Traditionally, performance horses are often fed high quantities of cereal grain-containing feeds to sustain their high-energy needs, and consequently, the forage portion of the diet is reduced. Unfortunately, such feeding practices are associated with adverse health effects, and thus, some investigations into alternative feedstuffs for performance horses have been undertaken. This article reviews the ability of high-energy fibrous feeds, such as haylage, sugar beet pulp, and soyhulls, to meet the energy requirements of performance horses, such as racing Thoroughbreds and Standardbred horses. Furthermore, it explores the effects of these feedstuffs on bodyweight and other parameters generally associated with performance including glycogen utilisation, glycogen replenishment, and muscle lactate clearance. The literature suggests that high-energy fibrous feeds may have digestible energy contents comparable to grain as a consequence of enhanced volatile fatty acid production. The inclusion of high-energy fibrous feeds in performance horse diets does not appear to adversely affect glycogen utilization or muscle lactate clearance during intense exercise, and an alkalizing effect of a haylage-only diet may offset acidosis induced by intense exercise. Increases in bodyweight have been observed with haylage-only diets, but to a lesser extent than those observed for hay, and this has been attributed to the water holding capacity of the fibers. Although considerable research is required to fully understand the effects of high-energy fibrous feeds in the diets of performance horses and its ability to sustain their nutrient requirements, the literature currently supports the replacement of at least a portion the grain fraction of the daily feed ration with high-energy fibrous feed products. The implementation of such a change would likely improve the welfare of the performance horse.