What Size Bush Hog Do You Need for a 40 HP Tractor? Do the PTO Math First
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Figuring out what size bush hog you need for a 40 HP tractor sounds like a five-second lookup, but the number that actually decides it isn’t the one stamped on your tractor’s hood. Getting it wrong means either a cutter that bogs down in anything thicker than pasture grass, or a gearbox that doesn’t survive the first season.
The Number on the Hood Isn’t the Number That Matters
“40 HP” almost always means engine horsepower — what the diesel produces at the flywheel before it loses anything to the transmission, hydraulic pump, and PTO drive. What actually spins the cutter is PTO horsepower, and that’s routinely 15–20% lower. A Kubota L4060, one of the more common tractors people mean when they say “40 HP compact,” is rated 42 gross / 40 net engine HP by Kubota’s own spec sheet — but only 34 PTO HP on the gear transmission, 32.5 PTO HP on the hydrostatic. A TractorByNet member sizing a cutter put the rule of thumb plainly: PTO output runs about 15% under engine HP, so a 50 HP tractor nets roughly 42.5 at the PTO. Whatever your tractor’s badge says, subtract before you shop.
Running the Math on the Real Number
Once you’ve got a real PTO figure, the standard guideline is 5–6 HP per foot of cutting width — some manufacturers and dealers lean conservative at 5, others closer to 6 depending on the cutter’s build quality. Applied to a true 40-engine-HP tractor (call it 33 PTO HP), that lands you at roughly a 5.5 to 6.5-foot cutter, not the 8-foot unit the raw “40” number might suggest.
| Engine HP | Est. PTO HP (~83%) | Recommended width (5–6 HP/ft) |
|---|---|---|
| 25 | ~21 | 3.5–4 ft |
| 30 | ~25 | 4–5 ft |
| 35 | ~29 | 5–5.5 ft |
| 40 | ~33 | 5.5–6.5 ft |
| 45 | ~37 | 6–7 ft |
| 50 | ~42 | 7–8 ft |
Terrain and vegetation still move that number around. Flat pasture grass mowed a couple of times a year lets you push toward the top of the range; steep ground or woody brush pulls you back toward the bottom.
Pull-Type vs. Lift-Type Changes the Answer
The same cutter can carry two different HP requirements depending on how it’s hooked up. Bush Hog’s 2107 model, an 8’3″ multi-spindle cutter, is rated 40 PTO HP minimum for pull-type (drawbar) use but 50 PTO HP for 3-point lift-type — because lifting the cutter’s weight on the hitch draws more from the tractor than just dragging it. If your 40 HP tractor is right at the edge on paper, a pull-type hookup buys you headroom a lift-type doesn’t.
Two Rules That Disagree With Each Other
The 5–6 HP/ft guideline isn’t the only one in circulation. Some dealers instead use a 10% rule — keep the cutter’s HP rating within about 10% of the tractor’s — which can produce a different answer than the per-foot math. There’s also a common mix-up worth clearing up before you buy: a cutter’s gearbox HP rating is usually a ceiling on what the box can absorb, not a floor you have to hit. A MyTractorForum thread on the Bush Hog 3008 makes the point directly — you can run a 40 HP-rated gearbox behind a 100 HP tractor without harm, as long as the shear bolt or slip clutch is correctly sized to protect it. Go the other direction, and it gets less forgiving: a TractorByNet member destroyed a 40 HP-rated gearbox running it behind a tractor putting out more than 40 PTO HP, and the thread’s consensus was blunt — don’t run a cutter rated at or below your tractor’s actual PTO output without the right overload protection in place.
What to Check Before You Buy a Used One
A used cutter is where a lot of these numbers get tested for real. Pull the gearbox dipstick and check the oil condition and level before anything else — a dry or metal-flaked gearbox is a bigger problem than any cosmetic issue on the deck. Check the driveline U-joints for play, and expect the blade bolts and spindle pins to be rusted solid if the cutter’s sat outside; a can of penetrating oil earns its keep here before you start swinging a breaker bar. Once it’s running, plan on greasing the U-joints and spindle bearings every 8 hours of use — a dedicated grease gun makes that a two-minute job instead of an excuse to skip it.
How I’d Choose
Two market forces are worth knowing about if you’re shopping rather than just researching. AEM’s sales data showed the under-40 HP tractor segment down 22% year-over-year as of June 2026 — the steepest drop of any horsepower class — which tracks with more owners holding onto existing tractors and buying implements to match rather than trading up. On the cutter side, ag equipment tariffs dropped from 25% to 15% in June 2026, but Farm Progress reported dealers still expect prices to climb through the year because the tariff now applies to the equipment’s full value, not just its steel content. Buying used, and buying to the size your actual PTO number supports rather than the size your ego wants, matters more this year than most.
Get your tractor’s actual PTO horsepower from the owner’s manual — not a guess, not the engine number — and work the 5–6 HP/ft math from there before you decide what size bush hog a 40 HP tractor like yours can really run.
