Both articles refer to forage fiber or acid detergent fiber (ADF) as being indigestible. The fiber fractions ADF and neutral detergent fiber (NDF) include lignin, which is indigestible and which lowers the digestibility (in ruminants) of the cellulose to which it is linked.
What is often misunderstood is that forage fiber is partially digestible in ruminants and varies (from roughly 25 percent to 75 percent) with the concentration of lignin in the fiber.
Some commercial laboratories offer analyses of fiber digestibility (dNDF and NDFD) that are based on lignin concentration or incubation of forage samples in buffered rumen fluid for up to 48 hours.
The articles refer to relative feed value (RFV) or relative forage quality (RFQ), and one correctly indicates that these indices are meant to express level of potential daily digestible dry matter (DM) intake, relative to that of full-bloom alfalfa hay (which has a value of 100). One article correctly points out that RFQ treats grasses more fairly than is the case with RFV.
The reason for this is that RFQ includes information on fiber digestibility, while RFV does not. Excellent additional information on modern forage testing, fiber digestibility and advantages of the RFQ approach can be found on the University of Wisconsin Team Forage website.
Lastly, one article indicates that all energy estimates used in forage testing, including total digestible nutrients (TDN), are calculated from acid detergent fiber (ADF) alone.
While this continues to be the case with many labs and is considered by some to be an obsolete approach, there are commercial labs that calculate TDN and other energy estimates with summative equations that incorporate information on fiber digestibility.
One such equation is presented in the 2001 NRC Nutrient Requirements of Dairy Cattle publication.
Cost-effective tests of fiber digestibility can be performed via wet chemistry or NIRS and deserve more attention, given the wide variation in energy availability from forage fiber that is not indicated by ADF or NDF alone. FG
Thomas C. Griggs
Assistant Professor
Forage and Grassland Agronomy
Division of Plant and Soil Sciences
West Virginia University
