5.11: Feed Evaluation Procedures

Accurate feed evaluation integrates chemical composition with biological availability and productive value. Feed analysis aims to determine the composition, digestibility, palatability, and productive value of feedstuffs.

Animal trials are biological procedures used to determine how a feed affects an animal. Animal trials are a necessary component of animal nutrition research, complementing chemical analyses of feedstuffs to determine their value to animals.

Nutritionists commonly employ three major types of biological trials: feeding trials (to test palatability and performance in functions like growth, lactation, or endurance), digestion trials (to measure digestibility by quantifying the nutrient difference between intake and excretion), and more advanced metabolism trials (which measure the body's total intake and excretion, including losses via urine and gas, to calculate nutrient retention or balance).

Although time-consuming, labor-intensive, and expensive, these methods provide essential data for accurate ration formulation and understanding how animals metabolize and utilize feed components.

This method, developed in the 1860s, provides a standardized way to compare feeds and formulate balanced diets. While not perfect—it doesn't account for digestibility or nutrient quality—proximate analysis remains widely used because it's relatively simple, inexpensive, and provides a consistent baseline for feed evaluation. Proximate Analysis partitions feed into six fractions:

• Dry Matter (DM): what remains after all water is evaporated by oven drying at 105°C
• Crude Protein (CP): calculated from the total nitrogen (N) content multiplied by 6.25 (since protein is roughly 16% N)
• Ether Extract (EE): fat extracted by organic solvents (high-caloric density)
• Ash: all mineral content remaining after burning off all organic matter at 500-600°C
• Crude Fiber (CF): insoluble carbohydrates (cellulose, hemicellulose, lignin) remaining after acid/alkali digestion
• Nitrogen-Free Extract (NFE): a calculated estimate of readily available carbohydrates (sugars, starch)

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The Van Soest method is a more accurate method for measuring fiber in animal feeds, developed to overcome the limitations of crude fiber analysis. Instead of using harsh acids and bases that dissolve some fiber components, the Van Soest method uses detergent solutions to separate feed into two main fractions:

Neutral Detergent Fiber (NDF): The total cell wall material, including cellulose, hemicellulose, and lignin. This represents the bulkiness of the feed and the amount of space it occupies in the animal's digestive tract.

Acid Detergent Fiber (ADF): The less digestible portion, containing only cellulose and lignin. This indicates how much of the fiber is poorly digestible.

Hemicellulose: Calculated as NDF minus ADF, represents the most digestible fiber fraction.

The key advantage of this method is that it more accurately predicts the digestibility of a feed, especially for ruminant animals. Higher NDF means the animal can eat less because the feed is bulkier. A higher ADF means a lower energy value because more of the fiber is indigestible. This information helps nutritionists formulate more precise diets, particularly for dairy cattle and other ruminants that rely heavily on forage.

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