Keep in mind though that feeds are not constant. The best way to balance a diet is to have your feedstuffs analyzed, especially your hay. Sometimes hay looks/smells great, but has little nutritive value.
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Here's parts of the article that you may find interesting.
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 are called "typical values." They aren't averages of published information, since judgment was used in arriving at some of the values in the hope that these values will be realistic for use in formulating cattle and sheep diets.
In addition, 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.
Chemical constituents vs. biological attributes of feeds
Feeds can be chemically analyzed for many things that may or not be related to an animal's response 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; that 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, and its ultimate efficiency of use depending 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 (total digestible nutrients 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. However, they are more difficult to precisely determine because there is an interaction between the feed's chemical composition and the animal's digestive and metabolic capabilities.
The 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.
Source of table information
Several sources of information were used in arriving at the typical values shown in the table. Where information wasn't available, but a reasonable estimate could be made from similar feeds or stage of maturity, this has been done; after all, it isn't too helpful to have a table with considerable missing information. Where zeros appear, the amount of that item is so small that it can be considered insignificant in practical diet formulation. Blanks indicate that the value is unknown.