.

Frank Forcella works for the USDA North Central Soil Conservation Research Laboratory in Morris, Minn. He and his wife, Jessica, have been growing apricots since 1991. In 2007 they had a very productive year, and Forcella wanted to find a way to use the pits left over from making jams and dried fruit instead of letting them go to waste.

A little online research taught him that ground shells of apricot pits are used in “sandblasting.” In fact, many modern sandblasting techniques use plant-based products. He was discussing this with Dean Peterson when both men had the same idea of sandblasting weeds.

He and Peterson spent about $50 on a handheld sandblaster, connected it to an air compressor and started testing it in the greenhouse. “We’d have corn and weeds growing in a pot and we learned right away that we could knock down the weeds and the corn would be okay. We repeated the experiment many times and had reasonably good success,” he said. When aimed at the bases of crop plants growing in rows, the propelled grit kills weed seedlings through abrasion.

To test the method on corn growing in the field, they bought a bigger sandblasting unit, and hauled the air compressor around on an ATV. “We learned if we made two or three passes at the right times we could get full-season weed control,” Forcella recalled.

Researcher Frank Forcella applies abrasive grit to corn rows using an air compresser hauled by a graduate student on an ATV. Forcella found that just two to three passes at strategic times provided full-season weed control. Photo by USDA-ARS

Forcella admits he is not “mechanically astute.” So when it came time to build a prototype of a larger unit he needed help. He applied for and received a grant from the North Central Sustainable Agriculture Research and Education (NC-SARE) for an “Air-Propelled Abrasive Grit Management” (PAGMan) system.

Forcella said it took a little while to find someone who was an expert ag engineer who worked with steel, but they were fortunate to find Dan Humberg, a professor of ag engineering at South Dakota State University (SDSU).

SDSU graduate student Cory Lanoue took an interest in the project and started working on it with CAD (computer aided design) software. Then, using the design he developed over nearly a year, he and fellow grad students, under the supervision of Humberg, put together their abrasive grit applicator machine in about two months.

“It was an impressive feat in my eyes,” said Forcella. “Cory did a fantastic job.”

They developed PAGMan “for selective, post-emergence, in-row weed control in corn, soybean, and other crops that are grown in widely spaced rows.”

The 2013 and 2014 growing seasons were their first using the four-row, eight-nozzle unit. Forcella admitted that the unit may seem small to many of today’s large-scale farmers, but the size is realistic for a family organic farm.

The field tests were performed on organically certified land at the West Central Research and Outreach Center of the University of Minnesota. These experiments are being led by SDSU graduate student Mauricio Erazo-Barradas under the direction of Sharon Clay, a weed scientist.

The idea is that farmers will clear weeds from between crop rows like they traditionally have for years with inter-row cultivation, flaming or mowing, but they will use the sandblasting method to kill the weeds next to the corn plants. The field tests include all of these treatments.

Ground corn cobs make an effective grit for “sandblasting” weeds. Any gritty material that is about 0.5mm works with the PAGMan system. Photo by USDA-ARS

“I think of using abrasive grit for weeds as one more tool in the toolbox for controlling weeds,” Forcella said. “It’s not the only tool—you need lot of them—but this definitely can be one more tool for organic producers to use.”

Through their trials, they learned that the system works best when the nozzles are about two feet from the plants using air pressure of 70 to 100 psi. If the nozzles are too far away or the pressure too light it doesn’t kill the weeds. If the nozzles are too close or there’s too much pressure, it can damage the corn.

Forcella said they have been focusing on the control of the most common annual weeds, such as lambsquarters, pigweed, and foxtail (Chenopodium album, Amaranthus retroflexus, and Setaria viridis), which are common throughout the nation and especially abundant in the Corn Belt.

“Timing is crucial,” Forcella said, and the weeds must be seedlings no more than two or three inches tall. The two necessary times to blast the weeds are once at the 1-leaf stage of the corn (4-6 inches) and again at the 3-leaf to 5-leaf stage (about 8-12 inches) in order to provide season-long weed control. This also maintains corn yields comparable to those in weed-free crops.

Using sandblasting along with cultivation offers 80 to 90 percent weed control, which Forcella described as “quite reasonable, especially for organic production.” Weed control at 80 to 90 percent is enough to prevent yield loss in field corn. A weedy corn field can easily lose 20 to 50 percent yield.

Although blasting works well with annual broadleaf weeds, it is not as effective with seedlings of grassy weeds because their growing points are below ground. When foxtail is the most common weed, three applications may be needed for good control; at the 1-leaf, 3-leaf, and 5-leaf stages of corn. The system likely will not be effective on perennials, as these weeds are simply too tough-stemmed, and they tend to have large root reserves, which allows them to sprout continuously.

Forcella and his colleagues have tried many different types of abrasive grit: walnut shells, ground up corn cobs and even plain sand. “As long as it’s the right size (about 0.5 mm) and gritty, it will work,” he said. Most of their experiments have been done with corn cob grit because it is inexpensive and readily available.

When they presented the idea to organic producers, they quickly realized it was more likely to be adopted if the applicator could be doing “double duty” when moving across the field. Luckily, Forcella said, many nitrogen-rich organic fertilizers have a gritty texture as well. They started experimenting with corn gluten meal, alfalfa meal, cotton seed meal, canola seed meal and even lime to neutralize acidic soil. “All the grits work equally well to control small weed seedlings,” he said. Their planned trials include looking at how and when the plants uptake nitrogen after gritty organic fertilizers are applied to control weeds.

Other upcoming trials will include determining what type of nozzle and spray pattern work best and looking at how the idea can be applied to organic vegetable production.

The project has taken on an international angle as well. For several years Forcella has collaborated with a weed scientist, Professor Jose Urbano, from the University of Seville (UdS) in Spain. One of Urbano’s colleagues at UdS, Manuel Perez Ruiz, is an ag engineer. He was interested in the concept as well and secured funding from his government to determine how to apply the idea to olive orchards and vineyards. His group is investigating how to integrate GPS and robotic technology with the PAGMan system. This new work is in collaboration with Professor David Slaughter at the University of California-Davis and the USDA-Agricultural Research Service.

The four-row, eight-nozzle PAGMan provides selective, post-emergence, in-row weed control for corn, soybean, and other crops grown in widely spaced rows. Photo by USDA-ARS

One of the drawbacks of the system thus far is that the unit needs to move at a rather slow speed, about one to one-and-a-half miles per hour, so it is time-consuming. Forcella explained that maintaining the diligence needed to drive at a very slow speed and staying aligned as to not damage the corn plants is difficult. GPS and robotic technology could help eliminate this drawback for the farmer.

Still others are working to help make this project come together. Sam Wortman at the University of Illinois recently was awarded a grant from the National Institute of Food and Agriculture’s Organic Research and Extension Initiative. This new project will extend the technology to organic vegetable production. The aforementioned researchers at SDSU and USDA-ARS will collaborate on this venture.

Forcella said as a public employee working on a project like this he has no interest in trying to patent the idea. Since the idea has already been shared with so many others, the overall concept may not even be patentable at this point, but smaller things such as the nozzle design could be.

Even though the idea may not be patented in the future, the unit is not something that could easily be built by someone on their own, he added. Forcella and his colleagues hope that a larger equipment company, a few of which have expressed interest, will decide to build and market the unit. He hopes the unit will be commercially available within the next five years.

Kelli Boylen is a freelance writer with an agricultural background. She lives with her family on a homestead in Iowa. Her work has been featured in local, regional, and national publications, including the Progressive Dairyman.

From the November | December Issue.

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