"Glucose supplementation may be important in limiting fatigue in equine
endurance exercise. Endurance time during treadmill running was prolonged by
the IV infusion of glucose solution during exercise. Plasma glucose was
higher during exercise in the horses receiving glucose, and plasma lactate
and body core temperatures were lower at fatigue. Plasma volume decreased
more slowly in treated horses. These results suggest that supplemental
glucose during exercise prolongs performance time in horses. This increase
may be due to increased glucose availability, reduced reliance on anaerobic
energy production, lower core temperature, and better maintenance of plasma
volume.
Glycogen concentration in skeletal muscle prior to performance is relevant
to fatigue during both short-term/intense and prolonged exercise. Depletion
of muscle glycogen causes a decrease in anaerobic power generation and
capacity for high-intensity exercise. Horses should not be depleted of
glycogen before short-term or endurance events. Intense or prolonged
exercise depletes the muscle glycogen stores, and it is sensible to allow at
least 48 hr for adequate postexercise glycogen resynthesis in horses. No
method of glycogen loading using adjustments to normal feeding has been
described in horses."  (Merck Vetrinary Manual)