To download the feed composition tables that accompany this article, please click here.
Typical composition of commonly used feeds for sheep and cattle.
Editor's Note: Since 1957, R.L. Preston has taught and conducted animal nutrition research in the areas of protein, minerals, growth and body composition. He also has conducted cattle feeding research on the energy value of feeds, growth promotants and nutrition management.
Preston has been a member of the National Research Council (NRC) Committee on Animal Nutrition and president of the American Society of Animal Science. Retired as Emeritus Professor from Texas Tech University, where he was Horn Distinguished Professor and occupied the Thornton Endowed Chair, Preston now lives in Pagosa Springs, CO.
Nutrition research spanning more than 100 years has defined the nutrients required by animals. Using the feed composition information in the accompanying table, 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 goal of feedstuff analysis is to predict the productive response of animals when they are fed diets of a given 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. Actual analysis should be obtained and used whenever possible. Often, however, it is 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.
When tabulated data are used, it should be understood that feeds vary in their composition. Using the data in the accompanying table, one can expect the organic constituents (i.e., 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 and 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.
New varieties of crops usually result in nutrient composition variation. Genetically modified crops will result in feeds with generally improved nutrient content and availability; the composition of these feeds, however, will require specific nutrient profiles.
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 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 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. 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.
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 that it can be considered insignificant. Blanks indicate the value is unknown. The table contains revisions and values for feeds not previously included.
Using Information In The Table Feed names: The most commonly used feed names are given in the table. "Fresh" feeds are feeds that are grazed or fed as fresh cut materials.
Dry matter: Typical dry matter (DM) values are shown. Since the moisture content of feeds can be the biggest reason for variation in feedstuff composition on an "as-fed basis," chemical constituents and biological attributes of feeds shown in the table are on a DM basis. Because 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." 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: Four measures of the energy value of feeds are shown in the table. Total digestible nutrients (TDN) is shown because there are more determined TDN values for feeds and because this has been the standard system for expressing the energy value of feeds for cattle and sheep. TDN has several technical problems, however.
As mentioned below, 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. Some have argued that energy is not measured in pounds or percent, and therefore TDN is not a valid measure of energy. This is more a scientific argument than a criticism of the predictive value of TDN.
Digestible energy (DE) values are not included in the table. The relationship between TDN and DE in cattle and sheep is constant. DE (Mcal/cwt.) can be calculated by multiplying the percent TDN content by 2. The ability of TDN and DE to predict animal performance is equal.
Interest in the use of net energy (NE) in evaluating feeds for cattle and sheep 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 problem in using 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 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 one 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 added to computer programs designed to formulate diets and predict performance.
In deciding on the energy system to use, the theoretical superiority of NE over TDN in predicting animal performance is unquestionable. 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 actually have 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. Crude protein does not give any information on the actual protein and non-protein nitrogen content of a feed. Digestible protein (DP) has been included in many tables of feed composition, but because of the contribution of microbial and body protein to the protein in feces, DP is more misleading than CP. To calculate DP from the CP content of the diet fed to cattle or sheep use the following equation:
%DP = 0.9(%CP) -3
where %DP and %CP are the diet values on a DM basis.
Undegradable intake protein (UIP; rumen "by-pass" or escape protein) values are shown. This value represents the percent of CP that passes through the rumen without being degraded by rumen microorganisms. These values are not constant. UIP values on many feeds have not been determined, and reasonable estimates are difficult to make.
How should these values be used to improve the predictability of animal response when fed various feeds? Generally, degradable intake protein (DIP) can supply CP up to 7% of the diet. If the CP required in the diet exceeds 7% of the DM, all CP above this amount should be UIP. 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 UIP. Once the relationships between UIP and DIP have been better quantified, CP requirements may be lowered especially at higher CP levels.
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. In the old scheme, the remaining carbohydrates (NFE) were thought to be more digestible than CF, even though many feeds have been shown to have a higher digestibility for CF than NFE. One reason CF remained in the analytical scheme was its apparent requirement for the calculation of TDN.
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 these measures relate more directly to predicted animal performance and thus 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 5-20% 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 (i.e., chopping, grinding, pelleting) and hydration (fresh forage, silages, high moisture grains) if these 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 to consider in most feeding situations. Potassium (K) becomes more important as the level of concentrate increases and when non-protein nitrogen 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 that preclude showing a single value in a table of feed composition. 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 than to a characteristic of the feed itself. Trace-mineralized salt and trace mineral premixes are recommended to supplement known trace mineral deficiencies.
Vitamins: Vitamins have been omitted from the table. The only vitamin of general importance in ruminant feeding is vitamin A. The vitamin A and carotene value in feeds depends largely on maturity and conditions at harvest, and the length and conditions of storage. Therefore, it is probably unwise to rely entirely on harvested feeds as a source of vitamin A. Where roughages are being fed that contain good green color or are being fed as immature fresh forages (i.e., pasture), vitamin A value will probably be sufficient to meet the animal's requirement. Other vitamins, if required, should be supplied as supplements.