Unfortunately, no single tool can quantify fiber adequacy in dairy rations and uniformly predict rumen health responses to diets. For NFFS-based diets, we advocate an approach similar to that proposed by NRC (2001), which uses a sliding scale of NDF from forage and total NDF concentrations (Figure 1). For example, a minimum of 18% NDF from forage is recommended if total NDF content of the diet is just 27%, but only 15% NDF from forage is considered necessary if total NDF is 33% of DM (NRC, 2001). This approach has been successfully extended to 12 to 13% NDF from forage with 31 to 35% total NDF without inducing milk fat depression (Miron et al., 2003; Mullins et al., 2010; Rezac et al., 2012). The tradeoffs between forage and total NDF reflect the concept that nonforage NDF is approximately half as effective as forage NDF at maintaining ruminal function and milk fat yield (Swain and Armentano, 1994).