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Commodities: Field Crops: Forages

Common Terms Used in Animal Feeding and Nutrition

Glossary: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


Carbohydrates: They are organic compounds and the main source of energy for animals. These are biochemical compounds composed only of the elements carbon, hydrogen and oxygen. Carbohydrates are polymers made of basic sugar units, such as glucose (the most abundant), fructose, galactose, etc. The two major classes of carbohydrates in plants are known as non-structural and structural. Those that serve as storage and energy reserves and that are available for more rapid metabolism to supply energy (e.g., sugars, starch, and pectin) are referred to as non-structural carbohydrates. Those carbohydrate fractions that are not used for energy storage and provide fiber and anatomical features for rigidity and even water transport are known as structural (e.g., fibrous cellulose and hemi-cellulose). Animals get the majority of their required energy from the carbohydrates in the feeds and non-structural carbohydrates are more available for energy metabolism than the structural carbohydrates.

Cellulose: This is a major structural carbohydrate that is present in the plant cell walls. Cellulose is an unbranched chain of 7,000 - 15,000 glucose molecules that are linked together by β-1,4 bonds. Cellulose is a major part of the structural fiber in forages and can be utilized by microorganisms in the rumen. When utilizing the chemistry associated with the Van Soest Detergent Fiber Fractions, cellulose is estimated as follows:

Cellulose = ADF – (ADL + Ash),

where ADF is acid detergent fiber and ADL is acid detergent lignin

Concentrates: Used to refer to animal feeds which are rich in energy but low in fiber, such as corn, soybean meal, oats, wheat, molasses, etc.

Crude Fat: Crude fat is an estimate of the total fat content of feeds from the older collection of methods known as proximate methodology. The crude fat is estimated using ether extraction. Crude fat contains true fat (triglycerides) as well as alcohols, waxes, terpenes, steroids, pigments, ester, aldehydes, and other lipids. See ether extract and fat.

Crude Fiber (CF): This older proximate method was used to divide carbohydrates into digestible and indigestible fractions. When CF content is higher, the energy content of the feed is lower because crude fiber is considered as indigestible. This was one part of the original system of analyzing the “digestible” fraction in feedstuffs. This method uses sequential acid and alkali extraction. It was developed by Henneberg and Sttohmann during the 1860s at the Weende Experiment Station in Germany, and is often referred to as the Weende System of proximate analysis. The CF extract was once used as a standard analysis for fibrous parts or the indigestible portion of carbohydrates in feeds. However, some of these substances are partially digestible by microorganisms in the rumen. Crude fiber accounts for most of the cellulose but only a portion of the lignin and no ash, so it underestimates true fiber and is less than acid detergent fiber (ADF). Thus, CF is not a good indicator of digestibility in ruminant animals, and the use of this parameter in feeds for ruminants is declining.

Even though CF is not a very useful parameter for quantifying forage fiber where lignin content is substantial, the CF is a reasonable estimate of the fiber in grains because of their low lignin content. Thus, it is still commonly used for analysis of feeds for nonruminants or monogastric animals (i.e., those do not have a chambered stomach or rumen; for example, horses and pigs). Crude fiber is still used today as the legal measurement of fiber in grains and finished feeds. See acid detergent fiber (ADF) and neutral detergent fiber (NDF) for contrast.  

Crude Protein (CP): Proteins are organic compounds composed of building blocks called amino acids. They are a major component of vital organs, tissue, muscle, hair, skin, milk, and enzymes. Protein is required on a daily basis for maintenance, lactation, growth, and reproduction.

The crude protein content of a feed sample represents the total nitrogen (N) in the diet which includes not only true protein but also non-protein nitrogen (e.g., urea and ammonia in a feed but nitrate is not included). Because N is an integral part of any amino acid, non-protein nitrogen has the potential to be utilized for protein synthesis by rumen microorganisms. In laboratory analysis, total N present in a feed sample is first determined and then the total amount of protein is calculated by multiplying the total N by 6.25. The basis for this is that leaf and stem issue protein contains 16 percent nitrogen, or 1 part nitrogen to 6.25 parts protein. Unless otherwise stated, protein values given in lab reports, feed tables, and feed tags are crude protein. 

Because the protein contents of forages, silages, or grains used in animal feeding are sometimes inadequate to meet the needs of the animal class, protein supplements become essential. Consequently, analysis for total protein or crude protein in a feed sample is important.

Crude protein in the feeds for ruminants can be further fractionated according to their rate of breakdown in the rumen as will be discussed below for neutral detergent fiber insoluble crude protein (NDFICP) and discussed earlier for acid detergent fiber insoluble crude protein (ADFICP).

No doubt, CP is an important indicator of the protein content of a forage crop, and even estimates of non-protein nitrogen are of value in evaluating the nutritive value. However, it is a false perception that protein is always the most limiting nutrient in the animal’s diet and CP is the ultimate measure of a forage quality. In fact, the energy value of forages is often the most limiting quality attribute to meet the animal’s requirements in most forage-based feeding. An overemphasis on CP may cause one to fail to pay due attention to meeting energy requirements. Furthermore, CP is merely an estimate of nitrogen content (N, % × 6.25 = CP, %) and must be considered in context of plant maturity, species, fertilization rate, and many other characteristics. For example, a high nitrate concentration in the forage will result in an artificially high CP level.