How fast can you mow?
Mowing speeds seem to be ever increasing. From the 135 with a finger bar mower cutting at a snails pace to a Big M chasing down Lewis Hamilton, speeds have increased with the years. So just how fast do you mow – and more importantly – how fast should you mow?
There are a few limitations for just how fast you can travel, most common of these is horsepower. Grasses and wholecrops are made of tough stuff, the cell structure is strong and resilient and this makes cutting them a high energy task requiring lots of engine horsepower. Obviously the state of the blades doing the cutting is a critical factor, but output is directly related to the horsepower at your disposal.
How much power does the mower need?
Disc mowers became the dominant design solution for most machines used in Europe from the early 1980’s. Mowing speeds increased massively with the switch from reciprocating knife and fingers to rotary cutting action. Back in the day a Taarup 307 was a common sight in much of the UK, from memory these were 9ft (or 2.7m) working width. Most were sitting behind 75hp to 90hp tractors meaning they had about 28-33hp per metre of cut at their disposal (not allowing for the power to drag it along).
Today that’s about the same power level as a 175hp to 200hp power unit with front and rear 3m mowers. The jump comes when we look at 400hp tractors running 9m-10m triples. These are pushing 40hp per meter of mower bed; a 30% increase results in much higher forward speeds being possible. Not only that, the self propelled mower market has grown from a tiny niche and today the Krone Big M 450 is pushing 45hp per metre into its 10m working width.
So just how fast can you mow?
Assuming you have enough (too much) horsepower for the job, there has to be a limit for just how fast you can travel. The maths (for anyone interested) is quite simple. Assume the mower is stationary with the discs spinning, as soon as the machine inches forward one of the blades will come into contact with a grass stalk and cut through. The machine continues to move forward until the next blade comes round to the leading edge to cut the next stalk.
Most manufactures design the discs to rotate at around 3000rpm. As there are two blades on each disc, that’s 6000 knife presentations at any one point, per minute. We need to work out how far forward the mower is each time the blade comes around. If the machine is travelling at 10 km/h that equates to 10,000m per hour or 166m per minute. Reducing the time period some more makes the sums easier, so the mower is mowing forward at 2.78m per second or 2780mm/sec. The disc is spinning at 50 revs per second, giving us 100 cuts per second so each blade is being presented with 27.8mm of fresh crop each time it comes around.
If you look closely at a new blade the cutting edge is around 50mm long. We now know that for every 1km/h of forward speed, each blade has to cut 2.78mm of crop, so if we divide 50mm by 2.78mm we 17.98 – or 17.98 km/h. That means if you are cutting over 18 km/h then some of the crop isn’t getting cleanly cut, it’s getting ripped off instead. Unless you get a clean cut, there is greater risk of contamination in the swath leading to potential fermentation issues. So 18 km/h is the absolute maximum you can look at, given there will be some yaw and sway in the mower on the back of the tractor, 15-16 km/h is probably a “safer” maximum.
It’s not all about the engine
As anyone who has ever sat on a tractor knows so well, the travel speed depends on your ability to stay in the seat as well as how much grunt there is under the bonnet. As tractor suspension has improved and progressed from seat to cab to axles, so speeds have increased. Mower bed suspension has also increased its ability to keep the bed at the desired height from the ground at faster forward pace. So now we have a maximum speed that you can mow:
But how fast should I mow?
This is a far more important question because it has a much greater impact on the potential feed value losses. As previously discussed getting the dry matter correct is vital to making great silage. You will have a target dry matter and from that have decided the ideal wilting period. Your mowing speed then needs to be calculated to accommodate this wilting period.
In practice, this will be driven by the chopper output. If you plan to wilt for 24hrs, and the chopper is clearing ground at 10 acres per hour then you need to be mowing at 10 acres per hour – 24hrs ahead of it. Obviously if you can only cut at 8 acres an hour, by the time it’s picked up, some stuff will be too dry, some too wet and some just right. This will lead to greater losses in feed value over the optimum operation.
Working out the potential output of a machine is more complex than just taking the spot rate. The spot rate is the working width multiplied by the forward speed. To work out the potential output, you need to factor in the time spent:
· Opening up the field
· Turning at headlands
· Finishing the short work
· Transition to transport mode and vice versa
· Travel time between work
This is important stuff if you are a farmer mowing your own crop for a contractor to pick up, or for a contractor planning your fleet. If you want to discuss the influence of mowing speed with an independent consultant or to discuss any of the other aspects of silage making covered in this series – contact The Silage Consultant, at jeremynash1@btinternet.com