Evaluating Feed Components and Finished Feeds: Part 2

Tim Herrman
Extension State Leader
Grain Science and Industry
Kansas State University

Moisture content affects an ingredient's nutritional content and its performance during handling, storage, and processing. Both direct and indirect measures of ingredient and finished feed moisture are approved for feed industry use. Direct methods include oven drying and distillation while indirect methods include near infrared (NIR) spectral analysis, conductance, and water activity. The oven drying method involves the removal of free water from a sample through heating and measurement of weight loss. This procedure is based on the principle that the boiling point of pure water is 212 degs F (100 degs C) at sea level. The likelihood that a compound will decompose or volatilize (turn from solid to vapor) determines the type of oven used (convection, forced draft, or vacuum oven). A vacuum oven lowers the boiling point of water and allows the oven drying procedure to be performed at a lower temperature, thus reducing loss of dry matter through volatilization. Calculation of moisture content and total solids is performed as follows:

Feed evaluation formula

In the case of semi-moist products (e.g., dog food) the Karl Fischer method is preferred. Water is extracted with methanol from pet food that contains other volatile components, and an aliquot is titrated with Karl Fischer reagent. This test is good for products containing between 20 to 30 percent moisture (AOAC Official Method 991.02). Moisture content in heat-sensitive feed ingredients is measured using the distillation method. In this technique, the ingredient is boiled in a solvent and water is driven off from the sample, condensed, and measured (AOAC Official Method 925.04). Indirect moisture measurement for feed grains involves the use of an electrical moisture meter (AACC Method 44-11). Another indirect moisture measurement can be performed using a beam of light in the near infrared (NIR) frequency with a spectrophotometer. This method works well for feed grains, feed ingredients, and finished feeds.

Proteins are comprised of amino acids which are the building blocks of protein. When formulating a complete feed, the nutritionist creates a feed ration with a complete balance of amino acids. A shortage of one amino acid in a complete ration can cause animals to experience depressed growth rate, poor feed conversion, and reduced reproductive performance. Most protein tests evaluate the nitrogen (N) content of the sample; nitrogen is present in protein molecules at about 16 percent. The combustible nitrogen analyzer has grown in popularity as the preferred method for measuring N. This technique is reliable, quick, does not involve the use of highly corrosive acids and bases, and its cost is fairly reasonable. Additionally, the use of optical measurement of protein content using NIR technology works well for cereal grains, oilseeds, and finished feed. Assaying feed for individual amino acids is expensive and is seldom performed by a feed company. Thus, nutritionists use standard values for amino acid content in feed ingredients based on the National Research Council publications (NRC, 2001).

Crude fat content is measured by extracting fat with an ethyl ether solvent and then weighing the extracted fat in a vessel after the solvent has been evaporated. Crude fat is a term that refers to both fats and oils or a mixture of the two and all other organic soluble compounds. The melting point of most fats is such that they are solid at ordinary room temperature, while oils have lower melting points and are liquids at room temperatures. Fats are high-energy ingredients containing about 2.25 times the amount of energy as other nutrients. Fat analyses should include moisture, impurities, unsaponifiable materials (M.I.U.), and free fatty acids (FFA). FFA content should not exceed 15 percent. Additionally, NIR technology works well for measuring oil content in oilseed crops (e.g., soybeans), corn, and on complete feeds.

Crude fiber includes the materials that are indigestible to humans and non-ruminant animals. It is defined as the material that is insoluble in dilute acid and dilute alkali under specified conditions. Crude fiber is used as an index of an ingredient's feeding value since materials high in fiber are typically low in nutritional value.

Mineral analysis procedures are described in the National Feed Ingredient Association's (NFIA, 1991) Laboratory Methods Compendium, Volume I. Calcium constitutes about 2 percent of the body weight and is important for bones, teeth, and muscle contraction and relaxation, especially the heartbeat; has a role in the transmission of nerve impulses; is necessary for blood clotting; and activates a number of enzymes. Phosphorus is closely associated with calcium, thus, a deficiency or overabundance of one will interfere with the utilization of the other. Phosphorus is involved with bone formation and maintenance, teeth development, milk secretion, and building muscle tissue; it is an essential element in genetic material, metabolic functions, and osmotic and acid-base balance. Magnesium interacts with calcium and phosphorus. If extremely low, magnesium will cause calcium to be deposited in soft tissues forming calcified lesions. An excess of magnesium upsets calcium and phosphorus metabolism. Sodium helps control the osmotic pressure and acid- base balance in body fluids (upon which depends the transfer of nutrients to the cells and removal of waste material from cells). Sodium is associated with muscle contraction and nerve function.

Pepsin Digest
Pepsin digest is a procedure used to determine the protein digestibility of animal by-product meals. Animal by-product meal is processed under extreme temperature conditions that can cause the proteins to become denatured and indigestible. Results of a pepsin digest analysis are usually reported as a percentage of pepsin indigestible residue or percent of crude protein that is pepsin indigestible.

The AFIA Feed Ingredient Guide II lists the following recommendations for animal by-product meals:

  • Poultry Feathers. Not less than 75 percent of crude protein should be pepsin digestible.
  • Meat Meal. Not more than 14 percent indigestible residue and not more than 11 percent of crude protein should be pepsin indigestible.
  • Meat and Bone Meal. Not more than 14 percent indigestible residue and not more than 11 percent of crude protein should be pepsin indigestible.

Urease is an enzyme (present in soybeans) that acts on urea to produce carbon dioxide and ammonia. Urease is controlled by heating to denature the enzyme, and as such, is analyzed in soybean meal to assess if it has been properly processed.

All microscopic identification is based upon relating the items seen to known material. Through the use of low magnification (8 to 50 times) materials are exam- ined and identified based on physical characteristics such as shape, color, particle size, softness, hardness, and texture. Feed microscopy is a useful method for identifying impurities/contaminants and evaluating the quality of incoming ingredients. It also serves as a useful method for identifying missing ingredients in finished feed.

M.I.U. stands for moisture, impurities, and unsaponifiable material. Fat sources should be evaluated for these components and should not exceed the following levels: moisture less than or equal to 1 percent, impurities less than or equal to .5 percent, unsaponifiable material less than or equal to 1 percent.

Brix is a term commonly used to indicate the sugar (sucrose) content of molasses. This analysis is per- formed based on the optical properties of the molasses using a refractometer. Brix is expressed in degrees and is closely related to percent sucrose. The AFIA Feed Ingredient Guide II specifies a Brix reading of 79.5 degrees.

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