Report from the Sub-comittee on the environment and health

Results of calculations with
<strong>the</strong> crop rotation model for
crop rotations with beets
200
Scenarier
Ingen behandling
Model for frøpuljens størrelse efter 25 år med
kontinuert vårbyg
Plus
Nudrif t
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7000
5000
21000
18000
90000
86000
0 20000 40000 60000 80000 100000
Frø pr. kvadratmeter
Figure 10.7
Calculations with <strong>the</strong> seed pool model in continuous spring barley. The bars show
<strong>the</strong> seed pool after 25 years with an initial seed pool of 6,900 (white bars) and
22,000 (hatched bars) per square metre for present production and <strong>the</strong> +scenario,
respectively. 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 vårbyg =
Model for <strong>the</strong> size of <strong>the</strong> seed pool after 25 years in crop rotations
with spring barley
Scenarier = Scenarios
Ingen behandling = No treatment
Plus = +-scenario
Nudrift = Present production
Frø pr. kvadratmeter = Seeds per square metres)
In <strong>the</strong> beet crop rotation, as shown in figure 10.8, <strong>the</strong> calculations show
that, with a starting point of 6,900 seeds per square metre, <strong>the</strong> seed pool
would remain largely unchanged for 25 years in <strong>the</strong> +scenario, in which
mechanical weed control is combined with limited use of herbicides. For
<strong>the</strong> larger seed pool of 22,000 seeds per square metre, <strong>the</strong>re would be a
fall in <strong>the</strong> seed pool after 25 years. After only one year in <strong>the</strong> scenario
without treatment, <strong>the</strong> seed pool would reach its maximum of 60,000 and
95,000 seeds, respectively, per square metre with a starting seed pool of
6,900 and 22,000 seeds. That is because beets in <strong>the</strong> model have very
poor competitiveness, enabling wild plants to proliferate. The average
seed pool of approx. 40,000 and approx. 44,000, respectively, would
adjust itself after a couple of years and <strong>the</strong>n fluctuate in step with <strong>the</strong>
change in <strong>the</strong> crop rotation. In <strong>the</strong> +scenario, in which chemical control
of couch grass is only carried out every ten years, <strong>the</strong>re would be
problems with couch grass. With both starting points for <strong>the</strong> seed pool,
couch grass would reach its biggest biomass of each individual year on 1
June in <strong>the</strong> +scenario, since it would largely avoid competition <strong>from</strong>
o<strong>the</strong>r seed weed in this scenario. The scenario without any weed control
would not be feasible in practice.