Raising the combine cutterbar as high as 36cm (14in) and then using a wide rotary mower to chop stubbles could speed up harvest and cut fuel use.
That’s the conclusion reached by researchers at Dresden University in Germany following a series of trials over the past couple of summers. They were commissioned to do the work by UK manufacturer Spearhead, which has just launched a range of big stubble-mulching mowers with a working width of up to 12.1m (29ft 10in). The term “mulching mower” refers to the fact that each rotor has six pairs of blades. Each of the top blades cuts horizontally, but a second set – angled downwards – also cuts close to the ground.
The result, says the company, is that each straw stem gets chopped several times, giving an average chop length that’s shorter than most combine-mounted choppers could achieve. The lower blades also stir up the top layer of soil, part-burying the straw and starting breakdown of the stems.
Though most farmers would be reluctant to add an extra operation at an already ultra-busy time of the year, Spearhead claims that, overall, the cut-high-and-chop system saves time and money.
For a start, moving from a typical 15cm (6in) combine cutterbar height to a much higher 36cm (14in) means 50% less straw is going through the combine’s threshing and separating mechanism.
It also means that the combine chopper has only half the straw to deal with. Given that a chopper on a big combine can take 120hp to run (and take 20% of the combine’s available power), it could mean a big potential fuel saving.
Even more importantly, when the 9m wide combine used by the Dresden University researchers moved from a 15cm cut height to 36cms, they found its forward speed could be pushed up by 34% (from 5.1kph to 7.2kph) and workrate increased by 3.6ha/hr to 5.4ha/hr. At the same time average fuel consumption dropped by 38% (from 18.2 litres/hr to 11.2 litres/hr).
That amounts to a total saving of E36 /ha (£33/ha).
Of course, an extra 200hp tractor, mower and fuel is required. The cost of that, reckon the researchers, would be E10/ha (£9/ha) based on a fuel consumption of 2.5 litres/ha at a workrate of 12ha/hr.
Even more surprisingly, they calculated that the higher workrate of the combine would mean that, even including the mowing operation, harvest would still end slightly earlier than it would otherwise.
So on a 720ha harvest that would normally take 200 hours with an average workrate of 3.4ha/hr, the cut-high-and-mow system would see the combining finish in just 133 hours. Add the 60 hours of mowing required and the total time taken would be 193 hours – seven hours quicker than the conventional system.
Other benefits include the fact that the mulching mower gives a shorter chop length than the combine’s own chopper, meaning that the straw starts to break down more quickly. Also, the mower tends to give a more even distribution of the chopped straw across the full cut width than the combine’s own system. Plus the higher cutting height means much less risk of the cutterbar hitting stones, less chance of the table bulldozing when conditions are wet and less likelihood of pulling in the lower, damper part of the cereal stems.
Some German farmers have apparently made the system even more efficient by towing a cultivator directly behind the mower to chop and produce a stale seedbed at the same time.
The Starcut range goes from a 3m (9ft 10in) model that requires a tractor of just 70hp to a mighty 12.1m (29ft 10in) model with a substantial £70,000 price tag. The design of all models, says Spearhead, is similar to an aircraft wing, with the framework sandwiched inside the upper and lower steel sheets. All hydraulic lines and electric cables have been encased within this steel sandwich, making the mower strong, easy to clean and easy to transport.