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Feed nutrients

Back: Ruminant Anatomy and Physiology Table of Contents Next: Feedstuffs


Feed nutrients


Energy (includes primarily carbohydrates and fats, but protein also can be used as energy) in a feed may be separated into: 1) the losses that occur in digestion and metabolism, and 2) the remainder that is available to the animal for maintenance and production. Figure 4 shows this division of energy. Gross energy refers to the total energy in feed, which is determined by complete oxidation (burning) of the feedstuff and measurement of the heat produced. The energy value is expressed in calories. Common feedstuffs are similar in gross energy content, but differ in feeding value because of differences in digestibility. Digestible energy is gross energy minus fecal (manure) loss. These losses will be greater for high fiber rations than for low fiber rations. Other losses include those in urine and gas. In the rumen, considerable methane is produced, representing an energy loss because the animal cannot use methane and must eructate (belch) the gas. These losses, added to fecal losses, are considered in calculating the metabolizable energy. Heat is produced during digestion and metabolism. Other than during cold weather, this heat has no value and represents a loss of energy. The remaining energy is net energy (NE) available for maintenance and production. In the requirement and feed tables for growth, a net energy value for maintenance and net energy value for gain are given. These values differ because animals use energy for maintenance more efficiently than for growth. The efficiency of energy use by lactating cows for maintenance and lactation is similar. Therefore, only one net energy value, net energy of lactation, is used for these two functions.

Total digestible nutrients (TDN) is another method of expressing the energy content of feeds or the energy requirements of cattle. TDN is comparable to digestible energy. It has been in use longer than the net energy system and more values are available for feedstuffs.

TDN = Digestible nitrogen-free extract (carbohydrate)
+ digestible crude fiber
+ digestible protein
+ (digestible ether extract x 2.25)

Both NE and TDN are given in this publication. NE of lactation (NEL) can be calculated from TDN:

NEL (Mcal/lb DM) = (TDN, % of DM x .01114) - .054
Figure 5

Figure 5. Relationship among crude fiber,
acid detergent fiber, and neutral detergent
fiber analyses. Sources: Van Soest and Moore
(1966), and Harris (1975*).

Carbohydrates. Carbohydrates are the major source of energy in diets for dairy cattle. Between 50 and 80 percent of the DM in forages and grains is carbohydrate. Three major categories of carbohydrate exist in feeds: 1) simple sugars (glucose); 2) storage carbohydrates (starch); 3) structural carbohydrate or fiber (cellulose and hemicellulose). Sugars are found in the cells of growing plants and in such feeds as molasses. Starch is the main component of grains. Cellulose and hemicellulose are made up of sugar molecules, as is starch, but are bound together differently. Cellulose and hemicellulose are classified as fiber, giving structure and strength to plant tissues. Simple-stomach animals, such as pigs and poultry, cannot digest much fiber. Adult ruminants digest fiber because the microbial population in the rumen breaks it down into usable products. Lignin, which is also a component of plants, is not a true carbohydrate. This compound is virtually indigestible. Feed digestibility is lowered when lignin is present in large amounts, such as in mature forages.

Three methods are available to measure fiber content of feeds (figure 5). Crude fiber (CF) is the oldest method and is the residue of a feed that is resistant to successive boiling with dilute acid and alkali treatments. Crude fiber is not an accurate measure of total fiber or cell walls because much of the lignin and hemicellulose is lost during the analysis. Even cellulose is not totally recovered in the CF fraction. Many feed testing labs no longer report CF, but it is required on feed tags of purchased feeds.

Acid detergent fiber (ADF) consists of cellulose, lignin, lignified nitrogen compounds (heat damaged protein), and insoluble ash. Acid detergent fiber does not represent the total fiber content in feed, as it does not account for hemicellulose. It is a relatively quick method for measuring fiber, often substituting for CF. Equations used to predict the digestibility or energy content of feedstuffs are usually based on ADF or include ADF as a major component.

Neutral detergent fiber (NDF) consists of ADF plus hemicellulose, and is often called cell walls. Because NDF represents the total fiber in a feed, it is highly correlated to intake, rumination, and total chewing time. Corrected for physical form, NDF provides the best measurement of effective fiber for formulating dairy rations.

