flyer,
The JD 310 is a good sized backhoe that will take out stumps fairly easily once you get the hang of operating it. The size of your stumps isn't going to make too big a difference, as they are all pretty similar in size and species.
I've done hundreds of stumps and even more trees with my backhoe and feel pretty comfortable with what I'm doing. If I was to estimate how long it will take, I'd put it at an hour per stump.
You need to figure in getting to each stump, positioning yourself and digging out around the stumps. Roots close to the stump will be thicker, but also held in place by the dirt around them. This makes them harder to take out. Start out a few feet away from the stump where you can break throught the roots easily. Then work your way inwards. Once you get the roots cut, dig down about four feet on both sides of the stump. Change positions and do the other two sides the same way.
There is a sweat spot where the backhoe has more leverage then other spots. Too close or too far away and you don't have as much power and leverage. Get in the right spot, and the stump pops right out.
What are your plans for the area you are removing the stump? If it's just pasture, then you can turn the stump sideways and bury it. People will tell you that it will rot and the ground will settle, but I'm not so sure about this. I think 90% of the ground settling is from the ground not being compacted when it's buried. Over time, the dirt settles and you get a low spot. The stump isn't roting, it's just the soil filling in the voides from not being compacted.
There was a show on TV about landfills. A University in Southern California drilled down into the landfill to where it was in the early 1970's and was able to pull out newspapers when Nixon was peresident that they could still read. Over thirty years in the ground, and newspaper didn't decompose.
This was something they didn't expect, but started doing research on and found that if you bury something and it's not exposed to the elements, it will not decompose.
I've dug up lumber that has been buried for over a decade that was still in pretty decent shape, so I believe this to be true. I wouldn't build a house or barn over buried stumps, but I have allot of them in my fields without any depresions forming yet. I compacted the fill really good, so I don't expect anything to happen either.
If you feel you have to remove the stumps out of the hole, this will add to your time. If you are going to put them into a burn pile, it's faster to carry a stump then it is to push it. If you cannot get the stump into the bucke, a chain works good for this, but it takes some practice to get it to stay on your bucket.
I'd rent the backhoe for a full week if I was gonna rent a machine to have it done. My backhoe uses about half a tank of fuel a day. Figure 12 to 14 gallons a day of fuel, plus your rental and other fees for a week. Take that number and see if you can hire it out for that amount.
This isn't a fun job. After your first couple of stumps, it becomes work.
If you are thinking about a trackhoe, remember that it takes a big machine to a faster job then the backhoe. Compare hp to each. The JD 310 is around 75 to 80 hp. Don't rent a smaller trackhoe and think it will do the job faster because it's a trackhoe. It is better then a bachoe for digging, but useless for moving stumps around. It's also useless for moving dirt if you want to fill those holes again.
A big trackhoe with a thumb will pluck the stumps out in a few minutes. I don't think you can rent one big enough to do this, but if you can hire it out, it should be a day job for the right machine. My neighbor has a guy with a Cat 315 on his land and it's too small to pluck out stumps. He has to dig them out just like I do with the backhoe. He has more power and can dig them out faster, but then they use a backho to fill in the holes and carry the stumps to the burn pile.
Good luck,
Eddie
