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Sam Wortman, University of Illinois at Urbana-Champaign

Oct 08, 2015
Sam Wortman, Assistant Professor, University of Illinois at Urbana – Champaign

Sam Wortman, Assistant Professor, University of Illinois at Urbana – Champaign

Why do organic farmers need specialized technologies to control weeds?

While many conventional farmers can rely on just a handful of herbicides to control most weeds, there are no comparable “silver bullets” available to organic farmers. Instead, organic farmers must develop an ecological approach to weed management that includes a diverse mix of cultural, mechanical, and physical tactics to control weeds. Cultural practices like crop rotation, cover cropping, and delayed planting dates are helpful for disrupting weed growth, but mechanical tillage is the primary form of weed control on most organic farms. Fossil fuel use and soil disturbance associated with intensive tillage have raised questions about the environmental sustainability of organic farming. Thus, research is needed to develop novel mechanical and physical organic weed control tactics that reduce dependence on tillage without sacrificing crop productivity or farm profitability.

Figure 1. Project director Sam Wortman (left), applying abrasive grits by hand to peppers in a straw-mulch raised-bed.

Figure 1. Project director Sam Wortman (left), applying abrasive grits by hand to peppers in a straw-mulch raised-bed.

What methods have organic farmers been utilizing to control weeds in lieu of synthetic herbicides? 

Many organic farmers have tried cover crop mulches (e.g., roller-crimped rye), natural herbicides (e.g., citrus oil), and flame-weeding to reduce tillage. Each of these strategies can be effective in certain cropping situations, but they also have limitations. For example, cover crop mulches can harbor insect pests and interfere with crop emergence and nutrient availability. Natural herbicides are expensive and not effective against many weed species. Flame-weeding is effective against a broad range of weed species, but is fossil fuel-intensive and can cause significant crop damage and yield loss.

Figure 2. Project collaborator Dan Humburg (right), working on the prototype two-row grit applicator for vegetable cropping systems.

Figure 2. Project collaborator Dan Humburg (right), working on the prototype two-row grit applicator for vegetable cropping systems.

What is abrasive weeding, how does it work and what are the benefits?

Abrasive-weeding, or “weed blasting”, is a physical form of weed control that uses air-propelled abrasive grits to kill small weed seedlings within the crop row. Any small, gritty material can be used in abrasive-weeding (e.g., granulated walnut shells and corn cobs), but we are interested in using organic fertilizers as abrasive grits so that farmers can control in-row weeds and supplement crop nutrition in one field pass.

Abrasive grits are applied in a 4-6 inch band within the crop row via compressed air (100 PSI) either by hand (Fig. 1) or by a prototype grit applicator (Figs. 2 & 3). Abrasive grits defoliate and kill small weed seedlings (i.e., those with two leaves or less) if the growing point is above the soil surface. Thus, abrasive-weeding is most effective on broadleaf weeds and less effective on grasses because the growing point is often protected beneath the soil surface.

Like many organic weed control tactics, abrasive-weeding is most effective when there is a size differential between the crop (large) and weeds (small); thus, planting or transplanting into a weed-free seedbed is important for success. Abrasive-weeding can partially defoliate and slow the growth of larger weeds, but weed mortality decreases greatly when the weeds have three or more leaves at the time of grit application.

Figure 3. Prototype four-row abrasive grit applicator in an organic corn crop. This first-generation applicator was developed in Dan Humburg’s lab at South Dakota State University in collaboration with Frank Forcella at USDA-ARS in Morris, MN.

Figure 3. Prototype four-row abrasive grit applicator in an organic corn crop. This first-generation applicator was developed in Dan Humburg’s lab at South Dakota State University in collaboration with Frank Forcella at USDA-ARS in Morris, MN.

Are there limitations on the crops that it can be used on?

Thus far, we have demonstrated successful weed control with abrasive-weeding in corn, soybean, tomato, and pepper. Further research is being done in broccoli and kale. Abrasive-weeding will likely be effective in most crops with a vertical growth habit or those that are vertically trellised. However, the number of applications and the amount of grit required in a specific cropping system will determine the economic feasibility of abrasive-weeding. As a result, this technology may have the greatest potential in organic vegetable crops because economic returns per unit area are high, and grit applications can be paired with a straw or (bio) plastic mulch, which should reduce the quantity of grits required to achieve season-long weed suppression.

Is this technology available to organic farmers now? Where can they learn about it?

Hand-held application of abrasive-grits is currently available to organic farmers as it only requires a portable air-compressor and a hand-held siphon- or gravity-fed grit applicator (Fig. 1), both of which can be found at your local hardware store. Larger-scale mechanized abrasive-weeding is still in the research and development phase, but our research team hopes to publish blueprints for a four-row (grain crops) and two-row (vegetable crops) prototype grit applicator within the next two years (Figs. 2 & 3).

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