Weed blaster shows promise as alternative to herbicides

Frank Forcella is tackling the problem of weeds head-on.

A U.S. Department of Agriculture research agronomist in Morris, Minn., Forcella doesn't spray pigweed and foxtail with herbicides to shrivel them.

He blasts them to smithereens with corncob grit.

The tactic is gaining attention from organic farmers who don't use chemicals and from food companies seeking to market pesticide-free snacks and other products.

Forcella said the technology is experimental but shows promise. It uses an air compressor to spray gritty material on both sides of a crop that kills young weeds without harming corn or soybeans.

"It obliterates the weed, especially if it's a small broad-leaved weed like Lamb's quarters or pigweed that's one to 3 inches high," Forcella said. "The corn plants growing next to them are taller and thicker and can withstand the grit blast, but the weeds just disappear."

Forcella uses mainly dried corncob bits but has had similar success with other gritty textures such as ground walnut shells, corn gluten meal and soybean meal. He and others from USDA's Agricultural Research Service have been working on organically certified plots owned by the University of Minnesota at its West Central Research and Outreach Center in Morris.

Initially, Forcella used an air compressor mounted on an all-terrain vehicle and sprayed the rows by hand. Collaboration with an engineer at South Dakota State University has now yielded a unit mounted on a tractor that blasts the weeds four rows at a time from eight nozzles. High-speed particles of grit shred the weeds at 100 pounds per square inch of compressed air.

"We point the nozzles at either side of a corn row and blast about a 4-inch band on either side of the row and within the row," he said. Field trials typically hit the weeds twice: once when the corn is 4 to 6 inches high, and again when it's about a foot tall. The technique is called "propelled abrasive grit management."

"We've been getting season-long weed control of about 80 to 90 percent, which isn't perfect, but most organic farmers would be happy with that amount of weed control," Forcella said.

The "back of the envelope" cost is about five times what spraying an herbicide would cost per acre, Forcella said, but that price differential could shrink if the technology takes off.

Sam Wortman, assistant professor at the University of Illinois Department of Crop Sciences, said that abrasive weeding or blasting might have greater potential for other row crops that have higher value, such as fruits and vegetables. He heard Forcella present the idea at a conference in 2011.

Wortman used the technique on tomatoes last year and on peppers this season, and may expand to sweet corn, kale and broccoli in the future.

"In our initial work [with tomatoes], we were able to reduce the density of weeds by about 75 percent with just one application," he said. "And the weeds that we didn't kill we were able to reduce the overall height so that they wouldn't become competitive with the crop."

Wortman said tomatoes and peppers are often grown with plastic film or mulch, with seedlings planted into 4-inch square holes. So the weed blasting involves driving along rows and spot spraying the weeds that emerge in the "crop hole" next to the plants, he said, and that would otherwise probably need to be hand-pulled.

The process for tomatoes uses much less grit than the continuous spraying along corn and soybean rows.

"Some grit does hit the stem of the tomato plant, but as long as we're not hitting the growing part at the top of the plant, then we don't see any unacceptable levels of damage," he said. One member of the research team is a plant pathologist who is monitoring any potential infections of the plants from soil-borne pathogens, he said.

Organic and smaller farmers are excited about the possibilities, Wortman said, because they could make their own blasting kits with an air compressor, applicator and cart for $2,000 to $3,000.

What's most exciting to Wortman is the potential to piggyback weed blasting with fertilizing.

Organic farmers could use granular forms of fertilizer, such as corn gluten meal or soybean meal, to nourish their plants at the same time they're blasting weeds, Wortman said. Those materials contain about 7 to 9 percent nitrogen, he said, whereas corncob and other grit are essentially inert.

The weed-and-feed treatment would also be cost-effective, Forcella said. "If we start using fertilizers that organic farmers are putting on those fields anyway, there's really no added costs except for the machinery."

Companies including PepsiCo are following the research closely, Forcella said, for the potential to make and market pesticide-free snack foods. Farmers often sign contracts with food processors, especially for organic products, he said, so the industry has a vested interest in new technology that can improve productivity and profitability on specific farms.

Forcella said there may be potential international interest as well. He hosted a researcher from Spain recently who is planning to test the technology in vineyards and olive orchards where weeds have become resistant to conventional herbicides.

Wortman and Forcella said the technology is still in its infancy, and that a huge amount of work needs to be done to determine whether it has potential to be deployed on farms.

"It's not anything that's wide-scale yet for sure, but we're hoping," Forcella said. "We never thought it would work, but it did work."

Tom Meersman • 612-673-7388