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
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 (NEm), growth (NEg) 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 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)].
In deciding on the energy system to use, there's 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 best.
NE1 values are also shown but few have actually been determined. NE1 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. CP provides no information on the actual protein and non-protein nitrogen (NPN) content of a feed.
Digestible protein (DP) has been included in many feed composition tables. But because of the contribution of microbial and body protein to the protein in feces, DP is more misleading than CP. One can estimate DP from the CP content of the diet fed to cattle or sheep by 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 represent the percent of CP passing through the rumen without degradation by rumen microorganisms. Degradable intake protein (DIP) is the percent of CP degraded in the rumen and is equal to 100 minus UIP. Like other biological attributes, these values aren't constant. UIP values on many feeds haven't been determined, and reasonable estimates are difficult to make.
How should these values be used to improve the predictability of animal performance when fed various feeds? Generally, DIP can supply CP up to 7% of the diet. If the required CP 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 UIP.
Once the relationships between UIP and DIP have been better quantified, CP requirements may be lowered, especially at higher CP levels. For diets high in rumen fermentable carbohydrate, DIP requirements may determine the total CP required in the diet.
Crude, acid detergent and neutral detergent fiber: Crude fiber (CF) is declining in use as a measure of poorly digested carbohydrates in feeds. Its major problem is that variable amounts of lignin, which isn't digestible, are removed in the CF procedure. In the old scheme, the remaining carbohydrates (NFE) were thought to be more digestible than CF despite many feeds having higher CF digestibility than NFE. One reason CF remained in the analytical scheme was its apparent requirement for the TDN calculation.
Improved analytical procedures for fiber have been developed, namely acid detergent fiber (ADF) and neutral detergent fiber (NDF). ADF is related to feed digestibility and NDF is somewhat related to voluntary intake and the availability of net energy. Both measures relate more directly to predicted animal performance and thus are more valuable than CF. Lignification of NDF, however, alters the availability of the surface area to fiber-digesting rumen microorganisms; lignin, therefore, may be added to future tables.
Recently, effective NDF (eNDF) has been used to better describe the dietary fiber function in high-concentrate, feedlot-type diets. While eNDF is defined as the percent of NDF retained on a screen similar in size to particles that will pass from the rumen, this value is further modified based on feed density and degree of hydration.
Rumen pH is correlated with dietary eNDF when diets contain less than 26% eNDF. Thus, when formulating high-concentrate diets, including eNDF may help prevent acidosis in the rumen. In feedlot diets, the recommended eNDF levels range from 5-20% depending on bunk management, inclusion of ionophores, digestion of NDF and/or microbial protein synthesis in the rumen.
Estimated eNDF values are shown for many feeds. These should be decreased depending on degree of feed processing (e.g., chopping, grinding, pelleting, flaking) 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 feed's crude fat content.
Minerals: Values are shown for only certain minerals. Calcium (Ca) and phosphorus (P) are important minerals to consider in most feeding situations. Potassium (K) is more important as the concentrate level increases and when NPN is substituted for intact protein in the diet.
Sulfur (S) also becomes more important as the NPN level increases in the diet. High dietary S levels compounded by high S levels in drinking water, however, can be detrimental. Zinc (Zn) is shown because it's less variable and 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.
The level of many other 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. Iodine and selenium are required nutrients that may be deficient in many diets, yet their level in a 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 generally used to supplement trace minerals; their use is encouraged where deficiencies exist.
Vitamins: Vitamins are not included in the table. The only vitamin of general practical importance in cattle and sheep feeding is the vitamin A value (vitamin A and carotene) in feeds. This depends largely on maturity and conditions at harvest, and the length and conditions during storage. Thus, it's probably unwise to rely entirely on harvested feeds as a source of vitamin A value.
Where roughages are 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 animal requirements. Other vitamins, if required, should be supplied as supplements.
Future table revisions
A feed composition table is of value only if it's relatively complete and contains feeds commonly fed, and the data are constantly updated. Suggestions and compositional data to keep this table useful to the cattle and sheep industry are welcomed.
When sending compositional data, adequately describe the feed, indicate the DM or moisture content and if the analytical values are on an as-fed or DM basis. If more than one sample was analyzed, indicate the number of samples analyzed.
Editor's Note: Since 1957, Rod Preston, Ph.D., has taught and conducted animal nutrition research in the areas of protein, minerals, growth and body composition. He's also conducted cattle feeding research on the energy value of feeds, growth enhancers and nutrition management.
He was a member of the NRC Committee on Animal Nutrition and President of the American Society of Animal Science. He is Emeritus Professor from Texas Tech University, where he was a Horn Distinguished Professor and held the Thornton Endowed Chair. His address is 1495 E. Village Lane #B, Bellingham, WA 98226-8017.