Don,
over time green wood moves from wet,flexible, supple to stable, stiff and eventually brittle. As Tim and Craig say, keeping a cut pole for 5 yrs of use becomes a judgement call at each time of use for present conditions.
There was an excellent speaker at the TFG conf in NH, a man from eastern Europe who was/is a professor at a Univ in the USA. His expertise was wood cell structure, growth, fiber characteristics, all translated to practical use, strength, function etc. I would attend a presentation by him again. Did you ever have a science teacher who made it real interesting and understandable, and you didn't want class to end yet? He was like that. He enjoyed sharing his knoeledge too.

Steve,

Even though I appreciate you wanting permission to copy the bullhook, I don't own rights to the design...so go ahead!

Besides there would be some old time loggers that would be turning over in their graves or haunting me in the woods if I tried to take any "credit" for my design!

Some guys use heavy pipe instead of solid rod. I prefer rod, because I have had tools made out of pipe where the pipe wall has torn out on me and the tool needed to be fixed or replace.
Also, the hooks are ones that attach to the ends of log-chains. I just pulled the pins on them, ground down each end or the rod so that it would slide up into the bottom "fork" of the hook, which give more surfaces to weld together.

For fun I'll have to post a pic of a "real" bullhook...you know, for hooking the ring on a bull, from outside the boxstall or over the fence?...anybody what to fetch a bull without one? haha

Rooster

Steve, help yourself, if you blame me I'm going to deny it
Don't know if it'd be of any help but this gets into some of that.
http://windyhilllogworks.com/Calcs/linepullclc.htm

Yes on the teachers, most have been since school and I enjoy their classes.
I checked the chart I posted above from the Army Rigging Manual. It appears to check for me using some pretty nice spruce poles just about vertically. The table says they can be tipped down to 45 degrees at the stated load. I'll be behind the truck, unless I'm seeing it wrong they seem to be more optimistic than I'm comfortable with.