Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 193

surface layer, which species must be killed in the white thread stage, which
remain susceptible after emergence, and how does this relate to development
of the weeds' root systems? Which species can recover from shallow burial?
How does the rate of kill for particular species relate to soil moisture status at
the time of disturbance? To what extent are seedlings of particular species
killed by burial vs. dismemberment vs. desiccation, and does the cause of
death vary with the implement? Fogelberg & Dock Gustavsson (1999) showed
that a vertical-axis brush weeder killed more weeds by uprooting than by
burial, though both mechanisms contributed significantly to weed mortality.
Similar quantitative data are unavailable for most implements.
Much highly applied work is also needed to provide a scientific basis for
cultivation recommendations. Many implements have not been studied
systematically for performance under different speeds,angles,depths,or posi-
tion relative to the row for even a single crop, but such data are needed for a
variety of crops,each in a range of phenological stages.Quantitative European
work on weeding harrows,flame weeders,and brush weeders provides models
for the type of research that is needed (Rasmussen, 1991; Ascard, 1994 b ;
Melander, 1997). Similarly, anecdotal observations and arguments from first
principles have led to recommendations here and elsewhere regarding the
role of tilth, soil texture, and clod size on the performance of cultivation
implements,but scientific documentation to support these recommendations
is largely lacking. This lack of information is analogous to making herbicides
available to growers without supplying information on crop tolerance,appro-
priate rate, or timing of application.
Amore difficult subject in need of investigation is root pruning during cul-
tivation. Few data are available on how close or how deep various implements
can be used without damage to crop root systems. Obviously, this depends on
crop species and size, and probably also depends on growing conditions and
soil properties. Further work is needed that builds on Russel, Fehr &
Mitchell's (1971) study of soybean response to root damage by cultivators
under various environmental conditions.
Regarding the engineering of mechanical weeders, the critical challenge at
present is not to create implements based on new principles, but rather to
make the present equipment more usable. In particular, most tools must be
set for depth and distance from the row, and often angle as well. Provision of
cranks, rulers, and angle gauges on supporting shanks and brackets could
greatly simplify these adjustments (Mattsson, Nylander & Ascard, 1990).
Ideally, a cultivator should be tuned to the crop size and soil conditions each
time it is used, and this is essential for precision implements working in or
close to the row. However, the nut-and-bolt work required to adjust most