Endurance Exercise

Lots of great stuff in this .pdf If you are an endurance rider you should check this out! 

When I started Chief in endurance I wanted to see how he would do if I didn't electrolyte supplement him.  He has now done more than 5700 miles this way.  I think that all of my horses eat and drink better now that I do it this way.  They do get salt and/or elyte mix put in their food, especially before traveling and post ride.

Exerpts: 

Conclusion
Other than enhancing water intake, supplementation of endurance horses with high doses of NaCl and KCl did not provide any detectable competitive advantage. Furthermore, the elevated serum electrolyte concentrations induced with HD might not be appropriate for endurance horses.

and

Results
Before 2002, on each international event, at least one horse and most often several horses presented anorexia or stopped voluntary drinking during the ride. In the absence of electrolyte supplementation, no horse stopped drinking or eating; consequently, the horses were considered easier to manage by the riders. The frequency of elimination or withdrawals for metabolic troubles was significantly reduced (P < 0.05) since 2002. There was no statistical difference in the performance of the supplemented and non-supplemented horses. From 2002 to 2004, French team horses contributed to get 4 medals without any electrolyte supplementation.
Conclusion
Electrolyte supplementation in well-conditioned horses is not essential during long distance endurance rides.