Vivrette et al. (2001) evaluated the effect of E+ fescue on cardiorespiratory and thermoregulatory aspects of performance horses. Fourteen quarter horses grazed either E- or E+ tall fescue pastures. Subsequently, the horses were exercised at 4 m/s (9 mi/h), a brisk trot, for 0.5 h and a distance of 7.2 km (4.5 mi) over rolling terrain. Respiratory rates were higher in horses grazing E+ pasture during the 3.5 h post-exercise period. In horses that had grazed E+ pastures, heart rates and skin temperatures remained above pre-exercise levels for 2.5 and 3.0 h, respectively. Heart rates and skin temperatures of horses from E- pastures returned to pre-exercise level 1.5 h after exercise. Water consumption following exercise for E- and E+ horses was 21.2 and 35.9 L (5.6 and 9.5 gal), respectively. These data suggest a negative effect of E+ fescue on performance when horses have been grazing E+ pasture immediately before exercise.