Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 170

Mechanical weeding is guided by several simple principles.
1. Row-oriented cultivators should work the same number of rows as the planter, or a
simple fraction of this number. Otherwise,imperfect spacing between adja-
cent planter passes will lead to improper placement oftools relative to
some rows,with consequent damage to the crop and poor control of
weeds in the inter-row.
2. The action of the cultivator must be appropriate for the growth stages of the weeds
and crop .The timing and number ofcultivations required depend on the
growth rate ofthe target species and the size range over which it is sus-
ceptible to the implement.Based on many years offarming experience,
Bender (1994,pp.35-7) suggested that staggered planting ofcrops facil-
itates timely cultivation by reducing bottlenecks due to weather.
The degree to which precise timing is critical depends on how closely
the implement works to the crop row.In-row weeders and full-field
implements cannot dig deeply without damaging the crop.For these
machines,operations must be timed to catch the weeds after they have
germinated but before they become well rooted,and delaying cultivation
may allow many to escape (VanGessel et al .,1998; Fogelberg & Dock
Gustavsson,1999).Implements that work close to,but not in,the row
have a larger window within which the work can be performed,but still
require careful attention to timing.In contrast,timing is less critical
with most inter-row cultivators.For example,Mt.Pleasant & Burt (1994)
found that timing ofcultivation with a shovel cultivator had little effect
on either weed biomass or maize yield.
3. Creation and maintenance of a size differential between the crop and the weeds
facilitates effective mechanical weed control .Most sophisticated mechanical
weed management programs begin with a stale seedbed or pre-emer-
gence cultivation to delay emergence ofweeds relative to emergence of
the crop.Full-field,in-row,and near-row weeding can then increase in
depth and degree ofsoil movement as the crop grows larger.For many
row crops (e.g.,maize,sorghum,potato),once the crop is well estab-
lished,soil can be thrown into the row to cover small weeds.However,
because ofthe high growth rate ofmost agricultural weeds,this will only
be effective ifthe first cohorts that germinate following crop planting
have been killed previously.
4. The effectiveness of cultivation decreases as weed density increases .This occurs
for several reasons.First,some proportion ofweeds in the crop row will
escape even a well-planned and carefully executed cultivation program.
Dieleman,Mortensen & Martin (1999) found that the proportion escap-
ing was constant over a wide range ofdensities.Thus,ifthe density is
high,the escapes may cause yield reduction (Buhler,Gunsolus & Ralston,