In order to compare the metabolic responses to exercise in 2 similarly managed breeds of horses, 5 Arabian (AR) and 5 Thoroughbred (TB) horses, fed an identical diet with a similar diet and exercise training programme for the 2 preceding months, undertook 3 treadmill (3° incline) trials with a minimum of 7 days between tests: 1) an incremental test (MAX) for determination of aerobic capacity, VLA4 and lactate threshold (LT; the percentage of V̇O2max when plasma lactate = 4 mmol/1); 2) a single high-speed exercise test (SPR) at 115% V̇O2max for estimation of maximal accumulated oxygen deficit (MAOD) and 3) a 90 min test at 35% V̇O2max (LO).
V̇O2max (P<0.001) and running speed (P<0.05) at VO2max were higher in TB (mean ± s.e. 154 ± 3 ml/kg/min at 12.9 ± 0.5 m/s) than in AR (129 ± 2.5 ml/kg/min at 11.8 ± 0.2 m/s). Total run time during MAX was greater (P<0.05) in TB (10.5 ± 0.5 min) than in AR (9.3 ± 0.3 min). However, VLA4 and LT were not different between groups. Run time during SPR (TB 149 ± 16; AR 109 ± 11 s) and MAOD (TB 88 ± 4; AR 70 ± 6 ml O2/kg) were higher (P<0.05) in the TB group. During LO, FFA were higher (P<0.05) and the respiratory exchange ratio (RER) lower (P<0.05) in AR than in TB between 60 and 90 min, of exercise, indicating a greater use of fat for energy. These metabolic differences may reflect breed variation in muscle fibre types. However, further studies are needed to determine the mechanisms underlying the apparent breed differences in energy metabolism during exercise.