A Brief History of Microwave Weed Control Research

Interest in the effects of high frequency electromagnetic waves on biological materials dates back to the late 19th century (Ark & Parry, 1940), while interest in the effect of high frequency waves on plant material began in the 1920s (Ark & Parry, 1940). Many of the earlier experiments on plant material focused on the effect of radio frequencies (RFs) on seeds (Ark & Parry, 1940). In many cases, short exposure resulted in increased germination and vigour of the emerging seedlings (Nelson, Ballard, Stetson, & Buchwald, 1976; Nelson & Stetson, 1985; Tran, 1979); however, long exposure usually resulted in seed death (Ark & Parry, 1940; Bebawi et al., 2007; Brodie et al., 2009). Davis et al. (1971; 1973) were among the first to study the lethal effect of microwave heating on seeds. They developed a set of prototypes, called “Zappers”, which they tested in the field for their Company and federal and state researchers. Their final prototype, designated Zapper III, underwent tests underwent tests to provide the data necessary for the construction of the first semi-commercial prototype. In October 1971, the Company purchased all proprietary rights to a discovery made at Texas A&M University concerning the toxic effects of microwaves on plants Davis et al. (1971; 1973).


Introduction 6.1
Interest in the effects of high frequency electromagnetic waves on biological materials dates back to the late 19th century (Ark & Parry, 1940), while interest in the effect of high frequency waves on plant material began in the 1920s (Ark & Parry, 1940). Many of the earlier experiments on plant material focused on the effect of radio frequencies (RFs) on seeds (Ark & Parry, 1940). In many cases, short exposure resulted in increased germination and vigour of the emerging seedlings (Nelson, Ballard, Stetson, & Buchwald, 1976;Nelson & Stetson, 1985;Tran, 1979); however, long exposure usually resulted in seed death (Ark & Parry, 1940;Bebawi et al., 2007;Brodie et al., 2009). Davis et al. (1971;1973) were among the first to study the lethal effect of microwave heating on seeds. They developed a set of prototypes, called "Zappers", which they tested in the field for their Company and federal and state researchers. Their final prototype, designated Zapper III, underwent tests underwent tests to provide the data necessary for the construction of the first semi-commercial prototype. In October 1971, the Company purchased all proprietary rights to a discovery made at Texas A&M University concerning the toxic effects of microwaves on plants Davis et al. (1971;1973).
A meta-study of published data (Menges & Wayland, 1974;Wayland, Merkle, Davis, Menges, & Robinson, 1975) reveals that microwave treatment of emerged weed plants, of eleven species, can be described by equations of the form (Figure 6.2): When the weed species are separated into categories of broad leafed and grasses, it appears that grasses require slightly more microwave energy to achieve treatment efficacy, compared with broad leafed plants ( Figure 6.3). It also became apparent that microwave treatment of the soil could inactivate weed seeds at various depths (Menges & Wayland, 1974;Wayland et al., 1975). The efficacy of the treatment depended on the soil type, the seed burial depth, the microwave treatment energy density and whether the soil had been irrigated prior to treatment ( Figure 6.4). Irrigation prior to treatment resulted in shallower microwave heating; therefore, seed which were buried deeper in the soil profile were less affected by the microwave heating (Menges & Wayland, 1974;Wayland et al., 1975). The consensus from this data is that 300 -500 J cm -2 of microwave energy density at the soil surface, can control weeds and their seeds in the top 4 -6 cm of soil. This is equivalent to between 30 and 50 GJ ha -1 of microwave energy, making microwave treatment a little more energy expensive than steam treatment (see Chapter 4).
It is unclear, from the available literature, why this promising technology did not become more widely available as a commercial system. It is apparent that the ideas generated by this early work interest persisted into the 1990's, because Nelson (1996) used a theoretical argument to dismiss microwave soil treatment as a viable prospect for weed management. The high energy input required to achieve good weed and seed control was certainly a strong argument against the adoption of this technology.   Menges & Wayland, 1974;Wayland et al., 1975). Despite this, then there has been ongoing research interest in microwave soil treatment and weed management. Table 6.1 lists a subset of the papers that have been published on these and related topics. The consensus from these studies is that: microwave treatment can kill plants; moderate microwave treatment can break dormancy in some hard-seeded species; and high energy microwave treatment can kill seeds in the soil. Response weed seeds in the soil to microwave energy, as a function of applied energy density, burial depth and irrigation status (Sources: Menges & Wayland, 1974;Wayland et al., 1975). Table 6.1: Literature addressing the application of microwave technology to seed and weed treatment.

