A Question of Digestibility

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

         Evaluation of forage has been an evolutionary process over a number of years.  In earlier times, two wagon loads of hay may have been compared simply by a visual analysis and perhaps chewing on a stem or two.  If still a close call, it probably came down to kicking the tires on each of the wagons.  Eventually, the use of forage testing labs using chemical tests to analyze forage became more widely used but there was always the delay in results and it was costly.  Today, wet chemistry techniques are still widely used for animal ration balancing purposes, but for simply getting a quick and accurate read on major forage qualitative factors, near infrared reflectance spectroscopy (NIRS) is the analysis method of choice.

          Although forage testing has made much advancement in the past fifty years, there are still times when the productivity of the animal doesn't always match the forage analysis result.  In other words, cows may milk significantly more or less on two different forages with the exact same percent crude protein, acid detergent fiber (ADF), neutral detergent fiber (NDF), and relative feed value (RFV).  The question, of course, is "Why?"

          The answer lies in the digestibility of fiber.  Although ADF and NDF tell us how much and what type of fiber is in the forage, we can only guess as to the actual digestibility of the fiber fraction.  It's not that we can't measure digestibility.  Researchers do it all the time.  The problem is that it requires rumen fluid from a cow and a lot of effort.  It has never been practical for a commercial testing lab to run such a test in mass quantity.  However, all of that is about to change.  Correction:  it has changed.

          For the past five years, University of Wisconsin researchers have been at work developing NDF digestibility equations for use with NIRS.  We can now accurately measure NDF digestibility on a quick and routine basis.  As one car manufacturer advertisement states, "This changes everything".  First, the ADF percent becomes a nearly useless number because it is merely an estimation of forage digestibility.  Second, we can now calculate a meaningful number for Total Digestible Nutrients (TDN) by summing the actual digestibility of forage components rather than making educated guesses.  Third, we can now more accurately compare two forages or evaluate a group of forages on the merit of potential animal performance.  Finally, this new approach to forage analysis will require a change in the way that RFV is calculated.  That hasn't occurred yet, but it's soon forthcoming.  As the situation stands now, TDN percent (calculated with the actual NDF digestibility) will more accurately reflect forage value than RFV.

The potential for milk production from these alfalfa stems will be more accurately evaluated with the ability to measure fiber digestibility using near infrared reflectance spectroscopy (NIRS). 

         Keep in mind that NDF digestibility, by itself, is rather meaningless and total NDF content of the forage is still the most important factor in determining overall forage quality.  For example, a forage with 40 percent NDF is generally of higher quality than a forage with 60 percent NDF.  However, knowing both the NDF and its related digestibility percentage will be a great help in sorting-out the many forages with same or similar total fiber values.

          As a rule, legumes like alfalfa will have less total NDF and lower NDF digestibility than grasses.  Average NDF digestibility values for legume silage or hay are from 48 to 50 percent.  Grasses will have a much wider potential range of NDF digestibility because they are harvested over a wider range of maturity (for example, lush pasture versus feeding straw).  Corn silage also has a wide potential range of NDF digestibility but it is typically not seen because of the relatively short harvest window.  The average value for corn silage is about 58 percent.

          Several factors will influence the NDF digestibility of forages.  Most important is plant maturity.  As plants mature, there is a corresponding drop in digestibility.  Plant genetics also has an influence.  It will now be easier to screen alfalfa varieties with inherently higher digestible NDF.  An extreme example of genetic variation within a species is brown midrib corn (highly digestible) versus normal corn genetics.  Growing environment has a profound influence on NDF digestibility.  Plants grown under cool conditions will be more digestible than those grown under hotter temperatures.

          The world of forage testing is changing rapidly and the ability to measure NDF digestibility using NIRS is a big part of that change.  Take time to consult with your commercial forage testing laboratory and your nutritionist to see where they stand on offering some of these new analyses.

For more information on NDF digestibility, see Pat Hoffman's Focus on Forage.