Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 188

11 000 L ha 1 ) 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 dem-
onstrated the effectiveness of electric weeders relative to herbicide wipers,
recirculating sprayers, and mowers.
Cultivator guidance systems
Cultivation is an exacting task that is often tiring for the tractor oper-
ator. It causes crop loss if not done carefully. These problems are multiplied
when using tools that have to travel at a precise position relative to the crop
row.Consequently,some rear-mounted implements are designed to be steered
by a rider (Geier & Vogtmann, 1987; Melander, 1997), but this is a labor-
intensive solution. Fortunately, automation of implement and tractor guid-
ance is advancing rapidly.
The simplest approach is purely mechanical.Wheels mounted on the culti-
vator guide the implement by rolling along the sides of raised beds or ridges,
or else travel in furrows laid down by the planter. These systems are suffi-
ciently accurate for cultivating at high speeds with in-row tools (Mohler,
Frisch & Mt.Pleasant,1997),or for cultivating very close to small plants with a
conventional inter-row shovel cultivator (Melander & Hartvig,1997).They are
best adapted to rear-mounted machines since the implement must have some
lateral sway relative to the tractor.Mechanical guidance is inexpensive relative
to the electronic guidance systems discussed below, and can be used when the
crops are too small to sense electronically. However, furrow guidance requires
implements six rows or wider, since two wheels are needed for stability and
the tractor tires must not obliterate the furrows.Also, the furrow made by the
planter must be preserved during any early-season passes with a rotary hoe or
harrow, and recreated with a furrowing tool on the cultivator for subsequent
cultivations. Parish, Reynolds & Crawford (1995) found that cultivating