Report from the Sub-comittee on the environment and health

Models for calculation of
<strong>the</strong> seed pool and wild
plants in <strong>the</strong> field
Scenarios for analysis of
<strong>the</strong> flora in <strong>the</strong> field
Results of calculations with
<strong>the</strong> seed pool model
means that <strong>the</strong> value of <strong>the</strong>se biotopes would remain poor for
insectivores such as insects and vertebrates.
10.3.2 The flora in cultivated and uncultivated terrestrial ecosystems
The development of plants over 25 years has been calculated using two
different types of ma<strong>the</strong>matical models, a "seed pool" model and a "crop
rotation" model (Kjellsson, Madsen 1998b). The models have a number
of constraints and have not been fully validated, but <strong>the</strong>y provide a
preliminary estimate of <strong>the</strong> development trends. The seed pool model
was developed for continuous spring barley on sandy soil and does not
contain a crop rotation (Kjellsson, Rasmussen 1995). It uses five plant
species that frequently occur as weeds. The model was validated over
three years. The crop rotation model was developed to simulate crop
rotations with genetically modified sugar beet and with rape (Madsen et
al. 1996; 1999). A crop rotation with sugar beet – barley – winter wheat –
winter wheat is used in <strong>the</strong> sugar beet model, and a crop rotation with
winter rape – winter wheat – winter wheat – winter barley is used in <strong>the</strong>
rape model. The model tests 4-6 wild plants, toge<strong>the</strong>r with volunteers
that occur as weed. A rough estimate has also been made of <strong>the</strong>
development of <strong>the</strong> weed biomass over time. In all three models, two
levels of <strong>the</strong> seed pool are tested. The first level is an average seed
content of 6,900 seeds per square metre, which corresponds to <strong>the</strong>
median value for Danish fields in <strong>the</strong> last study (Kjellsson, Rasmussen,
1995). The o<strong>the</strong>r level is 22,000 seeds per square metre, corresponding to
<strong>the</strong> upper limit for 80 per cent of <strong>the</strong> fields.
The models have been used to simulate a scenario for how <strong>the</strong> seed pool
and weed biomass develop over time without any form of weed control.
It must be emphasised that <strong>the</strong> omission of spraying with herbicides is
not replaced by suitable crop rotation measures or mechanical/biological
weed control. The wild plants thus have <strong>the</strong> opportunity to increase
without hindrance. The scenario has been compared with present
production. An intermediate scenario has also been analysed,
corresponding approximately to <strong>the</strong> +scenario since it includes bandspraying
of beet crops, control of couch grass every tenth year, use of
mechanical weed control and cultivation of resistant varieties.
In <strong>the</strong> spring barley rotation, <strong>the</strong> calculations show that <strong>the</strong> seed pool in
<strong>the</strong> soil would steadily increase without weed control (see figure 10.7). In
<strong>the</strong> + scenario, <strong>the</strong> number of seeds of wild plants in <strong>the</strong> soil would
increase to approx. 18,000 per square metre, taking as <strong>the</strong> starting point
<strong>the</strong> Danish average (median) of 6,900 seeds per square metre. With
22,000 seeds per square metre, <strong>the</strong> +scenario would result in an
unchanged seed pool. The seed pool model shows that, in all <strong>the</strong>
scenarios, <strong>the</strong>re could be an improvement of <strong>the</strong> conditions for wild flora
and for <strong>the</strong> species of fauna associated with it without <strong>the</strong> wild flora
proliferating out of control provided mechanical control and limited
chemical control were practised.
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