I get a lot of questions to the tune of “how many acres per hour will that cut?” I get them from people cutting their lawns as well as people cutting hay. The answer is both simple and complicated – it really depends on the situation. But there is a simple formula you can use to estimate:
Mower width in inches times forward speed in miles per hour divided by 100.
This formula will give you the maximum mowing capacity, in acres per hour, assuming you are always cutting new material (no overlap). Since we all know that never happens you really need to add an efficiency factor to this formula which will take into account overlap between passes as well as doubling back and other times when the mower is not actually cutting. For example, if I were using a 60 inch mower, I might assume there is 6 inches of overlap between passes, or a 10 percent reduction. In addition, I might estimate that I spend 10 percent of my time turning around, slowing and/or trimming around obstacles, or moving from one area to another. So my actual efficiency is only around 80%.
Cutting width is obvious, but what about speed? Most tractors and mowers do not have speedometers. Here you may have to estimate, or time yourself for a given distance and do the math. Some rules of thumb would be
3 mph – walking speed
4 mph – brisk walk
6 mph – a decent jog. About the maximum speed a sickle bar mower can cut. Also any tractor with small wheels, like a subcompact, will start to get pretty uncomfortable to ride on in a bumpy hay field if you go much faster than this.
8mph – about the maximum speed a finish mower can cut before cut quality begins to suffer (assuming adequate horsepower).
10 mph – easily achievable by a mid-sized utility tractor with a disc or drum mower, assuming it has the horsepower.
15 mph – Possible with a large tractor (category 2 or larger) and drum mower. Probably too fast to be safe for anything smaller. This is FLYING for a tractor in a field.
Power is usually the limiting factor as far as speed goes. Most compact tractors and dedicated mowers find their sweet spot between 4 and 6 mph.
Back to our example, if I am able to cut at 6 mph, the calculation would look like this:
60 inches X 6 mph x 80% / 100 = 2.88 (round to 2.9) acres per hour.
I’ve found this formula to be very useful over the years. I will sometimes use it to check my efficiency by timing myself in a field of known size. Since I know my speed (my Carraro TTR4400 has a speedometer) I can back-calculate to get my mowing efficiency. I find 80% to be pretty good. It is possible to do better on a large, rectangular field, but is usually below that mark when mowing small irregular fields.
Try it yourself this spring, and let us know what you find. Can you beat 80%? Maybe you find you are able to mow faster than the guidelines above. Send us your comments at email@example.com, and we’ll share them here in a later post.