Effect of various levels of dietary starch on glycogen replenishment in the light working horse

Nine Quarter Horses were used in a 3 × 3 Latin square with replication to determine the effect of various levels of dietary starch on glycogen replenishment in the light working horse. Horses were fed Coastal bermudagrass hay at 1% BW/d with remaining calories met by a high (HS), medium (MDS), or low starch (LS) concentrate. Horses were transitioned to 1 of 3 diets over 7 d for a 14-d treatment period where they were then worked to fatigue in a standardized exercise test. Total diets provided an average of 1,206.67, 844.61, and 263.13 g of starch/d in HS, MDS, and LS, respectively. Horses were lightly exercised for 30 min 3 d/wk. Skeletal muscle biopsies were taken from the biceps femoris at rest, immediately after the standardized exercise test, and 24 and 48 h after exercise. Venous blood samples were taken at rest, immediately after exercise, 10 min after recovery, and 24 h after exercise. There was a greater resting muscle glycogen concentration (P = 0.009) when comparing HS with MDS (10.25 vs. 8.28 μg/mg wet weight). There was a greater concentration of glycogen 24 h after exercise (P = 0.04) when comparing LS with HS (9.52 vs. 7.68 μg/mg wet weight). High starch used more glycogen than MDS or LS. A slight reduction in glycogen after exercise for MDS and LS indicated that fat or protein may have been used as substrate for exercise. Results indicated that feeding 1,206.67 g of starch/d did not yield an advantage in recovery time over a MDS or LS diet.