I have an Agco st30x that has about 23 PTO horse power, I would like to mow some very tall 7' + cattails. should I buy a 4 foot or 5 foot rotary cutter ?
I ran a 5 footer behind 21.5 PTO hp, and found it lacking. A four footer would have been more efficient. Not sure if your extra horse and a half would have been sufficient. But - surely you're going to use it for more than just cattails(?) Before deciding, consider what ELSE you'll be using it for once the rushes are chopped up.
I run a 5' using 24.5 pto hp. without any problems. It is a light duty cutter rated for 1.5" diameter or less. I would match it to the width of the rear track of your tractor.
I use a 5 foot Rhino behind my 23.5 PTO hp Kubota L2800 HST and it does fine. In my particular case, power for the 5 foot cutter isn't an issue as much as the weight of a "heavy duty" model might be.