A new technology, under study in the U.S. since 2009 by Frank Forcella at the USDA/ARS in Morris, MN shows promise for mechanizing control of in-the-row weeds. Field studies of abrasive weeding in vegetable crops (which uses organic grit propelled by compressed air) headed by Samuel Wortman were made with tomatoes and peppers at the University of Illinois during 2013 and 2014 at the Illinois Sustainable Student Farm.
“A variety of weed control tactics are needed for varying situations and abrasive weeding shows promise for killing small, annual weeds in the crop row, which would otherwise require hand weeding. Abrasive weeding for in-the-row weeds does need to be coupled with mechanical cultivation or flaming to kill weeds between the rows,” Wortman stated.
The “grit” used can be almost any ground organic material that can be reduced to a sieve size of between 40 and 20 (.015-.035-inch diameter). Usable materials include ground corn cobs, ground walnut shells, soy bean meal, corn gluten meal, ground limestone or dried distiller’s grain. The grit shape doesn’t really matter, because the grit is propelled at such high speeds — over 700 mph. The best grit for your situation will be something inexpensive that is available locally. Some growers are interested in trying soybean meal, to precision-apply nitrogen while killing weeds, all in one pass.
Buying grit by the ton helps cut costs. Avoid very fine materials like rock phosphate and greensand, which can create an inhalation hazard. When applying any air-propelled grit, always wear protective eyewear.
Prototype 4-row, 2-row and one-row tractor-drawn air-propelled abrasive grit applicators, called Propelled Abrasive Grit Management devices (PAGMan) were developed and built in 2012 by South Dakota State University’s Department of Agricultural and Biosystems Engineering. This was carried out with the assistance of Dr. Daniel Humburg, who led the development and testing of the prototypes in organic and conventional field crops in 2013. These tests were monitored by plant scientists who studied the effects on both weeds and crop plants.
The PAGMan blows the abrasive grit through tubing at the base of the row, leaving small annual weeds with broken stems and shredded leaves, most of them too damaged to recover. The transplant-sized tomato and pepper plants, taller and sturdier, will have visible but minimal damage. Studies have shown that this damage to the crop plants will be quickly outgrown.
Although these tractor-drawn applicators are not yet commercially available, interested growers may be able to construct their own smaller devices from readily available, over-the-counter hardware store items. In addition, specially developed motor-driven grit meters and custom-designed two-orifice nozzles (to prevent clogging) can be obtained from Sam Wortman, PhD of University of Nebraska, Lincoln (firstname.lastname@example.org).
Information on how these devices were constructed can be found in Wortman’s 2015 article in “Crop Protection” entitled “Air-Propelled abrasive grits reduce weed abundance and increase yields in organic vegetable production.”
The tractor-drawn grit applicator uses a PTO-driven air compressor and blows grit at both sides of each of the crop rows, aimed at the base of crop plants. An eight-inch-wide band of shredded weeds in the crop row results. If certain guidelines are followed, according to Wortman, two abrasive weedings about 10 and 24 days after seeding or transplantation can reduce weed biomass in the treated area 69 percent to 97 percent (with variations depending on weather, the extent of a crop canopy and the speed of application, among other factors) while not affecting the health or marketable yield of the crop plants. The guidelines are summarized below.
First, to avoid serious damage, tomato and pepper plants should be larger than the weeds. Transplant-size, with three to four leaves is good. Wait a week before using abrasive weeding on newly transplanted plugs. The stem will hold the leaves above the grit application while the very young annual weeds, still low on the ground, will be shredded and hopefully killed in large numbers. Damage to the crop plants should be short-lived and minimal.
Second, two abrasive weeding treatments, both made while the annual weeds are quite small, seem to give the best results. “One treatment is usually not enough,” continued Wortman, “and a third generally does not produce additional benefits. In our studies, two applications did not damage crop plants to a point of observed yield loss.”
Third, be sure to aim the nozzles for the base of crop plants. You don’t want the abrasive grit hitting the top of crop plants, where the growing point would be in broad-leaf weeds. You don’t want to kill the apical meristem.
Fourth, keep in mind that small broad-leaved annuals are easier to kill with abrasive weeding than small annual grasses. This is because the apical meristem in young grass plants is typically still underground. The plant top can be abraded, but the growing point lives on undamaged underground. If you have a heavy infestation of grasses in a particular field, you may want to consider other weed control measures when the plants are very small. For the same reason, perennial plants cannot be easily controlled with abrasive weeding.
Fifth, remember that larger annual weeds can be very difficult to kill. Treat while weeds are small. “Weeds with fewer than three true leaves are the most susceptible.”
Sixth, Dr. Wortman’s continuing studies on other vegetable crop plants have shown that “most crops with an upright stem architecture seem to be compatible with abrasive weeding. We have used abrasive weeding successfully in tomato, pepper, broccoli, kale, edamame, snap bean, sweet potato and zucchini.”
Perennial crop plants in liners, such as raspberries, can safely be treated with abrasive weeding for removal of small annual in-row weeds right after planting.
In Wortman’s 2013-2014 studies, yields of both tomato and pepper plots that had been abrasively weeded were significantly higher than yields in the weedy control plots with up to 44 percent higher in tomatoes and up to 33 percent higher in peppers. They did not differ significantly from the hand-weeded plots.
At the time of abrasive weeding there was some visible stem and leaf damage and sometimes some curvature of the stems. But the stem abrasions did not harbor disease organisms and the plants quickly outgrew the damage, thrived and produced a good crop.
Earlier successful studies using abrasive weeding in direct seeded organic, transitional and conventional corn and soybeans may give vegetable growers some guidance on using and timing abrasive weeding in sweet corn and edible bean crops, such as edamame and snap beans. These studies were completed by plant scientists Dr. Frank Forcella and Dr. Sharon Clay, of SDSU’s Department of Agronomy, Horticulture and Plant Science.
Dr. Forcella has reported that grit can safely be applied to very young corn plants to kill in-the-row weeds without damage to the corn stand, at the V1 through V5 stages, as the growing tip is underground at those points. The leaves may be damaged, but the corn plant will regrow and can be expected to produce a normal crop. Forcella recommends two grit applications, probably at V1 and V5, or V1 and V3. In his studies in which cultivation or flaming was used to control weeds between the rows, in-the-row weed density was reduced by up to 80 percent.