Agriculture Reference
In-Depth Information
that influenced yields adversely. First, the enlargement of the area cropped
with silage maize may have taken place mainly on marginal soils, with
lower yield potential. Second, silage maize was practically free of diseases
and pests shortly after it had been introduced to the Netherlands.
Gradually, however, pests, weeds and diseases have spread, depressing
silage maize yields, and counteracting the simultaneous progress made by
higher yielding varieties. The two adverse developments together
apparently caused a reduction in yield by 0.73 % annually, so that the net
increase was only 0.15 %. Moreover, whereas cereal yield increases by 0.2
% annually owing to an increase in the atmospheric
concentration, this
response is negligible in maize, being a crop.
The average yield of silage maize in the period 1975-1996 was
12.1 t d.m. (Figure 7.2). According to Van der Werf
et al.
(1993) the
potential yield amounts to 16.6 t d.m.
so that in practice farmers
achieved 73 % of the potential yield level.
The conclusion from the analysis of statistical data on cereals and
silage maize is that plant breeding accounts for an annual increase in crop
yield of 0.7 to 0.88 %. The effect of crop management on crop yield ranges
from -0.73 % to +1.03 % per year. Biomass of the harvestable product
increases by 0.15 to 2.07 % annually. The lower bound of the range applies
to conditions where improvements by plant breeding are counteracted by
trends in land use or pest and disease development that have adverse
effects on yield; the upper bound of the range is reached when there is a
synergy between breeding and management.
3.2 Analysis of crop physiology
3.2.1 Salix viminalis
Partitioning
Model calculation according to Vleeshouwers (2001), applied to the period
1968-1997 in the Netherlands, showed that the potential stem growth of a
full-grown
S. viminalis
crop is 17.5 t d.m. Further simulation
revealed that, compared to the current pattern of partitioning, final stem
yield was higher when allocation to leaves was increased early in the
season, and decreased later in the season. Early establishment of the leaf
canopy advanced the start of radiation interception. Once the leaf canopy
had been established, harvestable stem biomass was increased by
maximizing the allocation to stems. The exact shape of the optimal
allocation pattern, and the concomitant increase in stem yield are
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