Nitrogen metabolism in mature horses at varying levels of work.

Nitrogen metabolism was measured in five mature geldings performing varying levels of work that simulated race training. Following an initial maintenance period without forced exercise, workload was increased in succeeding 18-d periods by doubling the distance the horses were galloped in each period from period 2 through 4. A 4-d N balance experiment was conducted at the end of each 18-d period. The maximum distance galloped daily, which was in period 4, was repeated in period 5. Then, workloads were decreased through period 7. Nitrogen balance was determined in two additional periods, 8 and 9, while the geldings were at maintenance with no forced exercise. Geldings were fed diets containing similar protein-to-calorie ratios in all periods. Nitrogen retention increased (P less than .05) as workload increased from period 1 to period 3 and remained elevated throughout the rest of the experiment. Nitrogen retention did not (P greater than .05) decrease from highest values, observed at maximum workload during periods 4 and 5, until period 9, which was 34 d postexercise during a maintenance period. Nitrogen retention was higher (P less than .05) following training than before training.