Slow chop reaps long hemp
24 September 1999
Slow chop reaps long hemp
Harvesting hemp efficiently
has set new challenges for
machinery manufacturers.
Edward Long takes a look at
a technique which uses
modified forage harvesters
A SIMPLE modification to a self-propelled forage harvester allows tall-growing hemp to be cut easily and quickly without the wrapping and tangling previously encountered. This break-through is expected to boost grower-interest in the fibre crop.
This year 1820ha (4500 acres) of the non-narcotic cannabis crop are being grown on Home Office licences in the southern half of the country. Its long fibres are now in increasing demand as raw material for the manufacture of a range of papers, lightweight car door panels, and insulation materials.
Hemp is spring sown and can reach a height of 4m (13ft) in just 12 weeks of being planted in late April/early May.
Until the modification was made, forage harvesters could not be used as they chop the fibre into short lengths making it unsuitable for modern markets. So the tall stems need to be cut with a mower or converted rape swather. After cutting, the green material is left in the field to rett – the process in which bacteria degums the sticky lignin which binds fibre to the pith core. After a few weeks the retted material is baled and carted into the UKs only hemp processing plant – Hemcores unit at Maldon in Essex.
It was Hemcores production director Ian Underhay who designed the forage harvester modification. "Harvesting was the choke point in hemp production, tall stems wrapped around the equipment making the job difficult, and it was obvious something had to be done to make it easier," he says. "It was clear that unless the forage harvester design was altered, it was not the machine for the job. I decided a simple modification was all that was needed."
On 27th July last year he started by inspecting a John Deere header. Five weeks later the modified machine was at work cutting hemp. Later he turned his attention to a 4.5m wide Claas forager.
A conventional forage harvester has a cutting cylinder, with blades on it, which is mounted on a shaft and driven from one end.
"But it operates at a constant speed, for hemp it needs to be four or five times slower than the normal 1000 to 1200rpm. Modification comprises a shaft with gears cut into it connected to a ring gear fitted inside the drum. The aim is the maintain the shaft speed, as it rotates power is transferred through two sets of gears to drive the inside ring gear which turns the cylinder."
This slows the cylinder speed to 200rpm for the John Deere and to 250 for the Claas. Then by removing 10 of the 12 sets of blades a chop length of between 30 and 45cm (12-18ins) can be achieved instead of the maximum 17mm (0.7ins). By taking out the unwanted blades without lowering the rpm would only increase the chop length to about 9.5cm (3.8ins) which is insufficient for hemp.
"Since June Claas UK has backed our project, the company has supplied a complete header and four spare cylinders. Already two have been modified. This season about half the UK crop will be cut by three modified machines, next year it will all be done this way."
Whole stems cut by mowers or converted swathers are long and retting takes six weeks, the shorter forage harvested sticks only need to be exposed to the elements in the field for three weeks. This allows the crop to be baled sooner when there is a greater chance of reasonable weather.
According to Claas UKs Trevor Tyrrell, the modification is simple but highly effective.
"It has proved to be a success, now we want to perfect the design to boost output," he says. "Currently the 4.5m wide Class RU450 header operating at a forward speed of 8 to 12kph is capable of cutting 30 to 60 acres a day, depending on crop height and density. We aim to boost this to 100 acres which requires increasing the cutting width to 6m, or the forward speed to 15kph."
As only the cylinder is modified, a standard forage harvester can be converted in less than two hours. This allows a contractor to slot the fibre crop in during late August and early September between grass and maize to keep the machine busy. *