heat fatigue and dark urine

[Steph Teeter <step@fsr.com>]

p.s. 

Any of the vets or experts out there want to comment on what effect over-heating
has on the horse's metabolism? We pulled our older Orlov (my husband was riding
him) at 40 miles. He pulsed down ok at the last check, but was very sluggish
at the trot out and just looked plain tired. Didn't seem worth it to subject him to
another 10 miles in the heat, but more important in our decision to pull him was
that he peed a small amount of very dark urine. The reduced volume and the
dark color looked like trouble.

I later talked to several others who saw the same thing in their horses - no obvious
muscle problems, just fatigue and dark urine... so what's going on to cause
the dark urine? Must be some muscle tissue breakdown somewhere...

comments??

Steph