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explained from the increase in total above-ground biomass, since this
amounted to 1.23 %. Apparently, the remaining increase of 0.84 % per year
is caused by a shift in the ratio of grain to total above-ground dry matter.
Comparison with data reported by Austin
et al.
(1989) helps to interpret
these trends. When winter wheat varieties introduced in the period 1830-
1985 were grown under optimal conditions in the UK, Austin
et al.
(1989)
found that old varieties produce the same amount of dry matter as the
modern ones,
i.e.
about However, the grain mass of modem
varieties made up a larger proportion of the total biomass produced. The
increase of the harvest index in Dutch agriculture that can be inferred from
Figure 7.1 is similar in size to that reported by Austin
et al.
(1989), and
may thus be attributed to the efforts of plant breeders. The increase in the
total above-ground winter wheat yield by 1.23 % in the period 1946-1993
(Figure 7.1) is caused by improved growing conditions. In crops, the
rising concentration in the atmosphere causes an increase in dry matter
production by 0.2 % annually (Goudriaan and Unsworth 1990). The
remaining 1.03 % is attributable to a better crop management, which
allowed farmers to attain a steadily increasing fraction of the yield under
optimal conditions. The ceiling for yield increase by improvements in
agricultural production methods is the potential production level. The
potential yield of the varieties introduced in the period 1980-1985 is 15.9 t
(Austin
et al.
1989). The average actual total above-ground dry
matter yield in the final 10 years of the studied period (
i.e.
1984-1993) was
(Figure 7.1), so that farmers achieved 72 % of the potential
winter wheat yield, in terms of total above-ground dry matter.
When considering all cereal crops grown in the Netherlands, there
is an average annual increase of 1.7 % in grain yield. An increase by 1 %
results from an increase in total above-ground dry matter, mainly by
improved crop management (0.8 %, with the remaining 0.2 % owing to the
increase in atmospheric and an increase by 0.7 % results from an
increase in harvest index, caused by advancements in plant breeding.
Similar estimates for the relative contributions of management and
breeding were made by Evans (1993), who reported that half of the yield
increase over the years can be attributed to introduction of higher yielding
varieties and half to improved cultural practices.
3.1.2 Silage maize
Silage maize was introduced in the Netherlands in the early 1960s, but
statistical yield data are only available from 1975 onwards. Figure 7.2
shows the trend in silage maize yield on farmer's fields in the period from
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