. 1992. Effects of time and level of energy supplementation on intake and digestibility of low-quality tall fescue hay by sheep Can. J. Anim. . Eff'ects of time and level of energy supplementation on forage intake, nutrient digestibility and ruminal pH were investigated with sheep fed low-quality tall fescue hay either alone (control; C) or supplemented (S) with concentrate at 10 (low; L) or 20% (high; H) of ad libitum forage intake; supplement was offered at 08:00 or l5:00 h. Supplementation increased (P < 0.05) digestible dry matter intake (DMI), total tract organic matter digestibility, N intake, and N digestibility, but decreased (P < 0.05) mean and minimum ruminal pH compared with C. Offering supplement at 08:00 h increased (P < 0.05) forage and total DMI, N intake, fecal N excretion and maximum ruminal pH, but lowered (P < 0.05) total tract digestibility of acid detergent fibre (ADF) and organic matter, and mean and minimum ruminal pH compared with 15:00 h. A high level of supplementation increased (P < 0.10) digestible DMI, postruminal digestion of neutral detergent fibre (NDF), and hemicellulose (HC), but lowered (P < 0.0I) ruminal digestion of HC and NDF (P < 0.10), mean ruminal pH (P < 0.i0) and total tract ADF digestion (P < 0.05) compared with L. These data suggest that the substitution effects of concentrate f'eeding on forage intake can be mitigated by changing the time of day when supplement is off'ered in relation to forage. 15 h. La compl6mentation a accru (P < 0,05) la prise de matidre sdche digestible, la digestibilit6 globale de la matidre organique, I'ingestion et la digestibilit6 de N, mais elle a abaiss6 (P < 0,05) le pH moyen et le pH minimum du rumen par comparaison avec le r6gime t6moin. La distribution du compl6ment ir 8 h plut6t qu'ir 15 h a accru (P < 0,05) l'ingestion de foin et la consommation totale, l'ingestion de N, I'excr6tion f6cale de N et le pH maximum du rumen. En revanche, elle abaissait (P < 0,05) la digestibilit6 globale de la lignocellulose (ADF) et de la m.o., ainsi que le pH moyen et le pH minimum du rumen. Au niveau sup6rieur de compl6mentation, on a observd une augmentation (P < 0,10) de f ingestion de matibre sbche digestible et de la digestibilit6 post-ruminale de la NDF et de I'h6micellulose, en mOme temps qu'une diminution (P < 0,10 et P < 0,01) de la digestibilit6 ruminale respective de ces deux groupes de fibres et une baisse du pH moyen du rumen (P < 0,10) et de la digestibilit6 globale de I'ADF (P < 0,05), par rapport au niveau bas de rThis paper