Nutrition research spanning more than 100 years has defined the nutrients required by animals. With this information, diets can be formulated from feeds and ingredients to meet these requirements with the expectation that animals will remain healthy, 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

Table values

Unlike chemicals, which are “chemically pure” and thus have a constant composition, feeds vary in their composition for many reasons.

So what is the value of showing composition data for feeds? An actual analysis of a feed to be used in a diet is much more accurate than using 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 they will be realistic for use in formulating cattle and sheep diets.

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

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 given the feed. Thus, in the accompanying table, certain chemical constituents are shown. The response of cattle and sheep when fed, however, can be termed the “biological response” to the feed. It's a function of the feed's chemical composition and the animal's ability to derive useful nutrient value from it.

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 animal's nutrient status 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, a feed's “biological attributes” have much greater meaning in predicting the productive response of animals. Unfortunately, biological attributes are more difficult to precisely determine because of the 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.

Source of table information

Several sources of information were used in arriving at the “typical values” shown in the following 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's not 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.

Using table information

Feed names: The most obvious or commonly used feed names are used in the table. Feeds designated as “fresh” are feeds that are grazed or fed as fresh-cut materials.

Dry matter: Typical dry matter (DM) values are shown, but the moisture content of feeds can vary greatly. Thus, DM content can be the biggest reason for variation in feed composition on an “as-fed basis.” For this reason, chemical constituents and biological attributes of feeds shown in the table are on a DM basis.

Since DM can vary greatly, and one of the factors regulating total feed intake is the DM content of feeds, diet formulation on a DM basis is preferred to using an as-fed basis. However, to convert a value to an as-fed basis, simply multiply the decimal equivalent of the DM content times the compositional value shown in the table.

Energy: The table lists four measures of the energy value of feeds. TDN (total digestible nutrients) is shown because there are more determined TDN values, and it's been the standard system for expressing the energy value of feeds for cattle and sheep. There are several technical problems with TDN, however.

For one, the digestibility of crude fiber (CF) may be higher than for nitrogen-free extract (NFE) in certain feeds due to the location of lignin in the CF analysis. TDN also overestimates the energy value of roughages compared to concentrates in producing animals. Some argue that since energy isn't measured in pounds or percent, TDN isn't a valid energy measure. This, however, is more a scientific argument than a criticism of TDN's predictive value.

Digestible energy (DE) values aren't included in the table. There's a fairly constant relationship between TDN and DE in cattle and sheep; DE (Mcal/cwt.) can be calculated by multiplying the %TDN content by 2. The ability of TDN and DE to predict animal performance is equal.

Interest in using net energy (NE) in feed evaluation was renewed with the development of the California net energy system. This is due to the improved predictability of results depending on whether feed energy is being used for maintenance (NE_m), growth (NE_g) or lactation (NE₁). The major problem in using these NE values is predicting feed intake and thus the proportion of feed that will be used maintenance and growth. Some only use NE_g but this suffers the equal but opposite criticism mentioned for TDN; NE_g will overestimate the feeding value of concentrates relative to roughages.