Inversion Problems

[SandyDSA@aol.com]

Okay - all you experts - that means YOU! - our 14 year old mare who has been 
going very well since April in 25-35 milers, even earning a BC, and finishing 
with mostly A's on her cards, no lameness, no problems - we added corn to her 
diet two months ago or so, and shortly started seeing hanging HRs and the 
last month or so, inversion, up to 20 minutes after cessation of exercise. We 
took the corn out of her dit 6 days ago, and are watching carefully. She 
NEVER had this problem previously, and while some heat and humidity certainly 
can enhance a problem, we don't know what the PROBLEM is. She is otherwise 
fine - good hydration, Cap refill, Membranes, stools, everything is good 
except hanging about 80 HR after 5-8 minutes on 10 to 20 miles, and an 
inversion factor that shouldn't be there at all!  Any ideas? Mike? Heidi? I 
plan to have blood work pulled if she doesn't start coming around within the 
next week. I just have to think she has some aversion to corn - since that is 
the only thing we changed over that period.  Any and all ideas welcome!
San(who is READY for 50 but not on a horse with a prob!)