Agriculture Reference
In-Depth Information
as
roises
or
rôtière
) in order to start the retting
process. Female plants were then torn up,
dried and deseeded before also undergoing ret-
ting. Fibre from the male stalks was sold and
supplied small farmers with an important part
of the revenue generated by a hemp crop.
at the earliest opportunity, taking advantage of
favourable climatic conditions and freeing up
the fields early. In most cases, hemp is used in
a rotation system before autumn wheat.
In this context, monoecious varieties are
preferred. The sowing density is a compromise
between the price of the seed and the technical
constraints of obtaining a crop that is suffi-
ciently able to smother out the weeds and the
plants that fail to develop well, thereby allow-
ing the combine harvester to pass easily during
the harvesting of hemp seed. The amount gen-
erally used is approximately 50 kg/ha.
Hurd harvesting requires the use of plant
string (in order to avoid the risk of synthetic
fibre contamination). This currently makes the
use of high-density parallelipipedic bales (termed
square bales) impossible at the present time, as
these must be bound using synthetic string.
Fibre production for the paper industry
Processes originate essentially from the cultiva-
tion methods used in France:
This type of production requires that the
harvesting of hemp seed be combined with the
harvesting of the stems (combined).
Hemp cultivation competes with other
spring crops, including peas, sunflower, maize
and potatoes, for its place in crop rotation sys-
tems. It must yield similar revenue if it is to
attract the interest of farmers. In this respect,
hemp seed is an important part of the revenue
from this crop.
Another argument militating in favour of
the twin harvesting of hemp seed and hurds is
that it is easier to obtain high-quality hurds (yel-
low to grey white) when a mature crop is har-
vested than from plants that are still green and
have a higher water content. Statistically, there
is more than one chance in two of obtaining
poor-quality hurds when the crop is not com-
bined: the hurds will be either green, as weather
conditions have not allowed the minimum
amount of retting, or black, because reaping
has been followed by a lot of rain, resulting in
excessive retting. In both cases, quality suffers.
An epiphenomenon is also observed with hurd
conservation problems, as the seed attracts
rodents. Finally, the fibre extraction of non-
combined hurds will introduce small quantities
of seed into the hemp hurds. This can cause
problems where the hurds are used as animal
bedding (three-quarters of the hemp hurd mar-
ket) for the hemp seed renders it more appetiz-
ing and encourages animals to eat their
bedding. Where the hurds are used as mulch,
the appearance of hemp plants among flower-
beds is little appreciated.
Setting aside fibre colour, quality hemp
hurds will be clear in colour and will sell well in
the market place, particularly as bedding
material.
Precocity (earliness) is an interesting fac-
tor. It allows harvesting work to be undertaken
The production of technical fibres
These sources are recent and from Germany.
There are many similarities between the
production of fibres for the paper industry and
the production of technical fibres. The con-
straints are somewhat less demanding where
plastic contamination is concerned. This makes
the use of high-density parallelipipedic bales
possible, and therefore storage and transporta-
tion easier.
After fibre extraction, the fibres destined
for use in the production of temperature-
hardening plastics (a mat, or pre-form, of
mixed hemp fibres and plastic fibres such as
polypropylene, which can then be tempera-
ture fused and moulded) must have as low a
load of hurds as possible (<1%). By contrast,
those fibres destined for use in thermoplastics
(compounds made of very short fibres mixed
with polypropylene and thermo-injected) can
have a similar composition to those used in
paper manufacturing.
6.2.3 Harvesting techniques
The harvesting of hemp seed
Hemp seed is very fragile. Despite the shell that
protects the cotyledons, the shock required to
crack the seed is small. This then opens the
