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 will also 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.

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Table values for feed composition

Unlike chemicals that are “chemically pure” and thus have a constant composition, feeds vary in their composition for many reasons. What is 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. Actual analysis should be obtained and used whenever possible. But it is often difficult or impossible to determine actual composition; therefore, tabulated data are the best source of information.

In using tabulated values, one can expect organic constituents (e.g., crude protein, ether extract, crude fiber, acid detergent fiber and neutral detergent fiber) to vary as much as ±15%, mineral constituents to vary as much as ±30% and energy values to vary up to ±10%. Thus, these values can only serve as guides; that's why they are called “typical values.” They are 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 usually result in nutrient composition changes. Genetically modified crops will result in feeds with generally 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 may not be related to the response of an animal. Thus, in the table beginning on page 56, certain chemical constituents are shown. The response of cattle and sheep, 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 depends upon the nutrient status of the animal and the productive or physiological function it is performing. Thus, ground fence posts or 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, biological attributes of a feed have much greater meaning in predicting the productive response of animals, but 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 are generally more predictive, however, since they relate to the animal's response to the feed or diet.

Sources of table information

Several sources of information were used in arriving at the “typical values” shown in the table. Where information was not available, a reasonable estimate was made from similar feeds or stage of maturity wherever possible. Where zeros appear, the amount is so small it can be considered insignificant in practical diet formulation. Blanks indicate that the value is unknown.

Using information contained in the table

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 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 more sound than using “as-fed” basis. To convert a value to an “as-fed” basis, 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. 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 of a scientific argument than a criticism of TDN's predictive value.

Digestible energy (DE) values are not included in the table. There is a 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 (NEm), growth (NEg) or lactation (NEl). The major problem in using these NE values is predicting feed intake, and thus the proportion of feed that will be used for maintenance and growth. Some only use NEg, but this suffers the equal but opposite criticism mentioned for TDN; NEg will overestimate the feeding value of concentrates relative to roughages.

The average of the two NE values can be used, but this would be true only for cattle and sheep eating twice their maintenance energy requirement. The most accurate way to use these NE values to formulate diets is to use the NEm value plus a multiplier times the NEg value, all divided by one plus the multiplier. The multiplier is the level of feed intake relative to maintenance. For example, if 700-lb. cattle are expected to eat 18 lbs. of DM, 8 lbs. of which will be required for maintenance, the diet's NE value would be: NE = [NEm + (10/8)(NEg)]/[1 + (10/8)].

Such a calculation can be easily introduced into computer programs designed to formulate diets and predict performance.

In deciding on the energy system to use, there is no question on NE's theoretical superiority over TDN in predicting animal performance. But this superiority is lost if only NEg is used to formulate diets. If NE is used, some combination of NEm and NEg is required. NEl values are also shown, but few have actually been determined. NEl values are similar to NEm values except for very high and low energy feeds.

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