In a replicated 4 × 4 Latin square experiment conducted in summer, eight mature Thoroughbred horses received two diets, control and fat-supplemented, and were exercised at two fitness levels designed to increase daily digestible energy (DE) requirements to approximately 150% (fitness level I) and 200% (fitness level II) of maintenance. In a second experiment during the winter, horses received the two diets in a switchback design and exercised at fitness level II. After 3 wk adaptation to treatments, feed and fecal samples were collected and horses galloped a standardized exercise test (SET) designed to increase the heart rate above 185 bpm for 1200 m. Vital signs were monitored and blood samples were obtained. Feed intake increased as fitness level increased (P < .05). Horses on the fat-supplemented diet required less feed (P < .05) to meet the energy requirements at a given fitness level. Daily DE intake was higher (P < .05) for the horses exercised at fitness level II to meet the increased metabolic demands. Heart rate, respiration rate and rectal temperature all increased (P < .05) with exercise. No treatment effects were found for heart rate (P >. 10) or rectal temperature (P > .30). Horses exercised in the winter had lower (P < .05) respiration rates, indicative of less problems dissipating excess body heat. Plasma aldosterone concentrations increased (P < .05) with exercise, corresponding to an increase in plasma K concentrations. Horses exercised in the summer exhibited higher (P < .05) plasma aldosterone concentrations than horses worked in the winter. Plasma CI and Na concentrations did not change (P > .30) with any treatment or exercise, indicating that the horses were not sufficiently stressed to induce any significant dehydration.
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