Specifically, digestive tract and liver tissue mass, as well as O2 consumption, increase or decrease in accordance with increasing or deceasing amounts of dietary MEI (Burrin et al., 1989; Freetly et al., 1995; McLeod and Baldwin, 2000). Moreover, shifts in PDV O2 consumption associated with MEI and dietary forage: concentrate ratios have been ascribed to differences in digestive tract organ weights rather than ruminal or intestinal cellular oxidative capacity (Baldwin and McLeod, 2000; McLeod and Baldwin, 2000), thus equating changes inPDVO2 consumption to changes in organ mass. However, a direct cause-and-effect relationship