The purpose of this project was to determine the best time point for estimation of the indigestible fraction of NDF for many common non-forage fiber sources (NFFS). Recall, in order to calculate the digestible portion of NDF we need to first quantify the indigestible portion.
For many years we have relied on lignin analyses to make this estimation using the equation: 2.4 x % lignin. Through years of analyses of NDFD it's become clear that lignin alone doesn't accurately predict iNDF and that conducting time series analyses of ash free undigested NDF (uNDFom) does. The value of this improved estimation of indigestible NDF allows us to more accurately predict digestible NDF and therefore more accurately estimate the metabolizable energy and protein (ME and MP) and the microbial protein yields from feeds and forages.
Of the common feedstuffs in most of our dairy grains and rations it was clear that many are "done" digesting, have reached the indigestible NDF after 96 hours of fermentation, as noted by the minimal decrease in uNDFom from 96 to 120 hrs. A few such as beet pulp, corn gluten feed, soy hulls and wheat midds continue fermenting through 120 hrs and only citrus pulp continued to ferment for 240 hrs in this sample set.
From this data, it was concluded that uNDFom120, the 120 hr time point, was the single best time point across all NFFS to estimate the indigestible fraction of NDF. Comparing the uNDFom120 values to the 2.4 x lignin values, it's clear that for feeds such as canola, distillers, flaked corn, soybean meal and soy hulls that lignin greatly overestimates the indigestible fraction of NDF.
For rice hulls, wheat midds and cottonseed, lignin grossly underestimates the indigestible fiber fraction, thus overestimating digestible NDF.
Though only two samples of each feed were analyzed in this dataset, it provides clear indication that uNDFom120 better estimates the indigestible fraction of NDF allowing for better estimation of energy and protein estimates the cow can derive from these feeds.
Also to note is the low digestibility values/high uNDFom120 values of rice hulls, sunflower hulls, wheat midds and cottonseed. Something to consider when effective fiber is needed or when gut fill may be limiting. As the high uNDFom120 indicates slower digestion of the NDF, taking up rumen space and providing less energy.
This dataset is a good start for those interested in calculating the total uNDF of a ration and not just the forage uNDF. Most labs provide NIR uNDF values, but none have calibrations for NFFS.
This article is based on the work of Alessandro Zontini, who recently finished a PhD working in Mike VanAmburg's lab at Cornell. Much of his work focused on characterizing the NDF pools (fractions) of non-forage fiber sources (NFFS).