The following answer is provided by Dave Mertens, forage expert and president of Mertens Innovation & Research in Belleville, Wis. It is excerpted from a presentation he made at the recent Cornell Nutrition Conference.
Q: Does neutral detergent factor (NDF) content of the diet always regulate dry matter intake (DMI)?
A: No, but there are always specific situations where it is the controlling factor, and due to the nutrient demands of high levels of milk production, it is more often the controlling factor for the intake of lactating cows than in the past…
It is clear that the chemical concentration of NDF in feeds and in the ration is the single most important factor defining the effect of NDF on DMI. The NDF-Energy Intake System of Mertens (1987, 1992) uses this relationship to effectively adjust the (forage-to-concentrate ratio) of rations to define the upper boundary for intake at a given target of milk production and to describe the ration that maximizes the forage content of the ration for a target level of milk production.
Next in importance is particle size of the NDF because it not only affects the fill volume in the rumen, but it also has a dramatic affects on the passage of NDF through the animal. The simple mathematical descriptions of the physical and physiological mechanisms of intake regulation make it clear that it is the flows of nutrients or residues that define the intake constraint.
Finally, characteristics of the fiber that influence its digestibility and physical breakdown will have lesser impact that NDF concentration and particle size on the relationship between NDF and DMI, but these effects can be significant and should not be ignored. Increasing NDFD by first decreasing the indigestible fraction and second by increasing the rate of digestion will increase the intake of NDF or reduce its impact on DMI. Feeds with NDF that disintegrates more rapidly due to digestion or chewing activity will not only occupy less space in the rumen but also pass more quickly, leaving space for additional intake.
To use the knowledge we gain about the flow of NDF through the cow and its impact on DMI, we need quantitative feed formulation systems that build on the simple framework described by the NDF-Energy Intake System. Dynamic and steady-state models can provide additional information about the mechanisms that alter the basic relationship between NDF and DMI.