This is an older thread but I thought I would reply because: (1) I have a 2910 like Bill; (2) I have spent the plurality of my tractor time brush cuttering (everything from brush, to trees, to 10' tall grasses and cattails, to rocks, to buried iron fence posts, to rocks, to lawn, to air, to rocks, to mud, to water, and to rocks); (3) I bought the perfect size and weight cutter for the 2910; (4) I have the precise answer to Bill's primary question, which answer is 39.99%; and (5) I regret that no one has mentioned the potential connection between brush cutting and the Clancy Brothers.
Sure, you must consider the width of the cutter, the weight, and what you are cutting. The general rule is simple: Buy the biggest and strongest and heaviest cutter your tractor will handle. (That's the rule for all implements, which are the things that do ALL the useful work. The tractor itself does nothing except pull SOMETHING ELSE, pump oil to SOMETHING ELSE and spin pto's to power SOMETHING ELSE. Why bother selecting the "perfect" tractor and then buying cheap or small or light SOMETHING ELSES, the things that accomplish the tasks at hand.)
For the 2910, a four foot cutter is too small and a six foot too big. The five foot is ideal. Herewith, my CUT cutter width rule: a cutter should be the same width as your rear tires. Any smaller and you are wasting the size of your tractor. Any bigger and you won't be able to pull your cutter through openings (eg, trees) that your tractor can get through.
That leaves weight, which is a function of the strength and durability of the cutter. Herewith, my CUT cutter weight rule: the maximum weight for a cutter is 39.99% of the base weight of the tractor including (unloaded) tires. Buy the heaviest cutter you can handle under this rule and you will, ipso facto, have bought the strongest, most powerful and most durable cutter you can handle. Don't worry about pto horsepower. So you bog down occasionally when cutting King Kong grass or sequoia gigantica. Any cutter will bog down when the underdeck is completely filled up with vegetable or mineral matter, no matter the pto or engine horsepower. You just lift up, jiggle, empty, and go on with with your cutting. This is FUN, FUN, FUN. Once your jungle has been tamed, regular maintenance will be much easier--and you will then become a candidate for clinical depression because you have nothing left to annihilate and vaporize. (At this point you must give up all drugs--but not sex or rock & roll--else you may find yourself shredding your neighbor's rose garden at 2:00 am while belting out Clancy Brothers drinking tunes.)
My perfect cutter satisfied these Rules of CutCutterdom. It is a Woods MD-160, which was the 5' medium duty model in Woods' prior line of rotary cutters. It weighs 685 lbs. Since I have Freedom Hitches (another no-brainer and an Intergalactic Pleasurable Tractoring Requirement), which themselves weigh over 100 lbs. and which extend implements further back from the tractor, the effective weight of the cutter on the 2910 is greater than would be the cutter alone on the 3ph.
The 2910's 3ph can easily lift this cutter but the front is too light without the FEL on. If I had foam fill in my front tires, that would be the perfect front counterbalance to mow grasses without the FEL. (When cutting brush or backing over my steep creek banks, however, I always want my FEL on.)
The "medium duty" 5' cutter of Woods' new Brushbull line at 1039 lbs. would be too heavy for a 2910. The Woods "standard duty" has been beefed up, and the 5' Brushbull 60 at 554 lbs would be the best Brushbull choice. The Bushhog 5' medium duty Model 285 may be just too heavy at 794 lbs.
That's my tune and I'm sticking to it. Until I change my mind.
