Agriculture Reference
In-Depth Information
8.1
Seed-Rate or Seed-Density
Site-specific sowing has several objectives. A dominant objective is the control of
the
seed-rate
or instead of it the
seed-density
. The former is defined in kg per ha,
the latter in the number of seeds per unit area. Important is also the spatial placement
of the seeds in the field. This has to do with two objectives: the
seed distribution
over the area
and the
sowing depth
.
All these objectives depend heavily not only
on the respective crops, but on the seeding methods employed as well.
Different crops, soils and climates necessitate a very wide range of seed-densities
(Fig.
8.1
). The seeding methods depend largely on the respective seed density.
Precision drilling - targeted placing of every individual seed - can only be realized
for crops with rather low seed densities because of the costs. For crops with high
seed-densities - such as small grains, grasses, clover and alfalfa - it still is indis-
pensable to practice bulk-drilling.
However, adjusting machines for bulk-drilling via the seed-rate or seed-mass in
kg per ha should be replaced by a control that is oriented directly at the required
number of seeds per unit area, the seed density. Because it is this number and the
field emergence that define the
plant density per unit area
. Therefore, the aim
should be online and on-the-go control based on sensing the number of seeds that
pass through the seed tubes.
The traditional adjustment of the seed-mass in kg per ha with bulk-drilling can
cause substantial
deviations
from the number of seeds per unit area. These devia-
tions can arise from varying slip of the driving wheel within a field due to firm- or
soft ground as well as dry- or moist soil. Deficiencies can be due to the varying bulk
density of the seeds, since these drills are designed for bulk- or volume metering.
And the bulk density changes with the species, the variety, the provenance, and
because of vibrations in the hopper, which influence the settling of the seeds. Finally,
rather large deficiencies can arise from the varying average mass per seed. With
European wheat varieties, the mean mass per seed fluctuates between 40 and 55 mg.
All these deficiencies can be overcome by a reliable method of closed loop control
of the seed numbers per unit area. For this, sensing the number of seeds that are
passing through the seed tube is necessary.
Such a control of the seed density via optical sensors - which act as seed counters -
is state of the art with precision drilling. With these drills, sensing the seeds is facili-
tated because the distances between the falling seeds are rather even. Usually, the
coefficient of variation
for these distances with precision drills is only about 10-20 %.
With bulk-drilling, the corresponding distances of the seeds at the end of the seed
tube are very uneven because of more gaps as well as of seed clusters. The mean
coefficient of variation for the seed distances is 100 %. Sensing the number of seeds
online and on- the-go therefore is more difficult. But it is possible, at least for small
grains. It is necessary to adapt the seed sensing technique to the irregular seed
sequences. This adaptation can be done by either compensating for the seeds that
were not recorded as a result of clusters by a
computer program
(Feldhaus
1997
;
Heege and Feldhaus
2002
) or alternatively by more elaborate
optical sensors
for
