Effect of dietary corn silage inclusion on visceral organ mass, cellularity, and the protein expression of ATP synthase, Na / K -ATPase, proliferating cell nuclear antigen and ubiquitin in feedlot steers. Can. J. Anim. Sci. 89: 503Á512. Twenty-four steers (initial body weight 053595.0 kg) predominately of Angus breeding were used to determine the effect of dietary corn silage inclusion [20, 40, 60, or 80% of dry matter (DM)] on visceral mass, cellularity, and the protein expression of ATP synthase, Na ' /K ' -ATPase, proliferating cell nuclear antigen (PCNA) and ubiquitin. Steers were fed at similar energy levels (2.1 )NE m requirement). There were no significant treatment effects on specific visceral organ weights. Hepatic Na ' /K ' -ATPase expression linearly increased (P00.01) and ruminal Na ' /K ' -ATPase expression linearly decreased (P 00.01) with increasing corn silage inclusion. Hepatic PCNA expression was quadratically affected (P00.05) with a decrease when corn silage inclusion increased from 20 to 60%, and an increase when corn silage inclusion increased from 60 to 80%. Renal ATP synthase (P00.02) and ubiquitin expression (P00.01) were quadratically affected in a similar pattern with an increase when corn silage inclusion increased from 20 to 60%, and a decrease when corn silage inclusion increased from 60 to 80%. These results indicate that different dietary corn silage inclusions, at similar dietary energy intake, may alter rumen, liver, and kidney energy expenditure, at least in part, through changes in specific metabolism rather than mass. Incidence de l'inclusion d'ensilage de maı¨s sur la masse des visce`res, la cellularite´ainsi que l'expression de l'ATP synthe´tase, de la Na /K -ATPase, du PCNA et de l'ubiquitine dans les prote´ines chez les bouvillons d'engrais. Can. J. Anim. Sci. 89: 503Á512. Vingt-quatre bouvillons (poids corporel initial 0 535 9 5,0 kg) essentiellement de race Angus ont servi ap re´ciser les effets de l'inclusion de maı¨s d'ensilage dans les aliments (20, 40, 60 ou 80 % de la matie`re se`che) sur la masse des visce`res, sur la cellularite´, et sur l'expression de l'ATP synthe´tase, de la Na ' /K ' -ATPase, du PCNA (proliferating cell nuclear antigen) et de l'ubiquitine dans les prote´ines. Les bouvillons ont rec¸u la meˆme quantite´d'e´nergie (2,1 )NE m ). Le traitement n'a eu aucun effet significatif sur le poids des visce`res. L'expression de la Na ' /K ' -ATPase augmente line´airement dans le foie (P 0 0,01) et diminue line´airement dans le rumen (P 0 0,01) avec la hausse de la concentration de maı¨s d'ensilage. L'expression du PCNA dans le foie diminue de manie`re quadratique (P 0 0,05) quand la proportion d'ensilage de maı¨s passe de 20 % a`60 %, mais elle augmente lorsque la quantite´de maı¨s passe de 60 % a`80 %. L'expression de l'ATP synthe´tase (P 0 0,02) et de l'ubiquitine (P 0 0,01) dans les reins augmente elle aussi de manie`re quadratique quand la proportion de maı¨s d'ensilage passe de 20 % a`60 %, tandis qu'elle diminue quand la quantite´de maı¨s passe de 60...