Report from the Sub-comittee on the environment and health

Results of calculations with
<strong>the</strong> crop rotation model for
crop rotations with rape
Scenarier
Model for frøpuljens størrelse efter 25 år i
sædskifter med roer
Ingen
behandling
Plus
Nudrif t
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7000
6000
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14000
7000
44000
41000
0 10000 20000 30000 40000 50000
Frø pr. kvadratmeter
Figure 10.8
The development of <strong>the</strong> seed pool after 25 years in a crop rotation with beets. The
bars show <strong>the</strong> size of <strong>the</strong> seed pool after 25 years. In <strong>the</strong> +scenario, in which
mechanical weed control is combined with limited use of herbicides, <strong>the</strong> seed pool
remains unchanged when <strong>the</strong> starting point is 6,900 seeds per square metre (white
bars). For <strong>the</strong> larger seed pool of 22,000 seeds per square metre (hatched bars), it
will be seen that <strong>the</strong>re is a fall in <strong>the</strong> seed pool. In <strong>the</strong> third scenario, no form of
weed control is practised.
(Figure texts:
Model for frøpuljens størrelse efter 25 år i sædskifter med roer = Model
for <strong>the</strong> size of <strong>the</strong> seed pool after 25 years in crop rotations with beets
Scenarier = Scenarios
Ingen behandling = No treatment
Plus = +scenario
Nufrift = Present production
Frø pr. kvadratmeter = Seeds per square metre)
In <strong>the</strong> +scenario, a number of species of wild flora could occur more
frequently. A more varied plant community could <strong>the</strong>refore be expected,
providing food resources for a more varied animal community
(invertebrates and <strong>the</strong>ir predators). However, in view of <strong>the</strong> poor
competitiveness of beets, this crop rotation would hardly be profitable
unless new, alternative methods were found for controlling weed in beet
crops.
In <strong>the</strong> rape crop rotation, <strong>the</strong> results of <strong>the</strong> calculations of <strong>the</strong> seed pool
are shown in figure 10.9. It will be seen that <strong>the</strong> seed pool would increase
over 25 years if no form of weed control were practised. On <strong>the</strong> o<strong>the</strong>r
hand, <strong>the</strong> +scenario shows a distinct fall in <strong>the</strong> number of seeds in <strong>the</strong>
seed pool after 25 years compared with present production after 25 years,
whe<strong>the</strong>r <strong>the</strong> starting point is 6,900 or 22,000 seeds. One reason for this is
that chemical agents have little effect on shepherd's purse, which occurs
in this crop rotation, whereas mechanical control is more effective
(Kjellsson, Madsen 1998b).
201