Review: Keystone issues in ruminant science I. Feed intake control in ruminants.

In 1999, we were invited to review the literature on feed intake control in ruminants for the V^th International Symposium on the Nutrition of Herbivores (Pittroff and Kothmann, 1999). Our key findings were: (1) the dominating bi-phasic intake control model, with physical intake control at low DM digestibilities changing to metabolic control at higher DM digestibilities, is not supported by experimental evidence; (2) although main substrates in energy metabolism differ between ruminants and monogastrics, principal intake control mechanisms appear to be the same; (3) the most likely central intake control element is ATP generation in the liver; (4) peripheral anabolic and catabolic processes, which change fuel supply to the liver, produce predictable adjustments in feed intake: (5) tissue-available N for synthesis plays a pivotal role in the dynamics of nutrient allocation and therefore in fuel storage, and fuel oxidation in the liver and periphery. From these findings, it follows that multiple interactions between feed components, and between feed components and animal characteristics determine feed quality in the ruminant. Consequently, static factorial feed requirement systems cannot optimise the match between feed supply and animal requirements. We were asked again to review the literature on feed intake control for the 11th ISNH and conclude, 25 years later, that important new experimental evidence supports our original conclusions and hypotheses. However, although key elements of our new theoretical concept were taken up by other authors (albeit unreferenced), corresponding new developments in feed requirement prediction models did not occur and consequently, major progress in optimising feed resource utilisation in ruminant production is absent. We discuss the reasons and future research directions, including a brief critical review of 'big data' proposals for the optimisation of ruminant nutritional management.