Weende System: Proximate Analysis of Animal Feed

• The Weende system, also known as proximate analysis, determines the nutritional composition of animal feed.
• Wilhelm Henneberg and Friedrich Stohmann developed it in the mid-19th century at the Weende Experiment Station in Germany.
• The system separates feed components into six fractions: moisture, crude protein, ether extract, crude fiber, ash, and nitrogen-free extract.
• This wet chemistry procedure uses sequential extraction to isolate fractions.
• Despite its age and limitations, the Weende system remains a widely used and cost-effective method for feed analysis.
• It provides a basic understanding of the nutrient content of feedstuffs.

Components of Weende Analysis

• Moisture (Water): Determined by drying a sample in an oven until a constant weight is achieved, recording the loss in weight as moisture content.
• Crude Protein (CP): Estimated by determining the nitrogen content of the feed and multiplying it by a factor, typically 6.25.
  • It assumes that all nitrogen in the feed comes from protein and that protein contains 16% nitrogen.
  • CP = N x 6.25
• Ether Extract (EE): Measures the fat content of the feed.
  • The dried sample is extracted with ether, and the material that dissolves in the ether is weighed as ether extract.
  • Also known as crude fat.
• Crude Fiber (CF): Represents the indigestible carbohydrate fraction of the feed, consisting primarily of cellulose, hemicellulose, and lignin.
  • The residue remaining after boiling the ether-extracted sample in dilute acid and then dilute alkali is designated as crude fiber.
• Ash: Determined by burning a sample in a furnace at high temperature (500-600°C) and weighing the residue, which represents the total mineral content.
• Nitrogen-Free Extract (NFE): Calculated by subtracting the sum of the percentages of moisture, CP, EE, CF, and ash from 100.
  • NFE = 100 - (Moisture + CP + EE + CF + Ash)
  • Represents the soluble carbohydrates such as sugars, starches, and some hemicelluloses.
  • Also represents the errors in the analysis, and is the least accurate fraction.

Procedure

• Sample Preparation: The feed sample is first ground to a uniform particle size to ensure homogeneity.
• Moisture Determination: A known weight of the sample is dried in an oven at 100-105°C until a constant weight is achieved, and the loss in weight is recorded as moisture.
• Ether Extraction: The dried sample is then extracted with ether in a Soxhlet apparatus. The ether is evaporated, and the residue is weighed as crude fat (Ether Extract).
• Acid and Alkali Digestion: The ether-extracted sample is boiled in dilute acid and then in dilute alkali.
• Fiber Determination: The residue is dried, weighed, and then ashed in a furnace. The loss in weight due to ashing is recorded as crude fiber.
• Nitrogen Determination: A separate sample is used to determine the nitrogen content using the Kjeldahl method. The nitrogen content is multiplied by 6.25 to estimate crude protein.
• Ashing: Another sample is placed in a muffle furnace at 500-600°C until all organic matter is burned off, leaving behind the ash.
• NFE Calculation: NFE is calculated using the formula: NFE = 100 - (Moisture + CP + EE + CF + Ash).

Advantages

• Cost-Effective: Relatively inexpensive compared to more modern methods.
• Widely Available: Equipment and reagents are readily available in most laboratories.
• Standardized: The method is well-established and standardized, allowing for comparison of results across different laboratories.
• Provides Basic Information: Offers a basic understanding of the nutrient composition of feedstuffs.

Disadvantages

• Inaccuracy: The method is not very precise and can be subject to errors.
• Crude Estimates: Provides crude estimates of nutrient fractions rather than precise measurements.
• NFE Error: The calculation of NFE is subject to errors since it includes the cumulative errors from the other analyses.
• Incomplete Picture: Does not provide information on specific amino acids, fatty acids, vitamins, or minerals.
• Overestimation: Overestimates carbohydrate content because it includes other non-nitrogenous compounds.
• Nitrogen Assumption: Assumes all nitrogen comes from protein, which is not always accurate.
• Ignores Non-Nutritive Substances: Does not account for non-nutritive substances like tannins or alkaloids.
• Fiber Complexity: Crude fiber doesn't accurately represent total dietary fiber, as some fiber components are lost during the process.
• Environmental Impact: The use of ether and other solvents can pose environmental and safety concerns.

Limitations of Crude Protein

• The Kjeldahl method, used to determine nitrogen content and subsequently crude protein, does not differentiate between different forms of nitrogen.
• Non-protein nitrogen (NPN) sources, such as urea or ammonia, can inflate the crude protein value, even though they may not have the same nutritional value as true protein.
• The multiplication factor of 6.25 assumes that all proteins contain 16% nitrogen, which is not entirely accurate, as different proteins have varying nitrogen contents.

Limitations of Ether Extract

• Measures all substances soluble in ether, including fats, oils, waxes, pigments, and other non-nutritive compounds.
• It does not differentiate between different types of lipids, such as saturated, unsaturated, or essential fatty acids.

Limitations of Crude Fiber

• Underestimates the total fiber content of feedstuffs because some fiber components, such as hemicellulose and lignin, are partially lost during the acid and alkali digestion steps.
• It does not provide information on the specific types of fiber present, such as soluble or insoluble fiber.

Limitations of Ash

• Represents the total mineral content of the feed but does not provide information on the specific minerals present or their bioavailability.
• Some minerals may be lost during the ashing process due to volatilization.

Improvements / Alternatives

• Van Soest Fiber Analysis: Provides a more accurate determination of fiber fractions, including neutral detergent fiber (NDF) and acid detergent fiber (ADF).
  • NDF represents the total fiber content.
  • ADF represents the cellulose and lignin content.
• Amino Acid Analysis: Determines the specific amino acid composition of feedstuffs, providing a more accurate assessment of protein quality.
• Gas Chromatography: Determines the fatty acid profile of feedstuffs, providing information on the types and amounts of different fatty acids.
• Inductively Coupled Plasma (ICP) Spectroscopy: Determines the mineral composition of feedstuffs, providing information on the types and amounts of different minerals.
• Near-Infrared Reflectance Spectroscopy (NIRS): A rapid and non-destructive method for analyzing the chemical composition of feedstuffs.
  • NIRS uses infrared light to scan the sample and measures the reflected light.
  • The data is then used to predict the nutrient content of the sample.