[Excerpt] Introduction: The rumen has sometimes been described as a “superorganism”. “The rumen and its microbes” (the title of Robert Hungate’s 1966 seminal book) are so critical to all ruminants, especially the high-producing dairy cow. One of the dairy cow’s “superpowers” is rumination, as in Rick Grant’s recent article in Hoard’s. During my Master’s program in the 1980’s, I became interested in both subjects, and I’m still amazed that so much of the cow’s rumen is geared toward optimizing fiber degradability (measured as NDF degradability, NDFD). Again, the challenge is especially for the high-producing cow because increasing feed intake increases ruminal outflow, which would increasingly push out potentially degradable NDF (pdNDF), whereas fortunately forage genetics also have improved pdNDFD. Increased intake of degraded NDF has simultaneously diluted the cow’s maintenance energy while also diluting the proportion of energy used by microbes in parallel, which was the theme of my doctoral thesis. Both optimizing NDFD and passage of microbes increase supply of ruminal microbial protein (MicP) production to the small intestine. However, despite their importance, optimizing NDFD and MicP outflow are still hard to predict reliably because of so many challenges we face in measurement and in differences among cows.