Effects of Restricted Hay Intake on Body Weight and Metabolic Responses to High-intensity Exercise in Thoroughbred Horses

The purpose of the present study was to determine the effects of restricted hay intake on the metabolic responses of horses to high-intensity exercise. Four conditioned Thoroughbred horses were studied in a 2 x 2 crossover design. Initially, the length of time required for adaptation to ad libitum (AL) intake of grass hay was determined. Thereafter, the metabolic responses to sprint exercise (SPR) were examined in 2 dietary periods, each 5-days in duration: 1) AL, where horses had free choice access to hay; and 2) Restricted (RES), where hay intake was restricted (~1% of bwt) for 3 days before the exercise test. For SPR, feed and water were removed 4 h before exercise. After measurement of bwt, horses completed a warm up followed by 2 min at 115% of maximum oxygen uptake, then a 10-min walking recovery (REC). During the 3 days before SPR, hay intake in AL averaged (± SE) 10.1 ± 0.9 kg, whereas intake during RES was 4.3 ± 0.2 kg. Pre-exercise bwt was significantly lower in RES (528 ± 5 kg) than in AL (539 ± 4 kg). During SPR, total mass-specific VO2 was higher (P=0.02) in RES (243 ± 8 ml/kg/2 min) than in AL (233 ± 10 ml/kg/2 min). Conversely, accumulated oxygen deficit was higher (P<0.01) in AL (89.4 ± 2.2 ml O2/kg) than in RES (82.4 ± 1.7 ml O2/kg). Peak plasma lactate was also higher in AL (22.2 ± 1.2 mM) than in RES (19.1 ± 2.1 mM), and VO2 during REC was 10% higher (P=0.12) in AL. In conclusion, compared to ad libitum feeding, 3 days of restricted hay intake reduced bwt by 2%. This reduction in body weight was associated with an increase in the mass-specific rate of oxygen consumption during sprint exercise, with a corresponding decrease in anaerobic energy expenditure.