What is NDFD?

Many of us have seen over the past year or two a new measurement for fiber digestibility on our forage samples. NDFD is NDF digestibility with an hourly rate added to it. What is it? What does it measure? What does it mean for my ration?
NDFD is an incremented measurement of NDF fiber digested in the rumen. Efficiency and rate of digestion are both measured in this calculation.

The portion of your forage sample that is NDF is immersed in buffered rumen fluid to try to replicate the rumen environment. Then, the digestibility is measured at 30 hours, 120 hours and again at 240 hours. This gives us 3 pools of digestion rates.
That which is in less than 30 hours is rapidly turned into energy mostly in the form of rumen microbial protein. The slower pool that falls between 30 and 120 hours is only slightly digested. That which is in the 240 hour pool is basically not digested at all.

So, in using these three points of measurement we can determine the rate of digestion, and just as importantly, the rate of passage through the cow. The slow pool and the undigested portion are the indicators of gut fill and that is what limits total dry matter intake. So, the more NDF we have that falls in the greater than 30 hour digestibility measurement the less our cows can physically eat.
Corn silage NDFD rates can vary quite a bit on genetic differences between varieties. When choosing corn silage hybrids it is important to look closely at NDFD. Alfalfa genetics do not vary as much as do corn silage hybrids, so that makes timing and stage of maturity far more critical. Seed companies are slowly beginning to look at this in alfalfa genetics. Small grain silages and grasses also have a huge variation on NDFD mostly relating to maturity at harvest.

by Wilson Eberly, Agri-Basics, Inc. Nutritionist