Proximate Analysis/ Weende system
Proximate analysis is a system for estimating the nutritive value of feed or material for feeding purposes. The principle of the analysis is to separate the feed components into groups or fractions in accordance with their feeding value. The various components are moisture, ash, crude protein, crude fiber, and nitrogen free extract.
Moisture and Dry Matter:
Moisture in a feed is estimated by drying a sample in a laboratory oven at 100 oC until it reaches a constant weight. First of all sample is weighed and this weight is called wet weight. Then it is kept in oven and weighed after certain period of time. It is again kept in oven and again weighed after certain period of time. Sample is kept on keeping in oven and weighing until further change in weight of sample stops. The weight at this point is called dry weight. This is the amount of dry mater in sample. Subtract dry weight from wet weight which will give the amount of moisture in sample. The moisture free fraction is called dry matter.
|% Moisture =||Wet weight – Dry Weight||× 100|
|% DM =||Dry Weight||× 100|
Crude protein in feed is estimated by determining the nitrogen content of feed with Kjeldahl Digestion and Distillation Procedure. In this procedure feed is digested with sulphuric acid, which convert all nitrogen present in feed into ammonia except nitrate and nitrite. This ammonia is liberated by adding sodium hydroxide which is distilled off and collected in standard acid. The quantity so collected is determined by filtration or by an automated colorimetric method. It is assumed that the nitrogen is derived from protein containing 16 percent nitrogen and by multiplying nitrogen with 6.25 an approximate value of protein is obtained. This is not true protein as the method also determines nitrogen from sources other than protein such as free amino acids, amines and nucleic acids and the fraction is therefore digested crude protein.
|% CP =||Units of N × 6.25||× 100|
Ether Extract or Fat:
Ether extract refers to the fraction of the fat in the feed which is soluble in ether and is estimated by continuous extraction of a feed sample with diethyl ether for 6-9 hours in a Soxhlet Apparatus. The residue after evaporating of solvent is ether extract. Pigments and fat soluble vitamins, waxes, gums, resins are also included in this fraction. The ether is then evaporated and the extracted material is weighed.
|% EE =||Weight of Ether Extract||× 100|
Carbohydrates of the feed are obtained in two fractions, the crude fibre and the nitrogen free extracts.
In the laboratory crude fiber is measured by subjecting the residual feed from ether extraction to boiling with diluted acid and then with diluted alkali. The residue after filtration is considered the crude fiber.
|%CF =||Weight of fiber residue||× 100|
Nitrogen Free Extract:
This represents the more soluble carbohydrates or non structural such as starches and sugars. The nitrogen free extract is determined mathematically by difference:
% NFE = 100 – (% H2O + % ash +%EE + %CF)
This represents the mineral components of a feed. A dry sample is placed in a crucible and completely combusted in a furnace at 650 oC, the residue is the ash.
|% ash =||Weight of ash||× 100|
Van Soest Method of Feed Analysis
According to the concept of van soest there are two principal parts of dry matter of plant origin which are cell wall and cell contents. This method is highly efficient to take care of the defects in the principle of estimating crude fibre and NFE by proximate analysis. Plant cell contents consist of sugars, starch, soluble carbohydrates, pectin, non protein nitrogen, protein, lipids and water soluble materials including minerals and several vitamins.
The principal components of cell wall consist of cellulose, hemicelluloses, silica, lignin etc. This part is considered fibre. Fibre is classified as acid detergent fibre (ADF) and neutral detergent fibre. 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. 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.