The fineness at which forages are chopped during harvesting can alter the effectiveness of fiber for maintaining chewing activity. Hay-crop silages should be chopped at a minimum of 3/8 inch theoretical length of cut (TLC) to provide 15 to 20 percent (weight basis) of the particles greater than two inches long. Chopping at 1/4 inch TLC provides only about 10 percent of the forage particles greater than two inches long. Corn silage should be chopped at 1/4 to 3/8 inch TLC. Rations based on 1/4 inch TLC silage should include 5 pounds of long stem hay to provide adequate "effective" fiber. Hay-crop silage chopped at 3/16 inch TLC with less than 7 percent coarse particles should be fed with 8 to 10 pounds of long hay. Holstein cows need to chew about 11 to 12 hours per day or 12 to 14 minutes per pound of DM eaten to keep milk fat above 3.5 percent.

High fiber by-product feeds supply some "effective" NDF and can be used to partially replace NDF coming from forages in the ration. Whole cottonseed possesses the best forage NDF replacement value of commonly available by-product feeds fed in milking cow rations.

Starch, sugar, and pectin make up the highly digestible carbohydrate fraction in feeds termed non-fiber carbohydrates (NFC). Subtracting percent (DM basis) NDF, CP, ether extract or fat and ash from 100 provides an estimate of NFC percent in feeds.

(NFC% = 100 - [%NDF + %CP + %fat + %ash])

The term nonstructural carbohydrate is often used interchangeably with NFC but is analytically determined and may be slightly different from NFC.

Carbohydrate status of dairy rations has traditionally been evaluated with regard to measures of structural carbohydrates – ADF or NDF. However, optimum microbial growth in the rumen requires adequate amounts of NFC along with degradable intake protein (DIP) in the ration. Insufficient amounts of NFC in rations depress microbial growth and digestion of feed in the rumen, while excess NFC in rations causes acidosis and/or low milk fat tests.

Fat. The energy content of fat is about 2.25 times the energy in carbohydrates. Diets for baby calves that include large quantities of milk or milk replacer may contain 10 to 35 percent fat in the DM consumed. Fat may be added to the diets of adult ruminants to increase energy density and to reduce feed dustiness. Typical diets usually contain no more than 4 percent fat in the DM. Total fat and oil should be limited to less than 7 percent of the DM in lactating cow rations. Too much fat decreases feed intake, may depress fat and protein content of milk, and may cause scouring. Commonly fed sources of fat include whole cottonseeds, full-fat soybeans, sunflower seeds, tallow, and various rumen-inert fat products.

Fatty acid profile (table 2) is the most important characteristic of a fat source in determining how much to feed, as it is related to ruminal inertness and postruminal digestibility. Dietary fat comes as either fatty acids or triglycerides (three fatty acids attached to glycerol). The oils found in oilseeds and the fats in animal fats are examples of triglycerides. Rumen bacteria hydrolyze triglyceride to fatty acids and glycerol. The fatty acids in feeds and specialty fats are either saturated or unsaturated, and primarily range from 14 to 18 carbons in chain length. Saturated fatty acids (myristic C14:0, palmitic C16:0 and stearic C18:0) are solid at room temperature; hence the term "hard" fats. Tallow is considered a saturated fat, although about 50 percent of the fatty acids in tallow are unsaturated. Unsaturated fatty acids (palmitoleic C16:1, oleic C18:1, linoleic C18:2, and linolenic C18:3) vary in the temperature at which they become liquid, but as the amount of unsaturation increases, the temperature at which the fat becomes liquid decreases. Whole cottonseed and soybeans have 71 and 85 percent unsaturated fatty acids, respectively. However, feeding oilseeds whole encapsulates the fat and results in a slower digestion, allowing a slow release of fat into the rumen and more uniform conversion of the unsaturated fatty acids to saturated fatty acids by the bacteria. Unsaturated fatty acids interfere with rumen fermentation more than saturated fatty acids. Protected or ruminal inert fats are commercially produced fats that do not affect rumen fermentation.


Protein is essential for maintenance, growth, and milk production. The protein requirement of dairy cattle is really a requirement for amino acids by the animal tissues. Amino acids are supplied by the digestion of microbial protein, and by feed protein that escapes microbial breakdown in the rumen.

Protein requirements in this publication are expressed as crude protein (CP), either in amounts or as a percentage of the dietary DM. Crude protein is determined by multiplying the nitrogen content in a feed by the factor 6.25 (feed protein averages 16 percent nitrogen). Nonprotein nitrogen compounds, such as urea or ammonium salts, are considered to have a CP value, but supply no amino acids directly. Micro-organisms in the rumen convert nitrogen from NPN sources into amino acids for their use. Digestion of microbial protein in the small intestine releases amino acids for absorption and utilization, the same as amino acids released from the digestion of true proteins (composed of amino acids) in feeds.

