Four horses were exercised for 2 h on a treadmill at up to 200 m/min 4 h after feeding. The trial included 3 treatments, control (A) and electrolyte supplementation provided either in the diet at the morning feeding (B) or 1 h before start of exercise (C). The horses were fed 2 meals/day of hay and concentrate (6.5 g dry matter, 13.8 mg Na, 66.6 mg K and 54.2 mg Cl/kg bwt per meal). The supplement was a mixture of grass meal (47.6%), sugar beet syrup (33.3%) and NaCl (19.1%); additional intake: Na 79.7, K 15.5 and Cl 165.6 mg/kg bwt. Hourly urine and blood samples were taken.
Water intake and renal water excretion were significantly influenced by supplementary feeding (intake: A 35.5, B 49.9, C 55.6 ml/kg bwt/12 h; excretion: A 9.47, B 15.34, C 7.92 ml/kg bwt/12 h). The urinary Na and Cl excretion was significantly higher in treatment B (Na 24, Cl 55 mg/kg bwt/12 h) and C (Na 12, Cl 44 mg/kg bwt/12 h) as compared to A (Na 5, Cl 16.5 mg/kg bwt/12 h). Renal K output was less influenced by the different treatments, A43, B and C 61 mg/kg bwt/12 h. However, the renal Na and Cl excretion was lower in relation to intake when horses received the supplement. In these cases, losses of water, Na and Cl via sweat were compensated for at the end of exercise and a positive balance therefore maintained 12 h after feeding. The total plasma protein concentration increased during exercise, indicating a reduction in plasma volume. The Na and K concentrations in plasma during exercise showed minor changes whereas the Cl concentrations decreased significantly by 2.6 (A), 4.0 (B) and 1.4 (C) mmol/l. Changes in plasma Na and Cl in treatments B and C occurred at constant higher levels than in treatment A.
The results suggest a positive impact of NaCl suplementation on water and electrolyte metabolism in exercised horses. The increased water intake indicates that supplementary salt intake is more effective about 4 h than 1 h before exercise.