Agriculture Reference
In-Depth Information
validated for the prediction of N f fluxes in agroecosystems (Oomen and Habets 1998, Jensen
et
al.
2003), balancing tools were introduced for the estimation of nutrient and humus dynamics
in agricultural systems and the validation of sustainability of farming systems (Hülsbergen
2003, van Delden
et al.
2003, David
et al.
2004), and spreadsheet calculations have been pub-
lished by Stein-Bachinger
et al.
(2004) to support the estimation of N f fluxes on the various
levels of an organic farm (i.e. farm-gate, field, barn).
Weed management
According to an organic herb grower in Canada (Schimpf and Lundberg-Schimpf 2005):
We're weeding 12 hours a day, 6 days a week. It would be 7 days but we need to
irrigate every so often.
Are the weeds that bad in organic farming? In the seven years since the comment above was
made, and after trying a range of methods, those herb growers have developed an integrated
approach for managing the weeds so that they are not the major problem they were (Schimpf
and Lundberg-Schimpf 2005). However, weed management is still one of the major issues in
organic crop husbandry. Surveys of organic growers in many countries over several years have
regularly indicated that weeds are a prime constraint, especially during conversion (Baker and
Smith 1987, Peacock 1990, Beveridge and Naylor 1999, Walz 1999, Zinati 2002). Anecdotal
reports among certifiers from organic farmers and the certification bodies confirm that weed
control can be a key impediment in adopting, converting to and succeeding in organic produc-
tion (Dumaresq
et al.
1997). The conversion period can be particularly difficult because of
lack of experience and significant changes in weed population dynamics (Ngouajio and
McGiffen 2002).
As a result of the restrictions on synthetic herbicides, organic farming systems have to
manage the various crops and weeds using a combination of indirect and direct methods of
non-chemical weed regulation (Table 3.7). Indirect (cultural) methods consist of all techniques
that are aimed at improving crop performance, to diminish the distribution of weed seeds and
to suppress the development of weeds in the standing crop. Direct (physical) methods are
designed to regulate weeds mechanically, manually, thermally or biologically. These tech-
niques are used in conjunction with preventive and cultural methods for effective long-term
weed management (Bàrberi 2002).
Weed reproduction and dispersal strategies are important factors in determining both their
spread and their control. The problematic weeds in broadacre or intensive organic cropping
systems tended to be either heavy seeding annuals or perennials with persistent underground
parts such as stolons and rhizomes (Beveridge and Naylor 1999, Walz 1999). Heavy seeding
annuals such as fat hen (
Chenopodium album
) and grasses (Poaceae) may be effectively control-
led over time in organic cropping using a combination of seed bank depletion, avoiding condi-
tions favourable for weed germination and growth, competitive crops and preventing further
seed production (Bàrberi 2002). The common options available in organic agriculture such as
tillage, mulching and rotations are usually effective. However, perennial weeds with persistent
roots and shoots including couch (
Agropyron repens
), nutgrass (
Cyperus
spp.) and the docks
(
Rumex
spp.) pose a far bigger problem as the usual options for organic farmers tend to be inef-
fective or they even encourage weed dispersion and growth (Lampkin 1990).
Canadian thistle (
Cirsium arvense
) is one of the most serious root weeds in organic crop
husbandry (e.g. in cereals, potatoes and sugar beet), and is reported to be the fourth most
problematic weed for United States organic growers (Walz 1999). The outcome of a farm survey
of 140 farmers (Böhm and Verschwele 2004) clearly showed that thistles tended to become a
serious problem on farms with a high proportion of cereals and summer-annual crops in the
