ANN 111: Principles of Animal Nutrition and Feed Technology (2+1)

Introduction

FIBRE FRACTIONATION - DETERGENT METHOD OF FIBRE ANALYSES

• P.J. Van Soest and associates, working at the USDA station at Beltsville, Maryland, USA developed a rapid technique of separating feed carbohydrates on the basis of nutritional availability to ruminants and rumen bacteria. Essentially, the method divides feeds into two fractions; (1) plant cell contents, a highly digestible fraction consisting of sugars, starches, soluble protein, pectin and lipids; (2) plant cell wall constituents, a fraction of variable digestibility consisting of insoluble protein, hemicellulose, cellulose, lignin and bound nitrogen.
• The method involves boiling a sample in a neutral detergent solution. The soluble fraction is termed NDS - neutral detergent solubles (cell contents), while the fibrous residue is called NDF - neutral detergent fibre (cell wall constituents). Unlike crude fibre and NFE, both NDS and NDF accurately predict the proportions of more or less digestible fractions, respectively, found in a wide variety of feedstuffs.
• The Van Soest (or detergent) scheme has been further refined with the addition of acid detergent fibre (ADF) analysis, which breaks down NDF into a soluble fraction containing primarily hemicellulose and some insoluble protein and an insoluble fraction containing cellulose, lignin and bound nitrogen. Further more, the content of lignin in acid detergent fibre can be determined by either treating the fibre with H₂SO₄ to dissolve the cellulose or by oxidation with permanganate to degrade the lignin. This is quite important because lignin has been shown to be a major factor influencing the digestibility of forages.
  • Feed samples boiled in Neutral detergent solution
  • Neutral detergent fibre – (NDF:Cell wall contents)
  • Boiled in Acid detergent solution
  • Neutral detergent soluble – (NDS:Cell contents)
  • Acid detergent fibre  (ADF)
  • ADF Digested with 72 % H₂So ₄ gives Acid detergent lignin (ADL)
  • Acid detergent soluble - ADS
  • NDF – ADF = Hemicellulose
  • ADF -ADL = Cellulose

  Characteristics of some important forage cell wall constituents

Constituent	Characteristic
Lignin	Major non-carbohydrate portion of cell wall. Provides structural support for plant. Three-dimensional polymer of phenylpropanes. Associated with hemicellulose and cellulose and lowers their availability.
Cellulose	Major skeletal carbohydrate in plants. Provides structural support for plant. Polymer of glucose in β (1- 4) linkages. Cellulase enzyme not secreted by mammals. Digestibility varies with amount of lignin, silica, cutin.
Hemi cellulose	Polymer of xylose and other five carbon sugars (arabinose side chains). Provides structural support for plant. Digestibility depends an lignin, etc.
Pectin	Polymer of methyl D-galacturonic acid. Highly digestible and availability not greatly influenced by lignin, etc.
Cutin	Composed of waxes and waxy polymers. May be integrated with lignin and is measured as lignin in ADL. Lowers availability of cellulose and hemi cellulose.
Silica	Composed of SiO2 polymers. Taken up by grasses more so than by legumes. Content in plants may vary from less than 1 upto 22% of dry matter. Plants in sandy soils have higher levels. Has similar effect as lignin on digestibility of cellulose and hemi cellulose. May tie up some trace minerals needed by ruminal microorganisms.

Components of the detergent forage fiber analysis system and what they measure

Component	What it measures?
NDF	Primarily cellulose, hemicellulose and lignin. Availability depends on lignification
ADF	Primarily cellulose and lignin. Availability depends on lignificatiion
ADL	Primarily lignin, but some cutin and bound nitrogen (Maillard product). Unavailable.
AIA (Acid insoluble ash)	Fairly good estimate of silica content.