Miscanthus seems to have come of age in recent years – more so perhaps than the machinery used to plant and harvest it. Andy Collings spoke to an Oxfordshire contractor who last year planted more than 120ha (300 acres) of the crop
The government’s aim of generating 10% of the nation’s electricity from renewable energy sources by 2010 means that the interest in growing miscanthus (elephant grass) continues.
There are good reasons why this crop lends itself to be a top-notch biofuel. Not least is that it can grow to a height of more than 3.5m (11.5ft) and produce a dry weight harvest yield of between 12t and 16t/ha (5 and 6.5t/acre).
Better still, the plant can remain in the ground for more than 20 years and, after the second year, requires next to no inputs in terms of pesticide or fertiliser. It has been estimated that miscanthus produces 30 times more energy than it takes to grow it the average for other arable crops is put at 10 times.
Compared with coal, 20t of miscanthus is equivalent to about 8t, which, for coal-burning power stations, makes the crop very attractive. It can also be used for horse bedding and to produce biodegradable plastics and might prove to be a feedstock for transport fuels such as ethanol or hydrogen.
So what is the latest thinking on how to establish miscanthus? Contractor Phil Benson, based near Faringdon, Oxfordshire, plants and harvests miscanthus for biomass company Bical, which specialises in miscanthus and arranges contracts between growers and power stations.
“The first point to bear in mind is that miscanthus is going to be in the ground for a long time and this is the last chance available to ensure the ground conditions are correct,” he says. “And, in the same vein, existing grassweed problems should also be eliminated.”
Mr Benson recommends spraying the ground with glyphosate to control weed grasses and then to deep-plough the ground to a depth of 25cm (10in) or more.
“Most growers tend to want to put their worst land into miscanthus, just as they did with set-aside,” he says. “I’m not sure that is the right way forward, but it means that the ground really needs to be worked well prior to planting.”
Planted in the spring, the crop should be placed at a depth of about 10cm (4in), so a deep tilth – as for potatoes – is required, probably using several passes of a power harrow.
Miscanthus is not grown from seed. It starts life as a rhizome cutting 10-17.5cm (4-7in) long and with a variable number of shoots. Bical has developed its own planting machines and continues to refine them to ensure accurate distribution. But before looking at the planters, a word about the rhizomes, which are particularly vulnerable to drying out and dying as a result.
“To prevent this, the rhizomes are delivered in 1t bags, just before planting,” says Mr Benson. “If they need to be stored, even overnight, they have to be covered with moist sheeting and kept undercover.”
Ground conditions need to also offer a degree of moisture at planting time. The planter used by Mr Benson has a two-bag capacity hopper with a hydraulically-driven rubber floor that takes the rhizomes forward to the metering system. This comprises a powered smooth roller, which the rhizomes have to pass under before they encounter a second powered roller with flights on.
The flights pick up a few rhizomes on each revolution and deposit them down four chutes which lead to the four planting units. Each unit, spaced at 1m, has opening deflectors to provide a planting area at least 10cm (4in) deep into which the rhizomes are dropped. They are then covered by closing deflectors.
“The unit has a working width of 4m the outer planter units on each side slide out to their working position using hydraulic rams,” he says. “The aim when planting is to establish a crop which has a population of 10,000 plants a hectare, one every square metre.”
Just how many rhizomes need to be planted to achieve this appears to be down to experience. Bical’s technical product director Steve Croxton, says it is a matter of assessing the conditions at the time.
“Soil conditions, soil moisture, soil type, fertiliser indices and drainage can all make a difference to the number of rhizomes we plant,” he says. “It usually varies between 20,000 and 25,000 rhizomes a hectare, although on some extremely poor land, you just have to admit that it does not make financial sense to consider planting.”
With the crop planted, the ground is rolled to conserve moisture and then a week to six weeks later shoots start to appear. But before this a suitable pre-emergence spray such as pendimethalin can be applied to control grassweeds.
During the first year growers must suffer the inevitable ridicule of neighbours, as the crop can look puny and fragile. Ignore them, for there are better times to come.
When it dies down in the winter and becomes dormant there is the chance to apply a dose of glyphosate to kill off the green competition – including the important perennial weeds – that will have become established
Come the second year and things start to look better, with growth thicker and up to 2.5m (8.2ft) tall. By the third and subsequent years heights of 3.5m (11.5ft) are achieved.
Harvesting takes place from year two onwards, usually in the spring after the leaves have dropped off to provide a mulch and weed suppressant for next year’s growth and plant moisture level has fallen to as low as 30%.
“We use a self-propelled forage harvester with a cutterbar and a modified chopping unit to reduce the length of the crop,” says Mr Benson. “This chops the canes to a length that the baler can handle and then drops it in a swath. The length is quite critical. Too short and the baler’s pick-up tines can’t do their job too long and the stems wrap a round everything.”
Mr Benson says the miscanthus tends to grow in clumps that become proud of the ground, making life even more difficult for the big square baler.
“I understand there are moves to develop a self-propelled baler that can cut, chop and bale the crop in one pass,” he says. “This would prevent having to pick up the crop from the ground, along with all the stones and clumps.”
Once baled, the power station beckons. Ex-farm miscanthus is worth about £40/t for a gross return of about £600/ha. Mr Croxton says growers can expect a net margin of about £200/ha, but bear in mind this crop also attracts other payments.