Agriculture Reference
In-Depth Information
indirect methods are also used by organic growers to manage weeds in an integrated way.
Many of these techniques may not individually contribute to large reductions in weed impacts,
although they each have a role in an integrated weed management strategy, exemplified by the
phrase 'many little hammers' (Liebman and Gallandt 1997).
Various techniques or approaches for improving the competitiveness of the main crop
against weeds have been listed by Köpke (2000). Seed quality can strongly inf luence early crop
growth and establishment, so selecting bigger seeds generally increases emergence rates and
early vigour. Derrick and Ryan (1998) found that heavier wheat seeds had a higher P content
and that germination, root biomass and shoot biomass were positively correlated with seed P
levels. The morphological habit of cereal cultivars (e.g. erectophile v. planophile position of
leaves) can be selected to improve weed suppression. The shadowing caused by the position of
planophile leaves acts more against creeping dicotyledonous species, for example (Drews et al .
2002).
Germination in many weed seeds is phytochrome controlled. Seeds require light to stimu-
late germination and this is usually provided when soil is tilled (Scopel et al . 1994, Milberg
1997). Several researchers have investigated the impact of preventing light exposure during
tillage by cultivating at night or with covered implements, and have found that reductions in
density and biomass for a range of weeds are often obtained, although the results vary consid-
erably and are difficult to predict (Melander 1998a, Fogelberg 1999). Several factors have been
reported to inf luence variability in weed seed behaviour, including non-responsiveness to light
in certain species, interactions with chemical and physical conditions, intra-population
dormancy differences, land use history and current tillage practices (Milberg et al . 1996,
Milberg 1997, Botto et al . 1998, Gallagher and Cardina 1998).
When sowing crops, drilling in an east-west direction can provide better shadowing of
interrow space and light interception by the crop. The occurrence of Apera spica-venti could be
reduced more under erectophile cultivars drilled from east-west compared to north-south.
The effect of planophile cultivars was similar in both drilling directions tested (Köpke 2000).
Direct methods
Mechanical weed control
Whereas chemical weed control often achieves 100% efficacy against various monocotyledo-
nous or dicotyledonous plants, tools for mechanical weeding are much less effective and
reliable. Their effectiveness depends upon various factors such as soil type and status, type of
target plant and growth stage, and the technical design aspects of the implement. In addition
to limitations of poor accuracy, soil damage and promoting further weed f flushes, tillage imple-
ments cannot be used on time if climatic conditions are unfavourable (Fogelberg and Dock
Gustavsson 1999, Hatcher and Melander 2003). Nevertheless, mechanical cultivation remains
a prominent element of most organic weed management programs and Ehlers (1997) points
out that improved soil structure and accelerated seedling growth can occur after ploughing,
increasing the competitiveness of crops against weeds. See Special Topic 2 for the impact of
tillage on soils in organic farming.
According to Dierauer and Stöppler-Zimmer (1994) the chain harrow and the harrow comb
are very suitable for cereals and suitable for all other crops and crop groups (Table 3.8). Selec-
tivity of harrows can be inf luenced by the day of treatment, composition of weed f flora, site
characteristics and the degree of weed control (Rasmussen 1992). The use of cultivators
promises the best results in nearly all cultivated crops, although different harrow types fit
better for weed regulation in cereal crops. Finger weeders are best suited for potatoes, maize
and beet. High investment costs (brush hoes) and high energy costs (f (flame weeder) are only
justifiable in crops with high gross margins, such as market gardens or other high-value crops
 
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