. 2015. Effect of rumen degradable energy supplementation on forage utilization and performance of steers grazing stockpiled cool season perennial grass pastures. Can. J. Anim. Sci. 95: 255Á265. The objective of this study was to evaluate the effects of supplementing rumen degradable energy using blended by-product pellets that differed in starch and degradable fiber content on forage utilization and performance of steers grazing stockpiled cool-season perennial grass (CSPG) pastures. Forty-five yearling crossbred steers (334923.5 kg) stratified by body weight (BW) were randomly assigned to one of nine pastures (five steers/ pasture) of mature cool-season perennial grasses. Each pasture was randomly assigned (n 03) to one of three treatments: no supplement (CON); low-starch/high degradable-fibre (LSHF; 40.3% starch; 29.5% NDF) pellet and high-starch/lowdegradable fibre (HSLF; 48.6% starch; 22.8% NDF) pellet, both fed daily at 0.6% of BW (DM basis). The grazing period lasted 70 d (from early-August to mid-October). Over the 70-d study, forage CP decreased from 9.990.82 to 6.990.59% of DM and digestible energy content decreased from 2.690.04 to 2.290.04 Mcal kg(1 of DM. Treatment did not affect forage utilization (P00.50; 73.493.15%) or forage DM intake (P00.90; 9.091.54 kg d(1 ). Final BW and average daily gain (ADG) were not different (P0.05) between LSHF (43596.4 kg and 1.490.10 kg d(1 ) and HSLF (439910.4 kg and 1.590.14 kg d(1 ) treatments. However, supplemented steers had higher (PB0.05) final BW and ADG than the control steers (40295.6 kg and 1.090.09 kg d(1 ). These results show that unsupplemented beef steers grazing stockpiled CSPG pastures were limited in both energy and protein intake and that supplementation with by-product feed pellets formulated to supply protein and energy either as starch or degradable fibre equally improved animal performance.Key words: Stockpiled cool-season grass, by-products, starch, degradable fibre, grazing steers An˜ez-Osuna, F., Penner, G. B., Larson, K., Jefferson, P. G., Lardner, H. A. et McKinnon, J. J. 2015. Les effets des supple´ments d'e´nergie de´gradable dans le rumen sur l'utilisation des fourrages et la performance des baeufs broutant les paˆturages d'herbes vivaces de saison fraıˆche laisse´es sur pied. Can. J. Anim. Sci. 95: 255Á265. L'objectif de cette e´tude e´tait d'e´valuer les effets de supple´ments d'e´nergie de´gradable dans le rumen au moyen de granule´s de produits de´rive´s me´lange´s qui diffe´raient en teneur d'amidon et en fibres de´gradables sur l'utilisation des fourrages et la performance des baeufs broutant dans les paˆturages d'herbes vivaces de saison fraıˆche (CSPG -« cool-season perennial grass ») laisse´es sur pied. Quarante-cinq bovins croise´s d'un an (334923,5 kg) stratifie´s selon le poids corporel (BW Á « body weight ») ont e´te´assigne´s ale´atoirement a`1 de 9 paˆturages (5 bovins/paˆturage) de CSPG matures. Chaque paˆturage a e´te´assigne´ale´atoirement (n03) a`l'un de 3 traitements : sans supple´ment (CON) ; granule´s a`faible ten...