Nutrition research spanning more than 100 years has defined the nutrients required by animals. Using this information, diets can be formulated from feeds and ingredients to meet these requirements with the expectation that animals will not only remain healthy, but also will be productive and efficient. The ultimate goal of feed analysis is to predict the productive response of animals when they are fed diets of a given nutrient composition.

Unlike chemicals that are “chemically pure” and thus have a constant composition, feeds vary in their composition for many reasons. What’s the value then of showing composition data for feeds? An actual analysis of a feed to be used in a diet is much more accurate than the use of tabulated composition data, and actual analysis should be obtained and used whenever possible. But, it’s often difficult to determine actual composition in a timely way; therefore, tabulated data are the best source of information.

In using tabulated values, one can expect organic constituents (e.g., crude protein, ether extract and fiber) to vary as much as ±15%, mineral constituents to vary as much as ±30%, and energy values to vary up to ±10%. Thus, the values shown can only serve as guides. That’s why they’re called typical values. They’re not averages of published information since judgment was used in arriving at some of the values in the hope these values will be realistic for use in formulating cattle and sheep diets.

New crop varieties may result in nutrient composition changes. Genetically modified crops may result in feeds with improved nutrient content and availability, and/or decreased anti-nutrient factors. 

Download the 2012 Feed Composition Tables here.

Chemical constituents vs. biological attributes

Feeds can be chemically analyzed for many things that may or may not be related to the response of an animal when fed the feed. Thus, in the accompanying table, certain chemical constituents are shown.

The response of cattle and sheep when fed a feed, however, can be termed the biological response to the feed. This is a function of its chemical composition and the ability of the animal to derive useful nutrient value from the feed. The latter relates to the digestibility or availability of a nutrient in the feed for absorption into the body. Its ultimate efficiency of use depends upon the nutrient status of the animal and the productive or physiological function being performed by the animal. Thus, ground fence posts and shelled corn may have the same gross energy value but have markedly different useful energy value (TDN or net energy) when consumed by the animal.

Therefore, the biological attributes of a feed have much greater meaning in predicting the productive response of animals but are more difficult to precisely determine because there’s an interaction between the feed’s chemical composition and the animal’s digestive and metabolic capabilities. Biological attributes of feeds are more laborious and costly to determine and are more variable than chemical constituents. They’re generally more predictive, however, since they relate to the animal’s response to the feed or diet.