Paper Title
Reference Douglas-fir tree seed germination enhancement using microwave energy (Jolly & Tate, 1971) Microwave processing of tree seeds (Kashyap & Lewis, 1974) Increasing legume seed-germination by VHF and microwave dielectric heating (Nelson et al., 1976) Effects of low-level microwave radiation on germination and growth rate in corn seeds (Bigu-Del-Blanco, Bristow, & Romero-Sierra, 1977) Effects of Microwave Energy on the Strophiole, Seed Coat and Germination of Acacia Seeds (Tran, 1979) The effect of microwave-energy on germination and dormancy of wild oat seeds (Lal & Reed, 1980) The Effect of Externally Applied Electrostatic Fields, Microwave Radiation and Electric Currents on Plants and Other Organisms, with Special Reference to Weed Control (Diprose, Benson, & Willis, 1984) Control of field weeds by microwave radiation (Vela-Múzquiz, 1984) Effect of microwave irradiation on germination and initial growth of mustard seeds (Rao, Chakravarthy, & Panda, 1989) Inhibition of weed seed germination by microwaves (Barker & Craker, 1991) A possibility of correction of vital processes in plant cell with microwave radiation (Petrov, Moiseeva, & Morozova, 1991) Microwave irradiation of seeds and selected fungal spores (Cavalcante & Muchovej, 1993) Response surface models to describe the effects and phytotoxic thresholds of microwave treatments on barley seed germination and vigour (Stephenson, Kushalappa, Raghavan, & Mather, 1996) Energy Efficient Soil Disinfestation by Microwaves (Mavrogianopoulos, Frangoudakis, & Pandelakis, 2000) Microwave effects on germination and growth of radish (Raphanus sativus L.) seedlings (Scialabba & Tamburello, 2002) Report on the Development of Microwave System for Sterilisation of Weed Seeds: Stage I -Feasibility (Advanced Manufacturing Technologies, 2003) Design, construction and preliminary tests of a microwave prototype for weed control (Zanche, Amista, Baldoin, Beria, & Giubbolini, 2003) Thermal effects of microwave energy in agricultural soil radiation (Velazquez-Marti & Gracia-Lopez, 2004) Influence of low-frequency and microwave electromagnetic fields on seeds (Kalinin, Boshkova, Panchenko, & Kolomiichuk, 2005) An improved microwave weed killer (Vidmar, 2005) Microwave Weed and Soil Treatment Patents 6.3 The long-standing interest in applying microwave technology to weed and soil treatment has resulted in many attempts to capture the intellectual property through various patents (Tab. 6.2). It is evident that some of these are the same invention; however, they have been patented in different parts of the world. Patents have included two main methods of soil treatment: in-situ treatment systems that do not disturb the soil (Clark & Kissell, 2003;Haller, 2002;Joines, 2009); and tunnel treatment systems which use some mechanical method to remove the top soil, pass it through a microwave treatment chamber or tunnel and then return the soil to its original position after treatment (Wall, 2009). The in-situ treatment systems use various antenna systems or multi-mode cavities (somewhat like half of a microwave oven that is open to the soil) to apply the microwave energy (For example : Clark & Kissell, 2003;Haller, 2002). Several of these patents claim to control other crop pests as well as weeds and their seeds in the soil (Grigorov, 2003;Haller, 2002;Joines, 2009). There are also several companies that have developed microwave based weed management technologies, but have chosen not to apply for a patent to protect their inventions. There will be others that the authors are not aware of. Some of these Effect of microwave radiation on seed mortality of rubber vine (Cryptostegia grandiflora R.Br.), parthenium (Parthenium hysterophorous L.) and bellyache bush (Jatropha gossypiifolia L.) (Bebawi et al., 2007) Effects of microwave treatment on growth, photosynthetic pigments and some metabolites of wheat (Hamada, 2007) Microwave seed treatment reduces hardseededness in Stylosanthes seabrana and promotes redistribution of cellular water as studied by NMR relaxation measurements (Anand, S, Joshi, Verma, & Kar, 2008) Effect of microwave fields on the germination period and shoot growth rate of some seeds (Monteiro, Mendiratta, & Capitão, 2008) Germination of Chenopodium Album in Response to Microwave Plasma Treatment (Sera, Stranak, Sery, Tichy, & Spatenka, 2008) Work conditions for microwave applicators designed to eliminate undesired vegetation in a field (Velazquez-Marti, Gracia-Lopez, & de la Puerta, 2008)

Conclusion 6.4
It is clear from the number of papers, patents and other evidence that the basic principle of microwave weed management is of considerable interest and is well understood. Several system designs have been developed and protected; however, there is still scope to develop novel microwave applicator designs that better couple the microwave energy into the soil and weed plants. There is also opportunity to develop and implement better energy control systems that could reduce the energy required to achieve effective soil and weed treatment and automate the weed management process.
On the more cautionary side, in a theoretical argument based on the dielectric and physical properties of seeds and soils, Nelson (1996) demonstrated that using microwaves to selectively heat seeds in the soil "cannot be expected." He stated that seed susceptibility to damage from microwave treatment is a purely thermal effect, resulting from soil heating and thermal conduction into the seeds. He concluded that microwave weed management was not viable; however, his arguments ignored any effects of herbicide resistance on crop yields.