Approximately 60 percent of the CP in the typical dairy cow diet is broken down by microbial digestion to ammonia. The rumen microbes must convert the ammonia to microbial protein if the dairy animal is to receive any benefit. Fermentable energy must be available for the micro-organisms to grow and synthesize the necessary amino acids. If rumen ammonia levels are excessively high, the ammonia is absorbed into the blood and recycled or excreted in urine as urea. All feed protein sources are not degraded in the rumen to the same extent. Three protein terms describe the fate of dietary protein in the rumen. Degradable intake protein (DIP) is the portion of feed protein broken down to ammonia or amino acids by the rumen microbes. Soluble intake protein (SIP) is the portion of DIP that is rapidly degraded in the rumen. Generally, SIP is about half of the DIP. Undegradable intake protein (UIP) is the portion of feed protein that is not degraded by the rumen microbes and remains intact as it passes through the rumen. Other terms for UIP include by-pass protein and escape protein. Values for UIP, SIP and DIP can be expressed as either a percent of the dietary DM (for example, a feed may contain 17 percent CP and 6.8 percent UIP in the DM) or as a percent of the CP (for example, 40 percent UIP calculated by 6.8/17). The sum of DIP and UIP, expressed as percent of CP, must equal 100. Diets for high producing dairy cows should contain 19 percent CP with 38 percent of the CP as UIP, 62 percent as DIP and 30 percent as SIP; or 19 percent CP with 7.2 percent UIP, 11.8 percent DIP and 5.7 percent SIP in the dietary DM.

The optimal diet fed to dairy cattle will 1) meet the nitrogen requirement of rumen micro-organisms for maximum synthesis of micro-organism protein and 2) allow for maximum escape or bypass of high quality feed protein for digestion in the small intestine. Protein synthesis by rumen microbes will depend on feed intake, organic matter digestibility, feed type, protein level, and feeding system. Since 4.5 pounds of microbial protein synthesis per day is near the maximum, the remainder of the protein must be derived from UIP sources. Young, fast-growing heifers and high-producing cows may require additional UIP sources beyond their normal diet to meet their amino acid requirements. Brewers grain, distillers grain, corn gluten meal, or heat-treated soybeans are examples of UIP feeds that could be substituted in diets where excess rumen ammonia exists and less than optimal amounts of quality feed protein pass into the small intestine. Excess protein, above requirements, is used as a source of energy.


Vitamins are organic compounds needed in small amounts for life support. They contribute no measurable amount of protein or energy to the animal but function in chemical reactions involving other nutrients. If a vitamin deficiency occurs, definite symptoms appear and become more severe unless corrected.

The vitamins can be classified into two broad groups: 1) water soluble vitamins, and 2) fat soluble vitamins. Water soluble vitamins include the B vitamins: thiamine or B1, riboflavin or B2, niacin, pantothenic acid, pyridoxine or B6, vitamin B12, biotin, and folic acid. Choline is often included in the B complex group. Research indicates that under normal dietary conditions, B vitamins are synthesized in sufficient amounts in the rumen to meet the animal needs. Supplemental B vitamins may be required during diseased conditions, periods of stress, or for very young animals. Supplementing 6 grams of niacin from 2 weeks prepartum to 8 to 12 weeks postpartum can reduce ketosis and minimize fatty liver formation in high-producing cows. Another water soluble vitamin, vitamin C (ascorbic acid), is not required by dairy cattle because they are able to synthesize it within their tissues.

Table 3 shows fat soluble vitamins (with functions, deficiency symptoms, and associated problems) in common feed sources. Vitamin A or its precursor, carotene, is needed for normal vision, and for avoidance of reproductive problems and respiratory disturbances. Carotene is found in green forages and yellow corn. Synthetic forms of vitamin A are available. One milligram of beta carotene provides 400 international units (IU) of vitamin A.

Vitamin D, the sunshine vitamin, is needed for bone growth and development and for absorption and metabolism of calcium and phosphorus. Supplementation of vitamin D is essential when animals are confined without benefit of direct sunlight and receive primarily ensiled forages and grains. Animal sources of vitamin D (called D3) and plant sources (called D2) are biologically equivalent in dairy cattle.

Vitamin E is an antioxidant, reducing off-flavors in milk (1000 IU per cow per day), preventing white muscle disease in calves, lessening the severity and duration of subclinical mastitis, improving reproductive performance, and stimulating the immune system. One milligram of alpha tocopherol equals 1 IU of vitamin E.

Vitamin K normally is synthesized in adequate amounts in the rumen and intestine to meet the animal's needs. Hemorrhage, associated with moldy sweet clover disease, is caused by interference with the function of vitamin K. This problem is discussed in the herd health section.

