Flame weeders briefly expose weeds to a propane or butane flame at 800-1000°C (Ascard, 1995a), which disrupts cell membranes and leads to rapid dehydration (Ellwanger, Bingham & Chapell, 1973; Ellwanger et al., 1973). A bank of burners can flame a wide area to kill weeds before crop planting or crop emergence, or to defoliate plants prior to harvest (Tawczynski,
1990). Shielding such machines to contain the heat increases their efficiency. Irrigation a few days before planting ensures that the first flush of weeds will have emerged in time to flame before the crop is up. Alternatively, burners directed toward the row can control in-row weeds in crops like maize, onion, and cabbage that have a protected terminal bud (Figure 4.19) (Geier & Vogtmann, 1988; Ascard, 1990; Holm0y & Netland, 1994), and in cotton, which has a corky stem (Seifert & Snipes, 1996).
The effectiveness of a flame weeder is best described in terms of the amount of gas required to kill a certain percentage of weeds (Ascard, 1994b). For a given machine, gas consumption is usually regulated by varying ground speed. The amount of gas required for 95% control varies substantially with weed species and size. For example, Ascard (1994b) required 1.5- to 2-fold more propane per hectare to control Sinapis alba in the two-to-four leaf stage than to control it in the none-to-two leaf stage. Many common broadleaf species such as Chenopodium album, Stellaria media, and Senecio vulgaris can be well controlled by gas doses of less than 50 kg ha 1 when young (Ascard, 1995b). In contrast, grasses and broadleaf species with protected buds (e.g., Matricaria inodora, Phleum pratense, and Poa annua) are relatively resistant to flaming and can be controlled only with large gas doses, or not at all (Rahkonen & Vanhala, 1993; Ascard, 1995b).
Other thermal weed control approaches have been tested, including infrared irradiation and freezing with liquid nitrogen or carbon dioxide snow, but these appear to be less efficient than treatment with an open flame (Parish, 1989; Fergedal, 1993). Microwave heating of soil greatly decreases weed emergence, but appears impractical at field scales (Barker & Craker,
1991). Concentration of solar radiation with a Fresnel lens is energy-efficient and effective at killing young weeds (Johnson et al., 1989), but implements based on this technology are likely to be bulky and slow. Similarly, hot-water weeders are in use, but the large volume of water that must be heated (9000 to
11 000 L ha1) and the need to trail a hood to prolong the heating effect limits their application.
Electric discharge weeders are used primarily to kill escapes in low-growing row crops like sugar beet and soybean. They operate by bringing a high-voltage electrode into contact with weeds that stick up above the crop canopy. Electrical resistance of the weeds causes vaporization of fluids, which disrupts tissues (Vigneault, Benoit & McLaughlin, 1990). The proportion of weeds controlled decreases with weed density because more pathways for energy discharge result in a lower energy dose per plant. Energy use increases with weed density, which makes electrical discharge systems impractical as a primary weed management tool. However, energy use for electric discharge weeding and herbicides was similar for low-density populations (e.g., 5 m~2) that escaped other management measures (Vigneault, Benoit & McLaughlin, 1990). Rasmusson, Dexter & Warren (1979) and Diprose et al. (1985) have demonstrated the effectiveness of electric weeders relative to herbicide wipers, recirculating sprayers, and mowers.
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