100 mile completion rates

Every so often on the 'net, someone asks why 100 mile rides have completion rates that are so much lower than 50's.

I think in general it's common sense .  I do think the specific factors are different for each horse/rider team.  I've listed my top 3 reasons my completion rates for 100's are lower than 50's (specifically, with Farley), and I think it might actually be interesting to hear from you folks of what you think is YOUR reason you don't complete the same amount of 100's as 50's.  Or, the same amount of 50's as LD's. 

With Farley I have 3/5 100 completions, and 100% 50 mile (with a 65 and 55's and multi days thrown in) completions.

Here's the top 3 reasons I think my 100 mile completion rate is worse....

1.  Sheer distance.  At each 100 that I was pulled in, I finished more than 50 miles (65 miles at one, 92 at the other), thus if they had been standard 50 mile endurance rides, I would have finished.

2.  I'm more likely to take a horse to a 100 that doesn't work out perfectly weather or timing wise, because there's not as many of them.  For example, a 100 in the middle of July/beginning of August in California?  Insane!!!!!!  Riding 100 miles in the rain because your next chance isn't until July?  Insane!!!!!!!  In both of those cases, I probably wouldn't have done the ride if it was a 50 - there's too many of them to risk my horse in crappy weather or footing.  But a 100.....I have maybe 3 chances a year to ride a 100, IF my horse doesn't manage to kill herself before the next one. (But in my new philosophy of "I want to ride endurance until I'm 80", I would be more likely to give the ride - even a 100 - a pass, as a "fair weather rider").

3.  I think the 2 factors above are my major ones.  If I had to put a third, I would probably cite fatigue on the riders (my) part.  I've been having a tough time physically at the end of 100's, which just doesn't happen at a 50. There's not a good way of training for that, unless perhaps I was a medical resident in the ER, military stuff or something (hey!!!  maybe vet school will be excellent 100's trainig!!!  GREAT - $200K for 100's training!) like that.  I think that definitely takes the toll on the horse, whether it's how I'm riding, that fact I'm NOT getting off to run, or the things that I forgot to do for my horse during the last hold.  (one reason a crew is super importatnt for me at a 100).  I continue to figure out better ways to care for myself, just as I continue to figure out better ways to manage my horse, so I expect this to be less and less of a negative factor for my 100 mile rides. 

I know that other factors such as can impact 100 mile completions -  your tack has to fit even better, little things add up after 100 miles that were never a concern at 50, etc.  What's YOUR reason?

Update - for those of you not following me in Twitter, Thehorse.com has an article on endurance horse pulls at rides here.  It's VERY general, but could be interesting to keep an eye on.