Vitamins are most commonly supplemented in the diet, but are available in injectable forms. The recommended minimum and maximum vitamin content in diets for dairy cattle are listed in tables A-14 and A-15. Vitamin requirements or potency in feeds is expressed in either international units (IU) or United States Pharmacological units (USP). One IU per pound is equal to one USP per pound.


Minerals have both structural and regulatory functions. They are needed for bone and teeth formation, hormone actions, enzyme activation, and water balance. They are a structural part of hormones (iodine in thyroxine as an example) and hemoglobin (iron).

The minerals needed in largest amounts (macro minerals) by the dairy cow include: calcium, phosphorus, magnesium, sodium, chlorine, sulfur, and potassium. Sodium and chlorine usually are provided in the form of salt. Minerals required in small amounts (trace minerals) include iron, copper, manganese, zinc, iodine, cobalt, and selenium. Other minerals may be required in very minute amounts, including molybdenum and fluorine. Table 4 lists functions, deficiency symptoms, and feed sources of minerals. Feed tables A-9, A-10, and A-11 present the amounts of calcium and phosphorus in some common feeds. Table A-12 shows the composition of calcium and phosphorus supplements. Table A-13 gives major or macro mineral composition of some common feeds and mineral supplements. Tables A-14 and A-15 list recommended minimum and maximum mineral levels in diets for cows and heifers.

Sodium, potassium, chloride, and sulfur are strong ions and have a significant influence on acid-base regulation in the cow. When cations (minerals with a positive charge) exceed anions (minerals with a negative charge) in a solution the pH will be basic (above 7) and when anions exceed cations, the solution will be acidic (pH less than 7). The dietary cation-anion difference (DCAD) is the milliequivalent (meq) difference between the major cations and anions in a diet. The DCAD can be calculated when the percentages of Na, K, Cl and S (DM basis) in the diet are known:

DCAD, meq/100 grams of DM = [(%Na/.023) + (%K/.039)] - [(%Cl/.0355) + (%S/.016)]

Feeding a diet with a negative DCAD (anionic diet) in late pregnancy may reduce milk fever problems through alterations in calcium metabolism. Lactating cows need a high cationic diet to replenish the outflow of minerals in milk.

Many commercial mineral supplements are available. When a mineral supplement is chosen, mineral(s) needed to balance the diet should be considered. A supplement that properly meets the need should be selected. Cost per unit of the mineral needed should be the primary consideration. This can be determined by dividing the cost of the supplement (per hundred lb or per ton) by the number of pounds of element in the supplement. (Example: 15 percent phosphorus รท $15.00 per cwt = $1 per pound of phosphorus.)

Trace mineralized salt is a common source of salt and trace minerals. The adequacy of trace mineralized salt to meet requirements of an animal will depend on the potency of trace minerals in the salt and productivity level of the animal. Trace mineralized salt does not contain appreciable amounts of magnesium, sulfur, or potassium. In many diets, these minerals are present in adequate amounts and do not require supplementation. However, if they are needed, proper supplements will have to be provided. Magnesium oxide is a common source of magnesium. Sulfur can come from elemental sulfur, sodium sulfate, magnesium sulfate, or potassium sulfate. Potassium is available in potassium salts such as potassium chloride or potassium sulfate. Many commercially formulated feeds contain trace mineral information on the feed tag.

Mineral excesses should be avoided because of interaction with other minerals and possible toxicity. For example, even though copper is needed in small amounts, it can be toxic. Lead causes toxicity and can be a problem when stanchions and stalls are painted with a lead-base paint. Such paint should be avoided in painting barn equipment and other items to which cattle have chewing access.


Water is the nutrient required in the largest amount by dairy cattle. The amounts of water (gallons per day) needed by cattle for growth, maintenance, pregnancy, and milk production are estimated in Table 5. An equation can also be used to estimate water intake of lactating dairy cows.

Water intake (gal/day) = 4.22
+ (0.19 x DM intake)
+ (0.108 x pounds of milk)
+ (0.374 x ounces of sodium)
+ (0.06 x minimum daily temperature in F)

A lactating cow consuming 40 pounds of DM and 3 ounces of sodium, producing 65 pounds of milk and experiencing an average minimum temperature for the week of 50 degrees F should consume an average of 22.5 gallons of water per day [4.22 + (0.19 x 40) + (0.108 x 65) + (0.374 x 3) + (0.06 x 50)].

Fresh water should always be available for cattle. Water supplies contaminated with bacteria or high in nitrates and sulfates may cause health problems and reduce animal performance. A guideline for acceptable water quality standards for dairy cattle is in Table A-4.

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