Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 182

1995; Mohler & Frisch, 1997), so the utility of harrowing cereals is question-
able. Harrowing is unlikely to increase yield unless the predominant weed
species are sensitive to harrowing,their density is high,and the operations are
timely relative to phenology of the weeds. However, harrowing may be useful
for decreasing future weed pressure even when it does not increase yield of the
current crop.
In contrast with the situation in cereals, several studies have shown that
weed control with harrows increased yield of large-seeded crops like pea,
broad bean, and maize (Baumann, 1992; Rasmussen, 1992 b ; Mohler, Frisch &
Mt. Pleasant, 1997; but see Boerboom & Young, 1995).VanGessel et al . (1995 a )
found that harrowing pinto bean significantly improved weed control and
gave yields equivalent to the weed-free check. Deep planting and fast early
growth of large-seeded crops allows aggressive harrowing soon after emer-
gence, whereas this is not possible with cereals.
Although harrows are used for seedbed preparation in smallholder agricul-
ture (Intermediate Technology Publications, 1985, pp. 21-3), their use for
weed control is rare, possibly due to lack of appreciation for the benefits of
killing weeds in the white thread stage. The use of hand- or animal-drawn
chain harrows shortly after planting could substantially reduce the labor
requirements for later hand-weeding in some systems.
Several tools using different soil-moving principles are available for in-row
weed control in row crops. These are all precision tools that must be carefully
set for depth, distance from the row, and in some cases, angle, to achieve good
weed control without damaging crop plants. They are best used in a front-
mount or belly-mounted position, or with a guidance system. A smooth, flat
seedbed improves the consistency of weed control.
Torsion weeders are spring steel rods that reach within a few centimeters of
the crop row and travel 2 to 3 cm below the soil surface (Figure 4.15).The com-
pressive action of the springs causes the soil in the row to boil up, thereby dis-
turbing weeds in the row that are not yet well rooted.Larger weeds next to the
row may be sheared off as well. Spring hoes work in a similar manner, but are
more robust and aggressive (Figure 4.15). Both types of tools are usually
mounted on inter-row cultivators forward of the shovels or rolling gangs.
Several studies have shown improved weed control and crop yield by cultiva-
tors equipped with these tools in combination with spinners and spyders (see
below) relative to standard shovel cultivators (Figure 4.16) (Schweizer, Westra
& Lybecker,1994; VanGessel et al .,1995 b ,1998; Mohler,Frisch & Mt.Pleasant,
1997).
Stiff, heavy-duty spring hoes are available for work in orchards and vine-
yards. These scrape the soil surface free of weeds in and near the row. A castor
at the tip allows the tool to bend past trunks without scraping the bark.