Insist on NDF Digestibility as Part of the Forage Analysis

Dr. Ronald L Boman
USU Extension Dairy Specialist

I don�t want to beat a dead dog to death, but the importance of neutral detergent fiber (NDF) digestibility in the forage analysis needs to be emphasized until we include it as a standard operational procedure. I�ve attended two conferences in the last month where NDF digestibility was explained as the forage test of the future. Also numerous popular press articles and industry newsletters have focused on the importance of this procedure as a means to more accurately determine the energy content and feed quality of forages. The NDF digestibility varies greatly among forages, therefore if the laboratory you send your forage samples to doesn�t incorporate NDF digestibility in the forage analysis, you need to tell them to get with the program, or you should look for a laboratory that does provide NDF digestibility with a high degree of reliability.

There are numerous commercial and university forage testing laboratories that have the capability to determine NDF digestibility on forages. Laboratory in vitro techniques have been perfected where rumen fluid is incubated (in what becomes an artificial rumen) with buffers and other media to simulate the conditions found in the rumen of a live animal. A known amount of ground forage is subjected to fermentation for either 30 or 48 hours and the disappearance of the NDF is determined. The 30-hr fermentation is similar to what happens with high producing dairy cows with high intakes, while the 48-hr fermentation represents the values used to determine TDN and NEL in the 2001 Dairy NRC. NIRS equations have been developed based on in vitro or �wet chemistry� techniques from a sufficient number of alfalfa hay, haylage and corn silage samples of varying NDF content and digestibility. Reputable forage testing laboratories will have these NIRS equations and will be able to use the NIRS method to determine NDF digestibility with a high degree of confidence on alfalfa hay and haylage and on corn silage samples.

High-producing dairy cows will comsume more feed and produce more milk when fed forages that contain higher NDF digestibility. Besides having the NDF digestibility run on dairy forages, keep in mind the following principles when growing or purchasing alfalfa hay or haylage:
  1. The NDF in alfalfa is more digestible when formed during cool growing conditions with adequate crop moisture. Therefore, the first crop alfalfa grown in the Spring and early Summer will have a higher NDF digestibility than subsequent harvests made during the heat of the Summer. This explains why it is more difficult to get cows to produce at high levels when fed traditional 2nd crop or even 3rd crop hay that is harvested during the heat of the Summer. As ambient temperatures cool down in the Fall alfalfa NDF digestibility increases again for the last cutting.
  2. As a general rule, alfalfa grown at higher elevations will have higher NDF digestibility compared to that grown at lower elevations.
  3. To optimize the NDF digestibility of alfalfa the leaves must be retained in the hay or haylage. Leaves are more digestible than stems and extreme care should be exercised during harvesting and feeding operations to retain the leaves in the bale, in the silo and in the feed bunk.
  4. Relative Feed Value (RFV) has been a good measure of alfalfa feeding value in the past, but RFV must now be replaced with Relative Feed Quality (RFQ) which incorporates the digestibility of NDF into the calculation and gives a more accurate indication of forage quality for high producing dairy cows.
  5. Research has shown that alfalfa cut in the afternoon has a higher carbohydrate and lower NDF content than when cut in the morning. Cows will eat more of the afternoon cut alfalfa, whether in the form of hay or haylage, and also produce more milk. Although I can�t say for sure, I suspect that the NDF digestibility is also higher in the afternoon cut alfalfa.
For more information contact: Ron Boman at ronb@ext.usu.edu or (435) 797-2163. ©