Nutrition research spanning more than a hundred years has defined the nutrients required by animals. Using this information, diets can be formulated from feedstuffs 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 feedstuff analysis is to predict the productive response of animals when they are fed diets of a given composition. This is the real reason for information on feedstuff composition.

Table values for feedstuff composition Feedstuffs are not of constant composition. Unlike chemicals that are "chemically pure" and therefore have a constant composition, feeds vary in their composition for many reasons. Often, it is either impossible to determine actual composition or there is insufficient time to obtain such analysis and therefore tabulated data are the next best source of information after actual analysis.

When tabulated data are used, it should be understood that feeds vary in their composition. Using the data shown in the accompanying table, one can expect the organic constituents (e.g., crude protein, ether extract, crude fiber, acid detergent fiber and neutral detergent fiber) to vary as much as 15%, the mineral constituents to vary as much as 30% and the energy values to vary up to 10%. Therefore, values shown can only be guides. For this reason 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 that these values will be realistic for use in formulating cattle and sheep diets.

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 which 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 in a bomb calorimeter but have markedly different useful energy value (total digestible nutrients [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 accurately determine because there is an interaction between the chemical composition of a feed with the digestive and metabolic capabilities of the animal being fed. Biological attributes of feeds are more laborious to determine and are more variable than chemical constituents. They are generally more predictive, however, since they relate to the response of an animal being fed the feed or diet.

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

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

Dry matter: Typical dry matter (DM) values are shown; however, the moisture content of feeds can vary greatly. Therefore, DM content can be the biggest reason for variation in feedstuff 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 since 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." If one wants to convert a value shown to an "as-fed basis," multiply the decimal equivalent of the DM content times the compositional value shown in the table.

Energy: Four measures of the energy value are shown. TDN is shown because there are more determined TDN values for feeds and has been the standard system for expressing energy value. There are several technical problems with TDN, however. The digestibility of crude fiber (CF) may be higher than for nitrogen-free extract (NFE)in certain feeds. TDN also overestimates the value of roughages compared to concentrates in producing animals.

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. It should be apparent, however, that the ability of TDN and DE to predict animal performance is equal.

Interest in the use of net energy (NE) in evaluating feeds was renewed with the development of the California net energy system. The main reason for this is 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 for growing cattle and sheep is predicting feed intake and, therefore, the proportion of feed that will be used for maintenance and growth.

Some only use the NEg values but it should be obvious that 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 requirement. The most accurate way to use these NE values to formulate diets would be to use the NEm value plus a multiplier times the NEg value all divided by 1 plus the multiplier; the multiplier is the level of feed intake above maintenance 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, then the NE value of the diet 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 the theoretical superiority of NE over TDN in predicting animal performance. This superiority is lost, however, if only NEg is used in formulating diets. If NE is used, some combination of NEm and NEg is required. Net energy for lactation (NEl) values are also shown. Few NEl values have actually been determined. NEl values are similar to NEm values except for very high and low energy feeds.

Protein: Crude protein (CP) values are shown for each feed, which are Kjeldahl nitrogen times 100/16 or 6.25, since proteins contain 16% nitrogen on the average. Digestible protein (DP) has been included in many tables of feed composition but because of the contribution of microbial and body protein to the apparent protein in feces, DP is more misleading than CP. One can calculate DP from the CP content of the diet fed to cattle or sheep by the equation: %DP = 0.9 (%CP) -3 where %DP and %CP are the diet values on a DM basis.

Rumen "bypass" (escape or undegraded) protein values are shown. The value represents the percent of CP that passes through the rumen without being degraded by the rumen microorganisms. Like other biological attributes, these values are not constant.

How should these values be used to improve the predictability of animal response when fed various feeds? If the CP required in the diet exceeds 7% of the DM, all CP above this amount should be by-pass protein. In other words, if the final diet is to contain 13% CP, 6 of the 13 percentage units, or 46% of the CP should be in the form of bypass protein. Once these relationships have been better quantified, CP requirements may be lowered.

Crude, acid detergent and neutral detergent fiber: Crude fiber (CF) is declining in popularity as a measure of poorly digestible carbohydrates in feeds. The major problem with CF is that variable amounts of lignin, which is not digestible, are removed in the CF procedure.

Improved fiber analytical procedures have been developed, namely acid detergent fiber (ADF) and neutral detergent fiber (NDF). ADF is related to digestibility and NDF is also somewhat related to voluntary intake and the availability of net energy. Both of these measures relate more directly to predicted animal performance and, therefore, are more valuable than CF. Lignification of NDF, however, alters availability of surface area to fiber digesting rumen microorganisms; therefore, lignin may be added to future tables.

Recently, effective NDF (eNDF) has been proposed to better describe the dietary fiber function in high concentrate, feedlot-type diets. While eNDF is defined as the percent of NDF that is retained on a screen similar in size to particles that will pass from in the rumen, this value is further modified based on feed density and degree of hydration. Rumen pH was found to be correlated with dietary eNDF when diets contained less than 26% eNDF. Thus when formulating high concentrate diets, including eNDF will help to prevent acidosis in the rumen. The 1996 NRC Nutrient Requirements of Beef Cattle recommends eNDF levels for feedlot diets from 5 to 20% depending on bunk management, inclusion of ionophores and digestion of NDF and/or microbial protein synthesis in the rumen. Therefore, estimated eNDF values are shown for many feeds. These values must be modified, however, depending on degree of feed processing (e.g., chopping, grinding, pelleting) and hydration (fresh forage, silages, high moisture grains) if thes e feed forms are not specified in the table.

Ether extract: Ether extract (EE) shows the crude fat content of the feed.

Minerals: Values are shown for only certain minerals. Calcium (Ca) and phosphorus (P) are important minerals. Potassium (K) becomes more important as the level of concentrate increases and when non-protein nitrogen (N) is substituted for intact protein in the diet. Sulfur (S) also becomes more important as the level of non-protein nitrogen increases in the diet. Zinc (Zn) is shown because it is less variable and is more generally near a deficient level in cattle and sheep diets. Chlorine (Cl) is of increasing interest for its role in dietary acid-base relationships.

Several other minerals could logically be included in the table. The level of many trace minerals in feeds is largely determined by the level in the soil on which the feeds are grown or other environmental factors. Iodine and selenium are required nutrients that may be deficient in many diets, yet their level in feed is more related to the conditions under which the feed is grown. Trace mineralized salt and trace mineral premixes are generally used to supplement trace minerals; the use of these supplements is encouraged where there are known deficiencies.

Vitamins: The only vitamin of general practical importance in cattle and sheep feeding is the vitamin A value (vitamin A and carotene) in feeds which depends largely on maturity and conditions at harvest, and the length and conditions of storage. Where roughages are being fed that contain good green color or are being fed as immature fresh forages (e.g., pasture), there will probably be sufficient vitamin A value to meet the animal's requirement. Other vitamins, if required, should be supplied as supplements.

Future revisions to the table I welcome suggestions and compositional data to keep this table useful to the cattle and sheep feeding industry. When sending compositional data, please describe the feed, indicate the dry matter or moisture content and whether analytical values are given on an as-fed or dry matter basis. If more than one sample of a feedstuff was analyzed, the number of samples should also be indicated.