Quantification of energy expenditures of the gastrointestinal tract of steers fed three diets at two levels of intake
Forty eight yearling Holstein steers [257 ± 7.7 kg body weight (BW)] were fed switchgrass hay plus 10% soybean meal (diet S), orchard grass silage (diet O) and alfalfa silage (diet A) at 65 or 90 g DM kg–0.75 BW for 155 to 164 d in a 2 × 3 factorial experiment designed to evaluate the effects of dietary composition and level of feed intake on patterns of O2 consumption in gastrointestinal tract (GIT) tissues. At the end of the growth period, steers were stunned, exsanguinated and eviscerated. The various GIT components were then emptied of their contents and weighed. Ventral sac rumen, jejunal and large intestinal tissue samples were quickly acquired and placed in oxygenated M199 media. Mucosa and muscularis weights of these tissues were determined along with their total O2(TO2), ouabain-sensitive O2 (OSO2) and ouabain-insensitive O2 (OIO2) consumption. Oxygen consumption parameters were determined polarographically using a YSI Clark-style electrode. Total weights of rumen (P < 0.001), small intestine (P < 0.001) and large intestine (P < 0.05) were higher in steers fed the high level of intake compared to those fed the low level of feed intake. In all GIT tissues studied, increasing the level of feed intake (P <0.05) elevated mucosa and muscularis dry weights. Steers fed diet S had lower (P < 0.05) GIT tissue weights compared to those fed diets O and A. Weight-specific (i.e., expressed per unit tissue weight) TO2, OSO2 and OIO2 consumption for rumen, small intestine and large intestine were not affected by dietary composition and level of feed intake (P > 0.05), except for OSO2 consumption, which was higher (P < 0.05) in ruminal mucosa of steers fed diets S and A compared to those fed diet O. It is concluded that level of feed intake and dietary composition altered GIT O2 consumption via changes in visceral organ mass, rather than changes in weight-specific O2 consumption. Key words: Na+, K+-ATPase, gastrointestinal tract, Holstein steers