New Forage Analysis = Increased Feed Efficiency Potential

Dr. Ronald L. Boman
USU Extension Dairy Specialist
We’ve come a long way in forage analysis since the early to mid 1900's when Total Digestible Nutrients (TDN) was the standard for analysis of forages. This procedure required the chemical analysis of the various nutrients in the forage and feces to determine the digestibility of each nutrient. Steers were individually fed in metabolism crates, and the total amount of feed consumed was recorded and the total amount of feces voided was collected over a 7-day period. The feed and the feces were oven dried and then analyzed chemically. The % of each nutrient in the feed was multiplied by its digestibility to get the amount digested, and the sum of these digested amounts was expressed as TDN in 100 lbs of feed or as % TDN. This was a very labor and laboratory intensive procedure. Keeping the urine separate from the feces and collecting all of the feces was a real challenge (believe me I’ve done it more times than I care to remember).

In the late 1960's Peter Van Soest from USDA and Cornell University gave us the concept of cell contents (sugars, starches, fats, soluble proteins and pectin) which are readily digested and cell walls (cellulose, lignin, hemicellose and heat-damaged compounds) which are broken down much more slowly in the rumen by the actions of micro-organisms. We have used ADF (cellulose and lignin) of the forage as a measure of forage digestibility and NDF (cellulose, lignin and hemicellulose) as a measure of potential intake. Relative feed value (RFV) of a forage is based on the ADF and NDF content. As an example, an alfalfa hay that is 20% crude protein, 30% ADF and 40% NDF would have a RFV of 153. We’ve used relative feed value in the past as a measure of forage quality, and have paid more for higher RFV forages that have increased milk production and animal performance when included in a balanced ration. But now there is a better way than RFV of comparing forage quality. It is Relative Feed Quality (RFQ), which involves determining the digestibility of the NDF fraction of the forage. Frequently, dairy producers have told me that quite often two alfalfa hays of similar or near equal RFV actually feed differently, and that milk production either drops or increases when changing from one to the other. This might be due to many factors, such as rain damage, baling with too much moisture, or too little moisture causing the leaves to separate from the stems, etc. But not all NDF is digested at the same rate, and this most likely is the reason that two separate hays of equal NDF and RFV give different results when fed to milking cows. Data from Wisconsin with alfalfa and alfalfa/grass hays show a range of NDF digestibility (dNDF) from 35 to 70%. Recent information from USU with 5 different varieties of oats harvested at the boot stage of growth shows a range of dNDF from 66.6 to 76.4 %. Also, five different corn varieties harvested for silage had dNDF ranging from 53.3 to 62.3%. Forage testing laboratories that are now using the dNDF procedure are reporting similar ranges of digestibility. Clearly, there are opportunities to refine the quality measurements of our dairy forages by taking advantage of the NDF digestibility analysis. The dNDF procedure employs a 48 hour in vitro fermentation using rumen fluid. A number of forage testing laboratories are currently running NDF digestibility and others will most likely come on board soon. Some of those that I’m aware of are:

1) Bar Diamond, Inc, Parma, ID;
2) Cumberland Valley Analytical Lab, Hagerstown, MD;
3) Dairy One, Ithaca, NY;
4) Dairyland Laboratories, Arcadia, WI; and
5) University of Wisconsin Soil and Forage Analytical Lab, Marshfield, WI.

I encourage dairy producers and nutritional consultants to seriously consider having forages analyzed for dNDF in order to be able to select those forages that will improve efficiency of milk production and reduce feed costs.