Undigested Neutral Detergent Fiber (uNDF) a commonly misunderstood concept, and one the ruminant nutrition industry still has much to learn about through continued research and exploration. Historically defined as a measure to relate to fiber potential, uNDF is now described as a new measure of lignification.

Lignin ties cellulose and is typically explained as the “woodiness” factor within the diet. Historically, lignin has been used to estimate uNDF, however it has been found faulty in relating to uNDF. While we considered uNDF “the new lignin” when analogizing the term to help others understand the concept, uNDF is indeed superior to lignin. Lignin is only a small component of feeds, and hence has a greater coefficient of variation than uNDF. There is a wider range of uNDF in forages relative to lignin; the wider range of uNDF gives us better insight into this so-called “woodiness” factor that affects performance. 

uNDF is also described as it relates to what it effects – specifically as the functional fiber fraction that influences physical effectiveness, gut fill, and digestion/passage dynamics of forages (Cotanch et al., 2014 Cornell Nutr. Conference). While uNDF has potential for several possible uses, there are two main functions: 1) Relating to gut fill and, 2) modeling total digestible NDF. The first facet may hold potential in predicting dry matter intake limiting diets. Both Miner Institute and Cornell University researchers have suggested we should start to be concerned with possible gut fill limitations when uNDF intake approaches 0.35 to 0.40% of bodyweight.

uNDF is expressed in one of two ways: as a percent of dry matter (DM) basis; or as a percent of fiber (percent of the total fiber bound tight and not available to rumen fibrolytic bacteria).

uNDF’s impact on predicting total digestible NDF can be explored through a simple analogy borrowed from Prof. Combs (University of Wisconsin-Madison). Consider the question of how far you can drive. To understand and calculate the distance to empty, two other measures are necessary: your car’s fuel economy, and how much fuel is in the tank. uNDF is akin to knowing “How much fuel is in the tank”. We can think of total digestible fiber available for metabolizable energy (ME) and metabolizable protein (MP) being determined by knowing both the fiber digestion rate (NDF kd) as well as how much fiber is available (determined through uNDF).


Cotanch, K.W., R.J. Grant, M.E. Van Amburgh, A. Zontini, M. Fustini, A. Palmonari, and A. Formigoni. 2014. Applications of uNDF in ration modeling and formulation. Proc. Cornell Nutrition Conference. Syracuse, NY. Pgs. 114-131.

Author: John Goeser, PhD, PAS, and Dipl. ACAN JohnGoeser@rockriverlab.com