Introduction
Sweating is the major pathway for heat dissipation during exercise in both man and horses. However, equine sweat is hypertonic (McCutcheon et al. 1995) whereas the human sweat is hypotonic. Horses therefore lose large amounts of sodium (Na) during exercise. Since horse feed is poor in Na the Na-balance is constantly challenged in the athletic horse. Accordingly, recovery from sweat losses in the horse can only be achieved by Na supplementation. The Na appetite is well documented in many herbivores but to our knowledge there are no such studies in the exercising horse. We have investigated how different strategies of supplying NaCl to horses affect their Na balance and recovery from exercise-induced sweat losses.
Methods
Three experiments were performed (I, II and III). In I voluntary Na intake from a salt block was investigated for 50 days in Standardbred horses in training. In II voluntary intake of saline (0.9 % NaCl) was studied in endurance horses during a 62 km ride. In study III saline (10 L) was administered orally to Standardbred horses 1 h before they performed a simulated trotting race on a treadmill. Total daily collections of urine and/or faeces were made in studies I and III. Body fluid restoration was monitored by measuring either body weight (BW) and/or alterations in total plasma protein concentration and hematocrit.
Results
In I daily voluntary Na intake from the salt blocks were in the range of 0-62 mg/kg BW and in four of six horses it was less than 20 mg/kg BW. The horses with the lowest Na intakes had an increased plasma aldosterone concentration, a comparatively low faecal Na concentration and a high potassium concentration. They also showed signs of a reduced plasma volume. In II voluntary intake of saline was 18 ± 1 L during the ride and until 1 h post exercise compared to 14 ± 1 L in horses drinking only water. The following day horses drinking water still had a BW deficit of 1.4 % whereas the saline-drinking horses' deficit was negligible. In III horses performing a simulated race without saline the BW loss was 12 ± 2 kg and signs of dehydration was seen already during the exercise test. The urinary and faecal excretion of Na was low for at least 48 h post exercise, and the faecal potassium excretion was increased. In horses receiving saline prior to exercise the exercise-induced dehydration was mild and Na was excreted in amounts similar to those observed during days with out exercise.
Conclusions
The strategy used for supplying Na to exercising horses was critical for maintaining Na balance. Horses offered Na from a salt block may not even cover their maintenance requirement of Na (20 mg/kg BW). A voluntary intake of Na from a salt block is therefore not be recommended in horses that need to recover from frequent sweat losses. The voluntary intake of saline was enough to cover for the calculated loss of Na (60 g) during the ride. Saline seems therefore to be a good strategy, since it was absorbed during exercise and the horses had a rapid recovery of their body BW.
References
McCutcheon, L.J., R.J., Geor, M.J., Hare, J.K. Kingston & Staempfli H.R. 1995. Equine vet. J., Suppl. 18, 279-284.