Step-by-step guide to mowing tall grass and light brush with a rotary cutter. PTO speed, ground speed, deck height, and safety rules for compact tractor owners.
A rotary cutter (commonly called a bush hog after the original brand) is the right tool when the grass gets too tall for a finish mower. It chops weeds, brush, saplings up to about 2 inches, and tall grass into mulch. Done wrong, it throws rocks, scalps the ground, and stalls your tractor. Done right, it clears an acre an hour without breaking a sweat.
Walk the field. Pick up rocks, scrap metal, glass bottles, and anything bigger than a golf ball. A rotary cutter blade tip moves at over 17,000 feet per minute. A baseball-sized rock becomes a bullet. Mark stumps, t-posts, and irrigation valves with bright flags so you can see them from the seat.
For tall grass and brush, set the rear of the deck 4 to 6 inches off the ground and the front 1 inch lower than the rear. The forward tilt keeps the leading edge cutting cleanly and reduces the chance the cutter rides up and over thick patches. For a finish pass, lower to 3 inches with the deck level.
Engage the PTO at idle, then ramp engine RPM up to PTO speed (usually 540 RPM, marked on the tach). Pick a low gear. For thick brush, ground speed should be 1.5 to 2.5 mph. For tall grass, 3 to 4 mph. If the tractor RPM drops more than 100, you are going too fast. Ease off, let the PTO recover, and re-engage at a lower ground speed.
For very tall grass over 3 ft, mow the field twice. First pass at deck-high to knock the tops down, second pass at finish height to mulch the litter. Overlap each pass by 6 to 8 inches. Mow in a spiral from the outside in for fields, or in long parallel passes for paddocks. Always mow uphill on slopes greater than 10 degrees, never across.
The cutter must be sized to your tractor's PTO HP and lift capacity. A 5 ft cutter needs about 18 to 25 PTO HP. A 6 ft cutter needs 25 to 35 HP. A 7 ft cutter needs 40+ HP. Run a candidate cutter through our rotary cutter compatibility check to see which models fit your specific tractor.
Never operate without the chain guards intact. Never let anyone within 100 ft, especially children. Disengage the PTO and shut off the tractor before clearing a jam. Wear hearing protection (a rotary cutter at 540 RPM is 95+ dB at the seat). And lower the cutter to the ground or onto its parking stand before stepping off, never leave it in the air on the 3-point.
A standard 5 foot rotary cutter on a 25 to 35 HP tractor can handle grass up to roughly 4 feet tall in one pass if you slow down and raise the deck. For grass over 4 feet or woody brush, mow at maximum cut height first, then drop down for a second finish pass.
Start at 2 to 3 mph in tall or wet grass. Listen to the engine, if RPM drops, slow down further. Maintaining full PTO RPM (540) is more important than ground speed for a clean cut. Forcing speed in heavy material causes ragged cuts, blade damage, and tractor overheating.
Rotary cutter blades are designed to impact-cut, not slice, they don't need a razor edge. Replace blades when the cutting edge is rounded, chipped, or unbalanced. Always replace blades in pairs to keep the rotor balanced. Run a blade-balance check after replacement to avoid gearbox vibration damage.
Hearing protection, safety glasses, sturdy boots, long pants. Walk the area first to remove rocks, wire, and debris that the blades can throw. Keep bystanders at least 300 feet away on the sides and rear. Always disengage the PTO before dismounting and never reach under the deck while the rotor is spinning.
Published 2026-04-08, last updated 2026-07-18. Every figure quoted in this guide is taken from manufacturer documentation and linked in the text, and the tractors and attachments it references are records in the FitTractor compatibility database.