Agriculture Reference
In-Depth Information
different tasks such as ground preparation and removing rhizomatous weeds by pigs or post-
harvest weed seed removal by sheep (Clark
et al.
1995, Andresen 1998). However, the use of
grazing and livestock phases has not always been adopted by farmers converting to organic pro-
duction methods and, in many cases, farmers opt for stockless systems to reduce the complexity
of farm management and to focus on other plant-based commodities (Huxham
et al.
2001).
Thermal weed control
Flame and steam weeders are implements suited to annual and perennial horticultural systems
(Melander and Rasmussen 2001, Penfold 2001) as well as arable cropping systems (Rasmussen
2003). These tools have been gaining in popularity among organic growers and gas supply
companies as new designs are being built and tested (Daar 2002), and as basic knowledge about
dose-response patterns (Storeheier 1994, Ascard 1995, Bàrberi
et al
. 2002) and the wider eco-
logical impacts is acquired (Rahkonen
et al.
1999).
Flame and steam weeders are often used with good success in the pre-emergence and early
postemergence stages of a crop. However, the technique is limited by short suitable application
time periods and poor control in certain weed species, especially monocotyledonous plants
with shielded meristems (Ascard 1995). Steam weeders usually provide better heat transfer
efficiency than f flame weeders, although further research is needed to improve the fuel use and
heat transfer of thermal weeding units generally (Merfield 2002).
Biological methods
The use of weed-specific biological agents is a valid technique for managing weeds in organic
and other farming systems. However, despite the compatibility of biological control and organic
agriculture, these methods are not common practice within the organic farming movement
(Köpke 2000, Kristiansen 2003). Classical biocontrol may be less suited to annual cropping
systems in which the control agent's habitat may be disturbed or completely removed from time
to time, or if farming operations (e.g. tillage) interfere with the lifecycle of the control agent
(Hartley and Forno 1992). For weeds in organic cropping that commonly have perennial rhi-
zomatous organs with a strong regeneration capacity, biocontrol programs have been persistent
failures (Hartley and Forno 1992). In a review of the current development of biological weed
control methods in association with physical and cultural methods, Hatcher and Melander
(2003) recommend using combined control methods and stress the importance of strategic
timing to maximise damage to the weed and minimise damage to the biocontrol agent.
More research is needed to ensure the use of pathogen, dosage, environmental conditions
and potential success of applications. However, these new organisms have to be registered and
permitted as microbial products for certified organic weed management. Another group of
products are the mycoherbicides or bioherbicides requested by some organic farmers, but criti-
cised and disputed by organic organisations and some 'organic purists' (Verschwele 2005). If
these new products become listed as crop husbandry inputs in the organic standards, precise
restrictions will need to be included with allowance for their optional application (e.g. before
use, all indirect and partially direct weeding methods must have been tested).
Weeds as indicators
There are publications within the organic movement that relate the occurrence of specific
weeds to specific soil conditions and that refer to the possibility of weeds being 'indicators' of
soil status (e.g. Pfeiffer 1970, Walters 1996). The presence of a given weed therefore suggests
that the land has too much or too little of a certain soil feature (e.g. drainage, pH, copper
levels) that may be modified by the farm manager. Although many of the recommendations
are based on careful observation and commonsense, some of the relationships have not been
conclusively established.
