The endless complexity of the natural world makes laboratory science impossible and unsustainable. Here’s an example of the nonsense that results when we turn to laboratory science to teach us how to live out in the real world. This is a passage from the U. of Minnesota on determining the amount of fiber in a cow’s feed ration.
The real question is: “Who needs to know this?”
Three methods are available to measure fiber content of feeds.
Crude fiber (CF) is the oldest method and is the residue of a feed that is resistant to successive boiling with dilute acid and alkali treatments. Crude fiber is not an accurate measure of total fiber or cell walls because much of the lignin and hemicellulose is lost during the analysis. Even cellulose is not totally recovered in the CF fraction. Many feed testing labs no longer report CF, but it is required on feed tags of purchased feeds.
Acid detergent fiber (ADF) consists of cellulose, lignin, lignified nitrogen compounds (heat damaged protein), and insoluble ash. Acid detergent fiber does not represent the total fiber content in feed, as it does not account for hemicellulose. It is a relatively quick method for measuring fiber, often substituting for CF. Equations used to predict the digestibility or energy content of feedstuffs are usually based on ADF or include ADF as a major component.
Neutral detergent fiber (NDF) consists of ADF plus hemicellulose, and is often called cell walls. Because NDF represents the total fiber in a feed, it is highly correlated to intake, rumination, and total chewing time. Corrected for physical form, NDF provides the best measurement of effective fiber for formulating dairy rations.
The fineness at which forages are chopped during harvesting can alter the effectiveness of fiber for maintaining chewing activity. Haycrop silages should be chopped at a minimum of 3/8 inch theoretical length of cut (TLC) to provide 15 to 20 percent (weight basis) of the particles greater than two inches long. Chopping at 1/4 inch TLC provides only about 10 percent of the forage particles greater than two inches long. Corn silage should be chopped at 1/4 to 3/8 inch TLC. Rations based on 1/4 inch TLC silage should include 5 pounds of long stem hay to provide adequate “effective” fiber. Haycrop silage chopped at 3/16 inch TLC with less than 7 percent coarse particles should be fed with 8 to 10 pounds of long hay. Holstein cows need to chew about 11 to 12 hours per day or 12 to 14 minutes per pound of DM eaten to keep milk fat above 3.5 percent.
High fiber by-product feeds supply some “effective” NDF and can be used to partially replace NDF coming from forages in the ration. Whole cottonseed possesses the best forage NDF replacement value of commonly available by-product feeds fed in milking cow rations.
Starch, sugar, and pectin make up the highly digestible carbohydrate fraction in feeds termed non-fiber carbohydrates (NFC). Subtracting percent (DM basis) NDF, CP, ether extract or fat and ash from 100 provides an estimate of NFC percent in feeds.
|(NFC% = 100 – [%NDF + %CP + %fat + %ash])|
The term nonstructural carbohydrate is often used interchangeably with NFC but is analytically determined and may be slightly different from NFC.