Agriculture Reference
In-Depth Information
Where the seeding rate is low, however,
stalks are thicker and taller, and therefore
harder to harvest and process.
In the case where seed is to be harvested
with a combine harvester, the experimental
results indicate the optimal seeding rate at
approximately 20 kg/ha. Unfortunately, at that
density, canopy is insufficient to smother weeds
and infestation is a hazard of production.
Finally, where the crop is to be harvested
for both seeds and fibre, a compromise seed-
ing rate is required in order to optimize both
the seed yield and the fibre content. For farm-
ers wishing to produce fibre (for crops where
seeds are not harvested first), it is possible,
therefore, to advise them on the density of
sowing that will afford them an easy harvest
and processing. Recent trials (which would
benefit from additional replication) suggest a
seeding rate of 50-60 kg/ha.
When applied in excess, nitrogen is used
for leaf production rather than fibre produc-
tion. The plant then stays green longer. This
can lead to difficulties in cutting the crop,
increases the drying time and renders fibre
processing more difficult.
As a rule of thumb, the more nitrogen
applied, the greener the stems; and the more
difficult will be the processing of the fibre.
Influence of nitrogen on density,
height and colour of the crop
Increasing applied nitrogen results in a reduc-
tion in the density of the harvested crop due to
increased competition between plants during
the vegetative period. Nitrogen applied at
planting allows plants to gain a little in height
at harvest.
It was noted in these trials that heavy
application of nitrogen caused plants to stay
green longer even when colour variation was
minimal.
5.4.3 Nitrogen fertilization
Influence of nitrogen on yields
The absorption of nitrogen occurs primarily
between the three-leaf stage (50 cm) and the
end of flowering (FNPC, 2001).
Hemp's nitrogen requirements have been
estimated as between 14 and 15 u/t DM when
water resources are not limited (FNPC, 1999,
2000; Chambre d'Agriculture de l'Eure,
2001). Nitrogen assaying enables the pro-
ducer to adjust the fertilization rates for his or
her crop, depending on the residual nitrogen
levels in the fields.
Hurd and seed yields increase with the dose
of nitrogen and it would appear that excess
nitrogen in the soil promotes seed production
(FNPC, 1999, 2000, 2001). Hurd yield stabi-
lizes at a certain nitrogen titre (100-120 units),
regardless of the concentration in the soil
(FDGEDA de l'Aube, 1982; FNPC, 1984, 1986;
SPC, 2003; Fig. 5.10).
These figures are to be considered with
the amount of dry matter (DM) exported:
100
90
80
70
60
50
40
30
20
10
0
0
50
100
150
200
250
Dose of nitrogen (u/ha)
Fig. 5.10.
Relative hurd yields as a function of nitrogen fertilization of the sown crop (soil nitrogen content
unknown). Synthesis of the tests undertaken by ITC (2003) (SPC, FNPC) and FDGEDA de l'Aube (1982).
