Development and Demonstration of a New Method of Physical Weed Control

The goal of this project was to design, construct, and test a field-scale implement based upon a newly conceived method of weed control. The new method involved the postemergence application of gritty agricultural materials under high air pressures to selectively control within-row weed seedlings in growing corn, soybean, and other row crops. The gritty agricultural materials can be derived from any of several sources: corn cobs, nut shells, dried distillers grain, N-rich seed meals, ground limestone, poultry wastes, etc. Some of these materials are approved as organic fertilizers, which permits weed control and fertilizer application to occur simultaneously.

A four-row, field-scale, “propelled abrasive grit management” (PAGMan) applicator was designed and constructed by agricultural engineers at South Dakota State University in 2012-13. The applicator subsequently was tested in the field during 2013 and 2014 by SDSU and USDA-ARS plant scientists in Minnesota and South Dakota on both organically certified and conventional fields of corn and soybean. PAGMan design and development was reported formally in a Master of Agricultural Engineering thesis in 2012, and the field-testing will be detailed in a Plant Sciences PhD dissertation in 2015.

Field-testing of PAGMan indicated that for season-long control of within-row weeds, grit must be applied twice. As an example, in corn, the first grit application must occur at the 1-leaf stage of crop growth, and the second application should occur sometime between the 3-leaf and 5-leaf stages of crop growth. Greater than 80% control of within-row weeds results from these tandem applications of grit. (Between-row weeds can be controlled with a conventional interrow cultivator.) Crop yields equivalent to weed-free checks were maintained with two applciations of grit.

Using corn cob grit as an example, rates of grit application were about 500 lbs/acre (500 kg/ha) per pass, or 1000 lbs/acre (1000 kg/ha) when grit was applied twice. With the use of N-rich grit (e.g., seed meals), which contain about 10% N, the equivalent of about 100 lbs/acre (100 kg/ha) of N would be applied to the crop. Corn cob grit can be purchased for about $0.25/lb; thus, season-long control of within-row weeds using this material may cost $250/acre. Many improvements in efficiency are possible in this new system of weed control, which may lower costs. Moreover, recent experiments that succesfully used abrasive grit in vegetable crops indicate that grit costs are a minor expense for overall management of high-value crops.

Introduction:

Context, Background, Rationale, and Need

Context: Only labor availability typically surpasses weed control in its importance to attainment of high yields and positive economic returns for organic growers. Consequently, new forms of weed control are needed to improve organic farming systems. In addition, improved techniques also will ease the problems associated with the transition of conventional farms to organic methods. In other words, as the risk of weed control failure decreases, the fears of some growers to expand organic acreage will be dampened.

This proposal will engineer, field-test, and demonstrate a novel method of postemergence weed control; i.e., compressed air propelling abrasive grit derived from agricultural residues, such as corn cobs and nut shells. Proofs-of-concept by our team have reported weed control in greenhouse and small field plots (Forcella 2009 a, b). Development of demonstration-model prototypes with advice from organic growers, intensive field validation, and on-farm demonstrations now are needed to explore the efficiency and cost-effectiveness of this technique, which is fully expected to meet certified organic standards.

Background: National surveys of organic growers (Walz 2004) and Midwestern experiences (e.g., Posner et al. 2008) repeatedly show that weed management is the primary production-related concern in organic crops. The consistency of this outcome indicates that (1) weeds are a major worry of organic growers, and (2) concern with weed management has not improved over time and continues to be the principal issue related to organic crop production faced by growers.

The book “Non-Chemical Weed Management,” edited by Upadhyaya and Blackshaw (2007) reviews most techniques available to organic growers, such as crop rotation, cover crops, mulches, flaming, steaming, allelopathy, mycoherbicides, classical biological control, etc. Organically-compatible herbicides (Copping and Duke 2007), which are increasingly available, are mostly liquid products in the form of organic acids or oils that are derived from plants or microbes. These chemicals typically are applied in the same manor as conventional herbicides, but they usually are non-selective and expensive (> $200/A). Preemergence products are less common than postemergence chemicals and include corn gluten meal and mustard meal. These seed meals inhibit germination of small-seeded weeds (Webber and Shrefler 2007; Boydston et al. 2008). Prohibitively high costs for corn gluten meal, hundreds of dollars per acre, however, limit its adoption. Most corn gluten meal is sold to home-gardeners.

