The carbohydrate constituents of feeds can be divided into the fiber and nonfiber fractions. ADF and NDF are the most common terms used to describe the fiber fractions. NDF is becoming the most commonly used term in nutrition programming and ration evaluation. NFC is the term used to describe the nonfiber carbohydrate fraction when determined by calculation. NFC can be defined as 100 - (CP+Ash+Fat+NDF).

NDF is used to characterize the fiber content of feeds and forages. NDF includes the hemicellulose fraction that is not in the ADF fraction. The particle size and digestibility of the NDF fraction also need to be considered. A review paper examined the effect of NDF digestibility (NDFD) on DMI and milk production.[4] These authors concluded that a 1-unit increase in NDFD was related to a change of + 0.17 kg of DMI and +0.25 kg of 4% fat-corrected milk production. This relationship may not hold in all situations, but provides a good starting point to quantify the importance of fiber digestibility.

Forage particle size can also have an impact on DMI, chewing activity, and rumen function. The term peNDF (physically effective NDF) is used as an index of particle size. This system has been described.1-5-1 One method of determining the peNDF value of a feed is measuring the proportion of feed particles that are retained on a 1.18-mm screen after vertical shaking. Chewing activity decreases with smaller particle size feeds.

The NFC fraction of a feed is not uniform. This fraction can include sugar, starch, fructans, beta-glucans, and other compounds. The calculated NFC value will also include fermentation acids.

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