Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 186

Pleasant,1997).Generally,this stand loss appears as randomly spaced missing
individuals, rather than as blighted row sections. Consequently, growers may
find that a high planting rate improves yield and competitive pressure on sur-
viving weeds (Bender, 1994, p.39).

Thermal and electric weeders
Flame weeders briefly expose weeds to a propane or butane flame at
800-1000

C (Ascard, 1995 a ), which disrupts cell membranes and leads to
rapid dehydration (Ellwanger, Bingham & Chapell, 1973; Ellwanger et al .,
1973).Abank of burners can flame a wide area to kill weeds before crop plant-
ing 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; Holmøy & 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 (1994 b ) 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,
1995 b ). 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, 1995 b ).
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

°