Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 179

may work in wet soils (Weber, 1994). An additional advantage is that, like
basket weeders,they leave a loose,uniform soil surface that is not conducive to
weed germination. Horizontal-axis brush weeders require a flat seedbed for
consistent depth of operation (Geier & Vogtmann, 1987).
Full-field and in-row cultivation tools: weeding harrows,rotary
hoes,rubber-finger weeders,spinners,torsion weeders,and
spring hoes
The ecology behind all cultivators that attack weeds in the crop row is
essentially the same: disturb the shallow surface layer of soil above the rooting
depth of the crop, thereby killing very small weeds without uprooting crop
plants. All in-row and full-field cultivating tools are primarily successful
against small-seeded weeds that must germinate near the soil surface to
emerge, and in large-seeded crops that can be planted relatively deeply.
Conversely, these tools are ineffective against large-seeded and perennial
weeds that emerge from below the depth of operation, and they cannot be
used in small-seeded crops unless the crop is transplanted. Deeper planting
allows more aggressive weeding.
Weeds are fully susceptible to in-row and full-field tools only when in the
white thread and early cotyledon phases of development. Most weeds larger
than this will escape unless the tools are used so aggressively that the crop is
damaged. Consequently, successful in-row cultivation requires repeated
removal of weeds while they are still very small. Such repeated working of the
soil might seem contrary to good soil conservation. However, the operations
are so superficial that damage to soil structure by the implements themselves
is probably negligible, although repeated wheel traffic can contribute to com-
paction.
Full-field implements affect the crop row and inter-row areas equally.They
are primarily useful in close-planted crops like cereals, and for control of
weeds in the crop row of wider spaced crops like maize,sorghum,and soybean
when these will be cultivated with an inter-row machine later in the season.
Full-field cultivators are of two types: rotary hoes and weeding harrows.
A rotary hoe consists of two ranks of wheels, each bearing 16 spoon-like
projections. The wheels are attached to the tool bar by spring-loaded arms to
allow movement over obstacles.The ground-driven wheels typically penetrate
to a depth of 2-4 cm and flick up soil and small weeds as they turn.To disturb
the soil effectively, the machine must operate at high speed (11-21 km h 1 ).
This allows rapid weeding of large areas.
Lovely, Weber & Staniforth (1958) found that whereas three timely rotary
hoeings reduced subsequent weed dry weight by 72 % in solid seeded soybean,