Regardless of the availability of the aforementioned tactics, weeds continue to be a persistent issue for most organic growers. To date, no single tactic, except tillage, has been adopted extensively in any crop. Organic crop producers typically rely heavily upon soil tillage for weed control (e.g., Cloutier et al. 2007; van der Weide et al. 2008). Soil tillage, however, may not be sustainable in terms of soil health when viewed over the long-term. Consequently, for organically grown crops, a clear impetus exists for weed researchers and others to devise new management tactics that involve neither soil tillage nor synthetic herbicides. Development of new tactics must alleviate concerns with regard to soil degradation and coincide with the regulations of the National Organic Program (http://www.ams.usda.gov/AMSv1.0/NOP).

Little doubt exists that new, effective, and reasonably-priced tools for weed control in organic systems are needed and would be adopted quickly. Nørremark et al. (2006), who developed the Pneumat system of using compressed air to kill weeds, also postulated the use of air-propelled abrasive grit to control weeds, but they did not attempt to build and test the proposed system. Grits derived from agricultural residues, such as corn cobs and nut shells, are used in sand blasters, which are powered by air compressors, to strip old paints or oxidized surfaces from walls of buildings, hulls of ships, etc. Far more types of agricultural residues are produced than are processed for grit. Thus, new applications for grits may enhance values of agricultural residues.

Rationale: If agricultural residues or other natural products can be used in sand blasters to shred and kill weed seedlings, this may represent a new option for postemergence weed control in organic agriculture. We have conducted tests in greenhouses and determined that weed seedlings easily can be killed by grit derived from corn cobs and walnut shells under pressures of about 80 psi (Forcella 2009a), and field corn seedlings growing amongst these weeds are largely uninjured (Forcella 2009b). Preliminary experiments in small field plots during 2008 and 2009 using a single-nozzle sandblaster powered by a small air compressor verified the selectivity of the method for field corn and the sensitivity of weed seedlings. Single blasting treatments reduced weeds by about 50%. With two sequential blasting treatments occurring at the 1-leaf and 3-leaf stages of corn, total season-long weed control reached 87%, and this treatment did not injure corn plants nor affect maximum corn yields. Continued weed emergence after the treatments reduced apparent efficacy and suggested that three tandem blasting treatments may be needed. However, timing and frequency of blasting to maximize weed control and crop yield are not yet known.

The simple abrasive grit applicator used to generate the above results was constructed by plant scientists, not engineers, from easily available materials. Although conducted in the field, these experiments were small scale with blasting occurring only with a hand-held single-nozzle applicator with each side of a row treated individually. The nozzle was connected to an air compressor and a grit reservoir mounted on the back of an ATV. The goal of these field experiments was to test the concept rather than to perfect an implement. Whatever the case, this simple system successfully controlled weeds and justified the desire to build field-scale equipment. In other words, we now wish to harness the energy of a tractor, whose size is typical of those on organic farms (ca. 75 to 150 hp), to compress air and eject corn cob grit or other grits through four pairs of nozzles, with each pair directed at either side of a crop row.

The proposed grit applicator is not a panacea for weed management in organic crops. Most notably, it has little effect on large and well-established weeds. Because grit effectively abrades only small weed seedlings (< 4” tall), the need for sequential applications is anticipated due to the often prolonged emergence periods of many weed species. Consequently, timing of grit applications likely will be a critical issue. Unavoidable delays in grit application because of wet weather, labor shortage, etc. will lessen the technique’s efficacy appreciably. Many other possible limitations exist (e.g., energy capacity of tractor, availability of non-GMO grit, etc.), but most of these seemingly have engineering or agronomic solutions. The primary limitations are the timeliness issues mentioned above.

Need. The necessity for development of a grit applicator has been stated above, but for the sake of completeness it will be reiterated briefly here. Organic growers desire new and effective methods for weed control. In response to that need a grit applicator was conceived, and a simple prototype developed and tested successfully in greenhouse and small field-plot settings. A larger field-scale implement is needed to test its practicality and economic feasibility. Several organic growers, many of whom are skilled mechanics, have expressed interest in participating in the development and testing of the field-scale implement.